core/iter/traits/iterator.rs
1use super::super::{
2 ArrayChunks, ByRefSized, Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, FlatMap,
3 Flatten, Fuse, Inspect, Intersperse, IntersperseWith, Map, MapWhile, MapWindows, Peekable,
4 Product, Rev, Scan, Skip, SkipWhile, StepBy, Sum, Take, TakeWhile, TrustedRandomAccessNoCoerce,
5 Zip, try_process,
6};
7use crate::array;
8use crate::cmp::{self, Ordering};
9use crate::num::NonZero;
10use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
11
12fn _assert_is_dyn_compatible(_: &dyn Iterator<Item = ()>) {}
13
14/// A trait for dealing with iterators.
15///
16/// This is the main iterator trait. For more about the concept of iterators
17/// generally, please see the [module-level documentation]. In particular, you
18/// may want to know how to [implement `Iterator`][impl].
19///
20/// [module-level documentation]: crate::iter
21/// [impl]: crate::iter#implementing-iterator
22#[stable(feature = "rust1", since = "1.0.0")]
23#[rustc_on_unimplemented(
24 on(
25 _Self = "core::ops::range::RangeTo<Idx>",
26 note = "you might have meant to use a bounded `Range`"
27 ),
28 on(
29 _Self = "core::ops::range::RangeToInclusive<Idx>",
30 note = "you might have meant to use a bounded `RangeInclusive`"
31 ),
32 label = "`{Self}` is not an iterator",
33 message = "`{Self}` is not an iterator"
34)]
35#[doc(notable_trait)]
36#[lang = "iterator"]
37#[rustc_diagnostic_item = "Iterator"]
38#[must_use = "iterators are lazy and do nothing unless consumed"]
39pub trait Iterator {
40 /// The type of the elements being iterated over.
41 #[rustc_diagnostic_item = "IteratorItem"]
42 #[stable(feature = "rust1", since = "1.0.0")]
43 type Item;
44
45 /// Advances the iterator and returns the next value.
46 ///
47 /// Returns [`None`] when iteration is finished. Individual iterator
48 /// implementations may choose to resume iteration, and so calling `next()`
49 /// again may or may not eventually start returning [`Some(Item)`] again at some
50 /// point.
51 ///
52 /// [`Some(Item)`]: Some
53 ///
54 /// # Examples
55 ///
56 /// ```
57 /// let a = [1, 2, 3];
58 ///
59 /// let mut iter = a.iter();
60 ///
61 /// // A call to next() returns the next value...
62 /// assert_eq!(Some(&1), iter.next());
63 /// assert_eq!(Some(&2), iter.next());
64 /// assert_eq!(Some(&3), iter.next());
65 ///
66 /// // ... and then None once it's over.
67 /// assert_eq!(None, iter.next());
68 ///
69 /// // More calls may or may not return `None`. Here, they always will.
70 /// assert_eq!(None, iter.next());
71 /// assert_eq!(None, iter.next());
72 /// ```
73 #[lang = "next"]
74 #[stable(feature = "rust1", since = "1.0.0")]
75 fn next(&mut self) -> Option<Self::Item>;
76
77 /// Advances the iterator and returns an array containing the next `N` values.
78 ///
79 /// If there are not enough elements to fill the array then `Err` is returned
80 /// containing an iterator over the remaining elements.
81 ///
82 /// # Examples
83 ///
84 /// Basic usage:
85 ///
86 /// ```
87 /// #![feature(iter_next_chunk)]
88 ///
89 /// let mut iter = "lorem".chars();
90 ///
91 /// assert_eq!(iter.next_chunk().unwrap(), ['l', 'o']); // N is inferred as 2
92 /// assert_eq!(iter.next_chunk().unwrap(), ['r', 'e', 'm']); // N is inferred as 3
93 /// assert_eq!(iter.next_chunk::<4>().unwrap_err().as_slice(), &[]); // N is explicitly 4
94 /// ```
95 ///
96 /// Split a string and get the first three items.
97 ///
98 /// ```
99 /// #![feature(iter_next_chunk)]
100 ///
101 /// let quote = "not all those who wander are lost";
102 /// let [first, second, third] = quote.split_whitespace().next_chunk().unwrap();
103 /// assert_eq!(first, "not");
104 /// assert_eq!(second, "all");
105 /// assert_eq!(third, "those");
106 /// ```
107 #[inline]
108 #[unstable(feature = "iter_next_chunk", reason = "recently added", issue = "98326")]
109 fn next_chunk<const N: usize>(
110 &mut self,
111 ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>>
112 where
113 Self: Sized,
114 {
115 array::iter_next_chunk(self)
116 }
117
118 /// Returns the bounds on the remaining length of the iterator.
119 ///
120 /// Specifically, `size_hint()` returns a tuple where the first element
121 /// is the lower bound, and the second element is the upper bound.
122 ///
123 /// The second half of the tuple that is returned is an <code>[Option]<[usize]></code>.
124 /// A [`None`] here means that either there is no known upper bound, or the
125 /// upper bound is larger than [`usize`].
126 ///
127 /// # Implementation notes
128 ///
129 /// It is not enforced that an iterator implementation yields the declared
130 /// number of elements. A buggy iterator may yield less than the lower bound
131 /// or more than the upper bound of elements.
132 ///
133 /// `size_hint()` is primarily intended to be used for optimizations such as
134 /// reserving space for the elements of the iterator, but must not be
135 /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
136 /// implementation of `size_hint()` should not lead to memory safety
137 /// violations.
138 ///
139 /// That said, the implementation should provide a correct estimation,
140 /// because otherwise it would be a violation of the trait's protocol.
141 ///
142 /// The default implementation returns <code>(0, [None])</code> which is correct for any
143 /// iterator.
144 ///
145 /// # Examples
146 ///
147 /// Basic usage:
148 ///
149 /// ```
150 /// let a = [1, 2, 3];
151 /// let mut iter = a.iter();
152 ///
153 /// assert_eq!((3, Some(3)), iter.size_hint());
154 /// let _ = iter.next();
155 /// assert_eq!((2, Some(2)), iter.size_hint());
156 /// ```
157 ///
158 /// A more complex example:
159 ///
160 /// ```
161 /// // The even numbers in the range of zero to nine.
162 /// let iter = (0..10).filter(|x| x % 2 == 0);
163 ///
164 /// // We might iterate from zero to ten times. Knowing that it's five
165 /// // exactly wouldn't be possible without executing filter().
166 /// assert_eq!((0, Some(10)), iter.size_hint());
167 ///
168 /// // Let's add five more numbers with chain()
169 /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
170 ///
171 /// // now both bounds are increased by five
172 /// assert_eq!((5, Some(15)), iter.size_hint());
173 /// ```
174 ///
175 /// Returning `None` for an upper bound:
176 ///
177 /// ```
178 /// // an infinite iterator has no upper bound
179 /// // and the maximum possible lower bound
180 /// let iter = 0..;
181 ///
182 /// assert_eq!((usize::MAX, None), iter.size_hint());
183 /// ```
184 #[inline]
185 #[stable(feature = "rust1", since = "1.0.0")]
186 fn size_hint(&self) -> (usize, Option<usize>) {
187 (0, None)
188 }
189
190 /// Consumes the iterator, counting the number of iterations and returning it.
191 ///
192 /// This method will call [`next`] repeatedly until [`None`] is encountered,
193 /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
194 /// called at least once even if the iterator does not have any elements.
195 ///
196 /// [`next`]: Iterator::next
197 ///
198 /// # Overflow Behavior
199 ///
200 /// The method does no guarding against overflows, so counting elements of
201 /// an iterator with more than [`usize::MAX`] elements either produces the
202 /// wrong result or panics. If debug assertions are enabled, a panic is
203 /// guaranteed.
204 ///
205 /// # Panics
206 ///
207 /// This function might panic if the iterator has more than [`usize::MAX`]
208 /// elements.
209 ///
210 /// # Examples
211 ///
212 /// ```
213 /// let a = [1, 2, 3];
214 /// assert_eq!(a.iter().count(), 3);
215 ///
216 /// let a = [1, 2, 3, 4, 5];
217 /// assert_eq!(a.iter().count(), 5);
218 /// ```
219 #[inline]
220 #[stable(feature = "rust1", since = "1.0.0")]
221 fn count(self) -> usize
222 where
223 Self: Sized,
224 {
225 self.fold(
226 0,
227 #[rustc_inherit_overflow_checks]
228 |count, _| count + 1,
229 )
230 }
231
232 /// Consumes the iterator, returning the last element.
233 ///
234 /// This method will evaluate the iterator until it returns [`None`]. While
235 /// doing so, it keeps track of the current element. After [`None`] is
236 /// returned, `last()` will then return the last element it saw.
237 ///
238 /// # Examples
239 ///
240 /// ```
241 /// let a = [1, 2, 3];
242 /// assert_eq!(a.iter().last(), Some(&3));
243 ///
244 /// let a = [1, 2, 3, 4, 5];
245 /// assert_eq!(a.iter().last(), Some(&5));
246 /// ```
247 #[inline]
248 #[stable(feature = "rust1", since = "1.0.0")]
249 fn last(self) -> Option<Self::Item>
250 where
251 Self: Sized,
252 {
253 #[inline]
254 fn some<T>(_: Option<T>, x: T) -> Option<T> {
255 Some(x)
256 }
257
258 self.fold(None, some)
259 }
260
261 /// Advances the iterator by `n` elements.
262 ///
263 /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
264 /// times until [`None`] is encountered.
265 ///
266 /// `advance_by(n)` will return `Ok(())` if the iterator successfully advances by
267 /// `n` elements, or a `Err(NonZero<usize>)` with value `k` if [`None`] is encountered,
268 /// where `k` is remaining number of steps that could not be advanced because the iterator ran out.
269 /// If `self` is empty and `n` is non-zero, then this returns `Err(n)`.
270 /// Otherwise, `k` is always less than `n`.
271 ///
272 /// Calling `advance_by(0)` can do meaningful work, for example [`Flatten`]
273 /// can advance its outer iterator until it finds an inner iterator that is not empty, which
274 /// then often allows it to return a more accurate `size_hint()` than in its initial state.
275 ///
276 /// [`Flatten`]: crate::iter::Flatten
277 /// [`next`]: Iterator::next
278 ///
279 /// # Examples
280 ///
281 /// ```
282 /// #![feature(iter_advance_by)]
283 ///
284 /// use std::num::NonZero;
285 ///
286 /// let a = [1, 2, 3, 4];
287 /// let mut iter = a.iter();
288 ///
289 /// assert_eq!(iter.advance_by(2), Ok(()));
290 /// assert_eq!(iter.next(), Some(&3));
291 /// assert_eq!(iter.advance_by(0), Ok(()));
292 /// assert_eq!(iter.advance_by(100), Err(NonZero::new(99).unwrap())); // only `&4` was skipped
293 /// ```
294 #[inline]
295 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
296 fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
297 for i in 0..n {
298 if self.next().is_none() {
299 // SAFETY: `i` is always less than `n`.
300 return Err(unsafe { NonZero::new_unchecked(n - i) });
301 }
302 }
303 Ok(())
304 }
305
306 /// Returns the `n`th element of the iterator.
307 ///
308 /// Like most indexing operations, the count starts from zero, so `nth(0)`
309 /// returns the first value, `nth(1)` the second, and so on.
310 ///
311 /// Note that all preceding elements, as well as the returned element, will be
312 /// consumed from the iterator. That means that the preceding elements will be
313 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
314 /// will return different elements.
315 ///
316 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
317 /// iterator.
318 ///
319 /// # Examples
320 ///
321 /// Basic usage:
322 ///
323 /// ```
324 /// let a = [1, 2, 3];
325 /// assert_eq!(a.iter().nth(1), Some(&2));
326 /// ```
327 ///
328 /// Calling `nth()` multiple times doesn't rewind the iterator:
329 ///
330 /// ```
331 /// let a = [1, 2, 3];
332 ///
333 /// let mut iter = a.iter();
334 ///
335 /// assert_eq!(iter.nth(1), Some(&2));
336 /// assert_eq!(iter.nth(1), None);
337 /// ```
338 ///
339 /// Returning `None` if there are less than `n + 1` elements:
340 ///
341 /// ```
342 /// let a = [1, 2, 3];
343 /// assert_eq!(a.iter().nth(10), None);
344 /// ```
345 #[inline]
346 #[stable(feature = "rust1", since = "1.0.0")]
347 fn nth(&mut self, n: usize) -> Option<Self::Item> {
348 self.advance_by(n).ok()?;
349 self.next()
350 }
351
352 /// Creates an iterator starting at the same point, but stepping by
353 /// the given amount at each iteration.
354 ///
355 /// Note 1: The first element of the iterator will always be returned,
356 /// regardless of the step given.
357 ///
358 /// Note 2: The time at which ignored elements are pulled is not fixed.
359 /// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`,
360 /// `self.nth(step-1)`, …, but is also free to behave like the sequence
361 /// `advance_n_and_return_first(&mut self, step)`,
362 /// `advance_n_and_return_first(&mut self, step)`, …
363 /// Which way is used may change for some iterators for performance reasons.
364 /// The second way will advance the iterator earlier and may consume more items.
365 ///
366 /// `advance_n_and_return_first` is the equivalent of:
367 /// ```
368 /// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
369 /// where
370 /// I: Iterator,
371 /// {
372 /// let next = iter.next();
373 /// if n > 1 {
374 /// iter.nth(n - 2);
375 /// }
376 /// next
377 /// }
378 /// ```
379 ///
380 /// # Panics
381 ///
382 /// The method will panic if the given step is `0`.
383 ///
384 /// # Examples
385 ///
386 /// ```
387 /// let a = [0, 1, 2, 3, 4, 5];
388 /// let mut iter = a.iter().step_by(2);
389 ///
390 /// assert_eq!(iter.next(), Some(&0));
391 /// assert_eq!(iter.next(), Some(&2));
392 /// assert_eq!(iter.next(), Some(&4));
393 /// assert_eq!(iter.next(), None);
394 /// ```
395 #[inline]
396 #[stable(feature = "iterator_step_by", since = "1.28.0")]
397 fn step_by(self, step: usize) -> StepBy<Self>
398 where
399 Self: Sized,
400 {
401 StepBy::new(self, step)
402 }
403
404 /// Takes two iterators and creates a new iterator over both in sequence.
405 ///
406 /// `chain()` will return a new iterator which will first iterate over
407 /// values from the first iterator and then over values from the second
408 /// iterator.
409 ///
410 /// In other words, it links two iterators together, in a chain. 🔗
411 ///
412 /// [`once`] is commonly used to adapt a single value into a chain of
413 /// other kinds of iteration.
414 ///
415 /// # Examples
416 ///
417 /// Basic usage:
418 ///
419 /// ```
420 /// let a1 = [1, 2, 3];
421 /// let a2 = [4, 5, 6];
422 ///
423 /// let mut iter = a1.iter().chain(a2.iter());
424 ///
425 /// assert_eq!(iter.next(), Some(&1));
426 /// assert_eq!(iter.next(), Some(&2));
427 /// assert_eq!(iter.next(), Some(&3));
428 /// assert_eq!(iter.next(), Some(&4));
429 /// assert_eq!(iter.next(), Some(&5));
430 /// assert_eq!(iter.next(), Some(&6));
431 /// assert_eq!(iter.next(), None);
432 /// ```
433 ///
434 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
435 /// anything that can be converted into an [`Iterator`], not just an
436 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
437 /// [`IntoIterator`], and so can be passed to `chain()` directly:
438 ///
439 /// ```
440 /// let s1 = &[1, 2, 3];
441 /// let s2 = &[4, 5, 6];
442 ///
443 /// let mut iter = s1.iter().chain(s2);
444 ///
445 /// assert_eq!(iter.next(), Some(&1));
446 /// assert_eq!(iter.next(), Some(&2));
447 /// assert_eq!(iter.next(), Some(&3));
448 /// assert_eq!(iter.next(), Some(&4));
449 /// assert_eq!(iter.next(), Some(&5));
450 /// assert_eq!(iter.next(), Some(&6));
451 /// assert_eq!(iter.next(), None);
452 /// ```
453 ///
454 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
455 ///
456 /// ```
457 /// #[cfg(windows)]
458 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
459 /// use std::os::windows::ffi::OsStrExt;
460 /// s.encode_wide().chain(std::iter::once(0)).collect()
461 /// }
462 /// ```
463 ///
464 /// [`once`]: crate::iter::once
465 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
466 #[inline]
467 #[stable(feature = "rust1", since = "1.0.0")]
468 fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
469 where
470 Self: Sized,
471 U: IntoIterator<Item = Self::Item>,
472 {
473 Chain::new(self, other.into_iter())
474 }
475
476 /// 'Zips up' two iterators into a single iterator of pairs.
477 ///
478 /// `zip()` returns a new iterator that will iterate over two other
479 /// iterators, returning a tuple where the first element comes from the
480 /// first iterator, and the second element comes from the second iterator.
481 ///
482 /// In other words, it zips two iterators together, into a single one.
483 ///
484 /// If either iterator returns [`None`], [`next`] from the zipped iterator
485 /// will return [`None`].
486 /// If the zipped iterator has no more elements to return then each further attempt to advance
487 /// it will first try to advance the first iterator at most one time and if it still yielded an item
488 /// try to advance the second iterator at most one time.
489 ///
490 /// To 'undo' the result of zipping up two iterators, see [`unzip`].
491 ///
492 /// [`unzip`]: Iterator::unzip
493 ///
494 /// # Examples
495 ///
496 /// Basic usage:
497 ///
498 /// ```
499 /// let a1 = [1, 2, 3];
500 /// let a2 = [4, 5, 6];
501 ///
502 /// let mut iter = a1.iter().zip(a2.iter());
503 ///
504 /// assert_eq!(iter.next(), Some((&1, &4)));
505 /// assert_eq!(iter.next(), Some((&2, &5)));
506 /// assert_eq!(iter.next(), Some((&3, &6)));
507 /// assert_eq!(iter.next(), None);
508 /// ```
509 ///
510 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
511 /// anything that can be converted into an [`Iterator`], not just an
512 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
513 /// [`IntoIterator`], and so can be passed to `zip()` directly:
514 ///
515 /// ```
516 /// let s1 = &[1, 2, 3];
517 /// let s2 = &[4, 5, 6];
518 ///
519 /// let mut iter = s1.iter().zip(s2);
520 ///
521 /// assert_eq!(iter.next(), Some((&1, &4)));
522 /// assert_eq!(iter.next(), Some((&2, &5)));
523 /// assert_eq!(iter.next(), Some((&3, &6)));
524 /// assert_eq!(iter.next(), None);
525 /// ```
526 ///
527 /// `zip()` is often used to zip an infinite iterator to a finite one.
528 /// This works because the finite iterator will eventually return [`None`],
529 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
530 ///
531 /// ```
532 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
533 ///
534 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
535 ///
536 /// assert_eq!((0, 'f'), enumerate[0]);
537 /// assert_eq!((0, 'f'), zipper[0]);
538 ///
539 /// assert_eq!((1, 'o'), enumerate[1]);
540 /// assert_eq!((1, 'o'), zipper[1]);
541 ///
542 /// assert_eq!((2, 'o'), enumerate[2]);
543 /// assert_eq!((2, 'o'), zipper[2]);
544 /// ```
545 ///
546 /// If both iterators have roughly equivalent syntax, it may be more readable to use [`zip`]:
547 ///
548 /// ```
549 /// use std::iter::zip;
550 ///
551 /// let a = [1, 2, 3];
552 /// let b = [2, 3, 4];
553 ///
554 /// let mut zipped = zip(
555 /// a.into_iter().map(|x| x * 2).skip(1),
556 /// b.into_iter().map(|x| x * 2).skip(1),
557 /// );
558 ///
559 /// assert_eq!(zipped.next(), Some((4, 6)));
560 /// assert_eq!(zipped.next(), Some((6, 8)));
561 /// assert_eq!(zipped.next(), None);
562 /// ```
563 ///
564 /// compared to:
565 ///
566 /// ```
567 /// # let a = [1, 2, 3];
568 /// # let b = [2, 3, 4];
569 /// #
570 /// let mut zipped = a
571 /// .into_iter()
572 /// .map(|x| x * 2)
573 /// .skip(1)
574 /// .zip(b.into_iter().map(|x| x * 2).skip(1));
575 /// #
576 /// # assert_eq!(zipped.next(), Some((4, 6)));
577 /// # assert_eq!(zipped.next(), Some((6, 8)));
578 /// # assert_eq!(zipped.next(), None);
579 /// ```
580 ///
581 /// [`enumerate`]: Iterator::enumerate
582 /// [`next`]: Iterator::next
583 /// [`zip`]: crate::iter::zip
584 #[inline]
585 #[stable(feature = "rust1", since = "1.0.0")]
586 fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
587 where
588 Self: Sized,
589 U: IntoIterator,
590 {
591 Zip::new(self, other.into_iter())
592 }
593
594 /// Creates a new iterator which places a copy of `separator` between adjacent
595 /// items of the original iterator.
596 ///
597 /// In case `separator` does not implement [`Clone`] or needs to be
598 /// computed every time, use [`intersperse_with`].
599 ///
600 /// # Examples
601 ///
602 /// Basic usage:
603 ///
604 /// ```
605 /// #![feature(iter_intersperse)]
606 ///
607 /// let mut a = [0, 1, 2].iter().intersperse(&100);
608 /// assert_eq!(a.next(), Some(&0)); // The first element from `a`.
609 /// assert_eq!(a.next(), Some(&100)); // The separator.
610 /// assert_eq!(a.next(), Some(&1)); // The next element from `a`.
611 /// assert_eq!(a.next(), Some(&100)); // The separator.
612 /// assert_eq!(a.next(), Some(&2)); // The last element from `a`.
613 /// assert_eq!(a.next(), None); // The iterator is finished.
614 /// ```
615 ///
616 /// `intersperse` can be very useful to join an iterator's items using a common element:
617 /// ```
618 /// #![feature(iter_intersperse)]
619 ///
620 /// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
621 /// assert_eq!(hello, "Hello World !");
622 /// ```
623 ///
624 /// [`Clone`]: crate::clone::Clone
625 /// [`intersperse_with`]: Iterator::intersperse_with
626 #[inline]
627 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
628 fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
629 where
630 Self: Sized,
631 Self::Item: Clone,
632 {
633 Intersperse::new(self, separator)
634 }
635
636 /// Creates a new iterator which places an item generated by `separator`
637 /// between adjacent items of the original iterator.
638 ///
639 /// The closure will be called exactly once each time an item is placed
640 /// between two adjacent items from the underlying iterator; specifically,
641 /// the closure is not called if the underlying iterator yields less than
642 /// two items and after the last item is yielded.
643 ///
644 /// If the iterator's item implements [`Clone`], it may be easier to use
645 /// [`intersperse`].
646 ///
647 /// # Examples
648 ///
649 /// Basic usage:
650 ///
651 /// ```
652 /// #![feature(iter_intersperse)]
653 ///
654 /// #[derive(PartialEq, Debug)]
655 /// struct NotClone(usize);
656 ///
657 /// let v = [NotClone(0), NotClone(1), NotClone(2)];
658 /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
659 ///
660 /// assert_eq!(it.next(), Some(NotClone(0))); // The first element from `v`.
661 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
662 /// assert_eq!(it.next(), Some(NotClone(1))); // The next element from `v`.
663 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
664 /// assert_eq!(it.next(), Some(NotClone(2))); // The last element from `v`.
665 /// assert_eq!(it.next(), None); // The iterator is finished.
666 /// ```
667 ///
668 /// `intersperse_with` can be used in situations where the separator needs
669 /// to be computed:
670 /// ```
671 /// #![feature(iter_intersperse)]
672 ///
673 /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
674 ///
675 /// // The closure mutably borrows its context to generate an item.
676 /// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
677 /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
678 ///
679 /// let result = src.intersperse_with(separator).collect::<String>();
680 /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
681 /// ```
682 /// [`Clone`]: crate::clone::Clone
683 /// [`intersperse`]: Iterator::intersperse
684 #[inline]
685 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
686 fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
687 where
688 Self: Sized,
689 G: FnMut() -> Self::Item,
690 {
691 IntersperseWith::new(self, separator)
692 }
693
694 /// Takes a closure and creates an iterator which calls that closure on each
695 /// element.
696 ///
697 /// `map()` transforms one iterator into another, by means of its argument:
698 /// something that implements [`FnMut`]. It produces a new iterator which
699 /// calls this closure on each element of the original iterator.
700 ///
701 /// If you are good at thinking in types, you can think of `map()` like this:
702 /// If you have an iterator that gives you elements of some type `A`, and
703 /// you want an iterator of some other type `B`, you can use `map()`,
704 /// passing a closure that takes an `A` and returns a `B`.
705 ///
706 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
707 /// lazy, it is best used when you're already working with other iterators.
708 /// If you're doing some sort of looping for a side effect, it's considered
709 /// more idiomatic to use [`for`] than `map()`.
710 ///
711 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
712 ///
713 /// # Examples
714 ///
715 /// Basic usage:
716 ///
717 /// ```
718 /// let a = [1, 2, 3];
719 ///
720 /// let mut iter = a.iter().map(|x| 2 * x);
721 ///
722 /// assert_eq!(iter.next(), Some(2));
723 /// assert_eq!(iter.next(), Some(4));
724 /// assert_eq!(iter.next(), Some(6));
725 /// assert_eq!(iter.next(), None);
726 /// ```
727 ///
728 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
729 ///
730 /// ```
731 /// # #![allow(unused_must_use)]
732 /// // don't do this:
733 /// (0..5).map(|x| println!("{x}"));
734 ///
735 /// // it won't even execute, as it is lazy. Rust will warn you about this.
736 ///
737 /// // Instead, use for:
738 /// for x in 0..5 {
739 /// println!("{x}");
740 /// }
741 /// ```
742 #[rustc_diagnostic_item = "IteratorMap"]
743 #[inline]
744 #[stable(feature = "rust1", since = "1.0.0")]
745 fn map<B, F>(self, f: F) -> Map<Self, F>
746 where
747 Self: Sized,
748 F: FnMut(Self::Item) -> B,
749 {
750 Map::new(self, f)
751 }
752
753 /// Calls a closure on each element of an iterator.
754 ///
755 /// This is equivalent to using a [`for`] loop on the iterator, although
756 /// `break` and `continue` are not possible from a closure. It's generally
757 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
758 /// when processing items at the end of longer iterator chains. In some
759 /// cases `for_each` may also be faster than a loop, because it will use
760 /// internal iteration on adapters like `Chain`.
761 ///
762 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
763 ///
764 /// # Examples
765 ///
766 /// Basic usage:
767 ///
768 /// ```
769 /// use std::sync::mpsc::channel;
770 ///
771 /// let (tx, rx) = channel();
772 /// (0..5).map(|x| x * 2 + 1)
773 /// .for_each(move |x| tx.send(x).unwrap());
774 ///
775 /// let v: Vec<_> = rx.iter().collect();
776 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
777 /// ```
778 ///
779 /// For such a small example, a `for` loop may be cleaner, but `for_each`
780 /// might be preferable to keep a functional style with longer iterators:
781 ///
782 /// ```
783 /// (0..5).flat_map(|x| x * 100 .. x * 110)
784 /// .enumerate()
785 /// .filter(|&(i, x)| (i + x) % 3 == 0)
786 /// .for_each(|(i, x)| println!("{i}:{x}"));
787 /// ```
788 #[inline]
789 #[stable(feature = "iterator_for_each", since = "1.21.0")]
790 fn for_each<F>(self, f: F)
791 where
792 Self: Sized,
793 F: FnMut(Self::Item),
794 {
795 #[inline]
796 fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
797 move |(), item| f(item)
798 }
799
800 self.fold((), call(f));
801 }
802
803 /// Creates an iterator which uses a closure to determine if an element
804 /// should be yielded.
805 ///
806 /// Given an element the closure must return `true` or `false`. The returned
807 /// iterator will yield only the elements for which the closure returns
808 /// `true`.
809 ///
810 /// # Examples
811 ///
812 /// Basic usage:
813 ///
814 /// ```
815 /// let a = [0i32, 1, 2];
816 ///
817 /// let mut iter = a.iter().filter(|x| x.is_positive());
818 ///
819 /// assert_eq!(iter.next(), Some(&1));
820 /// assert_eq!(iter.next(), Some(&2));
821 /// assert_eq!(iter.next(), None);
822 /// ```
823 ///
824 /// Because the closure passed to `filter()` takes a reference, and many
825 /// iterators iterate over references, this leads to a possibly confusing
826 /// situation, where the type of the closure is a double reference:
827 ///
828 /// ```
829 /// let a = [0, 1, 2];
830 ///
831 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
832 ///
833 /// assert_eq!(iter.next(), Some(&2));
834 /// assert_eq!(iter.next(), None);
835 /// ```
836 ///
837 /// It's common to instead use destructuring on the argument to strip away
838 /// one:
839 ///
840 /// ```
841 /// let a = [0, 1, 2];
842 ///
843 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
844 ///
845 /// assert_eq!(iter.next(), Some(&2));
846 /// assert_eq!(iter.next(), None);
847 /// ```
848 ///
849 /// or both:
850 ///
851 /// ```
852 /// let a = [0, 1, 2];
853 ///
854 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
855 ///
856 /// assert_eq!(iter.next(), Some(&2));
857 /// assert_eq!(iter.next(), None);
858 /// ```
859 ///
860 /// of these layers.
861 ///
862 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
863 #[inline]
864 #[stable(feature = "rust1", since = "1.0.0")]
865 #[cfg_attr(not(test), rustc_diagnostic_item = "iter_filter")]
866 fn filter<P>(self, predicate: P) -> Filter<Self, P>
867 where
868 Self: Sized,
869 P: FnMut(&Self::Item) -> bool,
870 {
871 Filter::new(self, predicate)
872 }
873
874 /// Creates an iterator that both filters and maps.
875 ///
876 /// The returned iterator yields only the `value`s for which the supplied
877 /// closure returns `Some(value)`.
878 ///
879 /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
880 /// concise. The example below shows how a `map().filter().map()` can be
881 /// shortened to a single call to `filter_map`.
882 ///
883 /// [`filter`]: Iterator::filter
884 /// [`map`]: Iterator::map
885 ///
886 /// # Examples
887 ///
888 /// Basic usage:
889 ///
890 /// ```
891 /// let a = ["1", "two", "NaN", "four", "5"];
892 ///
893 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
894 ///
895 /// assert_eq!(iter.next(), Some(1));
896 /// assert_eq!(iter.next(), Some(5));
897 /// assert_eq!(iter.next(), None);
898 /// ```
899 ///
900 /// Here's the same example, but with [`filter`] and [`map`]:
901 ///
902 /// ```
903 /// let a = ["1", "two", "NaN", "four", "5"];
904 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
905 /// assert_eq!(iter.next(), Some(1));
906 /// assert_eq!(iter.next(), Some(5));
907 /// assert_eq!(iter.next(), None);
908 /// ```
909 #[inline]
910 #[stable(feature = "rust1", since = "1.0.0")]
911 fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
912 where
913 Self: Sized,
914 F: FnMut(Self::Item) -> Option<B>,
915 {
916 FilterMap::new(self, f)
917 }
918
919 /// Creates an iterator which gives the current iteration count as well as
920 /// the next value.
921 ///
922 /// The iterator returned yields pairs `(i, val)`, where `i` is the
923 /// current index of iteration and `val` is the value returned by the
924 /// iterator.
925 ///
926 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
927 /// different sized integer, the [`zip`] function provides similar
928 /// functionality.
929 ///
930 /// # Overflow Behavior
931 ///
932 /// The method does no guarding against overflows, so enumerating more than
933 /// [`usize::MAX`] elements either produces the wrong result or panics. If
934 /// debug assertions are enabled, a panic is guaranteed.
935 ///
936 /// # Panics
937 ///
938 /// The returned iterator might panic if the to-be-returned index would
939 /// overflow a [`usize`].
940 ///
941 /// [`zip`]: Iterator::zip
942 ///
943 /// # Examples
944 ///
945 /// ```
946 /// let a = ['a', 'b', 'c'];
947 ///
948 /// let mut iter = a.iter().enumerate();
949 ///
950 /// assert_eq!(iter.next(), Some((0, &'a')));
951 /// assert_eq!(iter.next(), Some((1, &'b')));
952 /// assert_eq!(iter.next(), Some((2, &'c')));
953 /// assert_eq!(iter.next(), None);
954 /// ```
955 #[inline]
956 #[stable(feature = "rust1", since = "1.0.0")]
957 #[cfg_attr(not(test), rustc_diagnostic_item = "enumerate_method")]
958 fn enumerate(self) -> Enumerate<Self>
959 where
960 Self: Sized,
961 {
962 Enumerate::new(self)
963 }
964
965 /// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods
966 /// to look at the next element of the iterator without consuming it. See
967 /// their documentation for more information.
968 ///
969 /// Note that the underlying iterator is still advanced when [`peek`] or
970 /// [`peek_mut`] are called for the first time: In order to retrieve the
971 /// next element, [`next`] is called on the underlying iterator, hence any
972 /// side effects (i.e. anything other than fetching the next value) of
973 /// the [`next`] method will occur.
974 ///
975 ///
976 /// # Examples
977 ///
978 /// Basic usage:
979 ///
980 /// ```
981 /// let xs = [1, 2, 3];
982 ///
983 /// let mut iter = xs.iter().peekable();
984 ///
985 /// // peek() lets us see into the future
986 /// assert_eq!(iter.peek(), Some(&&1));
987 /// assert_eq!(iter.next(), Some(&1));
988 ///
989 /// assert_eq!(iter.next(), Some(&2));
990 ///
991 /// // we can peek() multiple times, the iterator won't advance
992 /// assert_eq!(iter.peek(), Some(&&3));
993 /// assert_eq!(iter.peek(), Some(&&3));
994 ///
995 /// assert_eq!(iter.next(), Some(&3));
996 ///
997 /// // after the iterator is finished, so is peek()
998 /// assert_eq!(iter.peek(), None);
999 /// assert_eq!(iter.next(), None);
1000 /// ```
1001 ///
1002 /// Using [`peek_mut`] to mutate the next item without advancing the
1003 /// iterator:
1004 ///
1005 /// ```
1006 /// let xs = [1, 2, 3];
1007 ///
1008 /// let mut iter = xs.iter().peekable();
1009 ///
1010 /// // `peek_mut()` lets us see into the future
1011 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
1012 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
1013 /// assert_eq!(iter.next(), Some(&1));
1014 ///
1015 /// if let Some(mut p) = iter.peek_mut() {
1016 /// assert_eq!(*p, &2);
1017 /// // put a value into the iterator
1018 /// *p = &1000;
1019 /// }
1020 ///
1021 /// // The value reappears as the iterator continues
1022 /// assert_eq!(iter.collect::<Vec<_>>(), vec![&1000, &3]);
1023 /// ```
1024 /// [`peek`]: Peekable::peek
1025 /// [`peek_mut`]: Peekable::peek_mut
1026 /// [`next`]: Iterator::next
1027 #[inline]
1028 #[stable(feature = "rust1", since = "1.0.0")]
1029 fn peekable(self) -> Peekable<Self>
1030 where
1031 Self: Sized,
1032 {
1033 Peekable::new(self)
1034 }
1035
1036 /// Creates an iterator that [`skip`]s elements based on a predicate.
1037 ///
1038 /// [`skip`]: Iterator::skip
1039 ///
1040 /// `skip_while()` takes a closure as an argument. It will call this
1041 /// closure on each element of the iterator, and ignore elements
1042 /// until it returns `false`.
1043 ///
1044 /// After `false` is returned, `skip_while()`'s job is over, and the
1045 /// rest of the elements are yielded.
1046 ///
1047 /// # Examples
1048 ///
1049 /// Basic usage:
1050 ///
1051 /// ```
1052 /// let a = [-1i32, 0, 1];
1053 ///
1054 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
1055 ///
1056 /// assert_eq!(iter.next(), Some(&0));
1057 /// assert_eq!(iter.next(), Some(&1));
1058 /// assert_eq!(iter.next(), None);
1059 /// ```
1060 ///
1061 /// Because the closure passed to `skip_while()` takes a reference, and many
1062 /// iterators iterate over references, this leads to a possibly confusing
1063 /// situation, where the type of the closure argument is a double reference:
1064 ///
1065 /// ```
1066 /// let a = [-1, 0, 1];
1067 ///
1068 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
1069 ///
1070 /// assert_eq!(iter.next(), Some(&0));
1071 /// assert_eq!(iter.next(), Some(&1));
1072 /// assert_eq!(iter.next(), None);
1073 /// ```
1074 ///
1075 /// Stopping after an initial `false`:
1076 ///
1077 /// ```
1078 /// let a = [-1, 0, 1, -2];
1079 ///
1080 /// let mut iter = a.iter().skip_while(|x| **x < 0);
1081 ///
1082 /// assert_eq!(iter.next(), Some(&0));
1083 /// assert_eq!(iter.next(), Some(&1));
1084 ///
1085 /// // while this would have been false, since we already got a false,
1086 /// // skip_while() isn't used any more
1087 /// assert_eq!(iter.next(), Some(&-2));
1088 ///
1089 /// assert_eq!(iter.next(), None);
1090 /// ```
1091 #[inline]
1092 #[doc(alias = "drop_while")]
1093 #[stable(feature = "rust1", since = "1.0.0")]
1094 fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
1095 where
1096 Self: Sized,
1097 P: FnMut(&Self::Item) -> bool,
1098 {
1099 SkipWhile::new(self, predicate)
1100 }
1101
1102 /// Creates an iterator that yields elements based on a predicate.
1103 ///
1104 /// `take_while()` takes a closure as an argument. It will call this
1105 /// closure on each element of the iterator, and yield elements
1106 /// while it returns `true`.
1107 ///
1108 /// After `false` is returned, `take_while()`'s job is over, and the
1109 /// rest of the elements are ignored.
1110 ///
1111 /// # Examples
1112 ///
1113 /// Basic usage:
1114 ///
1115 /// ```
1116 /// let a = [-1i32, 0, 1];
1117 ///
1118 /// let mut iter = a.iter().take_while(|x| x.is_negative());
1119 ///
1120 /// assert_eq!(iter.next(), Some(&-1));
1121 /// assert_eq!(iter.next(), None);
1122 /// ```
1123 ///
1124 /// Because the closure passed to `take_while()` takes a reference, and many
1125 /// iterators iterate over references, this leads to a possibly confusing
1126 /// situation, where the type of the closure is a double reference:
1127 ///
1128 /// ```
1129 /// let a = [-1, 0, 1];
1130 ///
1131 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
1132 ///
1133 /// assert_eq!(iter.next(), Some(&-1));
1134 /// assert_eq!(iter.next(), None);
1135 /// ```
1136 ///
1137 /// Stopping after an initial `false`:
1138 ///
1139 /// ```
1140 /// let a = [-1, 0, 1, -2];
1141 ///
1142 /// let mut iter = a.iter().take_while(|x| **x < 0);
1143 ///
1144 /// assert_eq!(iter.next(), Some(&-1));
1145 ///
1146 /// // We have more elements that are less than zero, but since we already
1147 /// // got a false, take_while() isn't used any more
1148 /// assert_eq!(iter.next(), None);
1149 /// ```
1150 ///
1151 /// Because `take_while()` needs to look at the value in order to see if it
1152 /// should be included or not, consuming iterators will see that it is
1153 /// removed:
1154 ///
1155 /// ```
1156 /// let a = [1, 2, 3, 4];
1157 /// let mut iter = a.iter();
1158 ///
1159 /// let result: Vec<i32> = iter.by_ref()
1160 /// .take_while(|n| **n != 3)
1161 /// .cloned()
1162 /// .collect();
1163 ///
1164 /// assert_eq!(result, &[1, 2]);
1165 ///
1166 /// let result: Vec<i32> = iter.cloned().collect();
1167 ///
1168 /// assert_eq!(result, &[4]);
1169 /// ```
1170 ///
1171 /// The `3` is no longer there, because it was consumed in order to see if
1172 /// the iteration should stop, but wasn't placed back into the iterator.
1173 #[inline]
1174 #[stable(feature = "rust1", since = "1.0.0")]
1175 fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1176 where
1177 Self: Sized,
1178 P: FnMut(&Self::Item) -> bool,
1179 {
1180 TakeWhile::new(self, predicate)
1181 }
1182
1183 /// Creates an iterator that both yields elements based on a predicate and maps.
1184 ///
1185 /// `map_while()` takes a closure as an argument. It will call this
1186 /// closure on each element of the iterator, and yield elements
1187 /// while it returns [`Some(_)`][`Some`].
1188 ///
1189 /// # Examples
1190 ///
1191 /// Basic usage:
1192 ///
1193 /// ```
1194 /// let a = [-1i32, 4, 0, 1];
1195 ///
1196 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1197 ///
1198 /// assert_eq!(iter.next(), Some(-16));
1199 /// assert_eq!(iter.next(), Some(4));
1200 /// assert_eq!(iter.next(), None);
1201 /// ```
1202 ///
1203 /// Here's the same example, but with [`take_while`] and [`map`]:
1204 ///
1205 /// [`take_while`]: Iterator::take_while
1206 /// [`map`]: Iterator::map
1207 ///
1208 /// ```
1209 /// let a = [-1i32, 4, 0, 1];
1210 ///
1211 /// let mut iter = a.iter()
1212 /// .map(|x| 16i32.checked_div(*x))
1213 /// .take_while(|x| x.is_some())
1214 /// .map(|x| x.unwrap());
1215 ///
1216 /// assert_eq!(iter.next(), Some(-16));
1217 /// assert_eq!(iter.next(), Some(4));
1218 /// assert_eq!(iter.next(), None);
1219 /// ```
1220 ///
1221 /// Stopping after an initial [`None`]:
1222 ///
1223 /// ```
1224 /// let a = [0, 1, 2, -3, 4, 5, -6];
1225 ///
1226 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1227 /// let vec = iter.collect::<Vec<_>>();
1228 ///
1229 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1230 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1231 /// assert_eq!(vec, vec![0, 1, 2]);
1232 /// ```
1233 ///
1234 /// Because `map_while()` needs to look at the value in order to see if it
1235 /// should be included or not, consuming iterators will see that it is
1236 /// removed:
1237 ///
1238 /// ```
1239 /// let a = [1, 2, -3, 4];
1240 /// let mut iter = a.iter();
1241 ///
1242 /// let result: Vec<u32> = iter.by_ref()
1243 /// .map_while(|n| u32::try_from(*n).ok())
1244 /// .collect();
1245 ///
1246 /// assert_eq!(result, &[1, 2]);
1247 ///
1248 /// let result: Vec<i32> = iter.cloned().collect();
1249 ///
1250 /// assert_eq!(result, &[4]);
1251 /// ```
1252 ///
1253 /// The `-3` is no longer there, because it was consumed in order to see if
1254 /// the iteration should stop, but wasn't placed back into the iterator.
1255 ///
1256 /// Note that unlike [`take_while`] this iterator is **not** fused.
1257 /// It is also not specified what this iterator returns after the first [`None`] is returned.
1258 /// If you need fused iterator, use [`fuse`].
1259 ///
1260 /// [`fuse`]: Iterator::fuse
1261 #[inline]
1262 #[stable(feature = "iter_map_while", since = "1.57.0")]
1263 fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1264 where
1265 Self: Sized,
1266 P: FnMut(Self::Item) -> Option<B>,
1267 {
1268 MapWhile::new(self, predicate)
1269 }
1270
1271 /// Creates an iterator that skips the first `n` elements.
1272 ///
1273 /// `skip(n)` skips elements until `n` elements are skipped or the end of the
1274 /// iterator is reached (whichever happens first). After that, all the remaining
1275 /// elements are yielded. In particular, if the original iterator is too short,
1276 /// then the returned iterator is empty.
1277 ///
1278 /// Rather than overriding this method directly, instead override the `nth` method.
1279 ///
1280 /// # Examples
1281 ///
1282 /// ```
1283 /// let a = [1, 2, 3];
1284 ///
1285 /// let mut iter = a.iter().skip(2);
1286 ///
1287 /// assert_eq!(iter.next(), Some(&3));
1288 /// assert_eq!(iter.next(), None);
1289 /// ```
1290 #[inline]
1291 #[stable(feature = "rust1", since = "1.0.0")]
1292 fn skip(self, n: usize) -> Skip<Self>
1293 where
1294 Self: Sized,
1295 {
1296 Skip::new(self, n)
1297 }
1298
1299 /// Creates an iterator that yields the first `n` elements, or fewer
1300 /// if the underlying iterator ends sooner.
1301 ///
1302 /// `take(n)` yields elements until `n` elements are yielded or the end of
1303 /// the iterator is reached (whichever happens first).
1304 /// The returned iterator is a prefix of length `n` if the original iterator
1305 /// contains at least `n` elements, otherwise it contains all of the
1306 /// (fewer than `n`) elements of the original iterator.
1307 ///
1308 /// # Examples
1309 ///
1310 /// Basic usage:
1311 ///
1312 /// ```
1313 /// let a = [1, 2, 3];
1314 ///
1315 /// let mut iter = a.iter().take(2);
1316 ///
1317 /// assert_eq!(iter.next(), Some(&1));
1318 /// assert_eq!(iter.next(), Some(&2));
1319 /// assert_eq!(iter.next(), None);
1320 /// ```
1321 ///
1322 /// `take()` is often used with an infinite iterator, to make it finite:
1323 ///
1324 /// ```
1325 /// let mut iter = (0..).take(3);
1326 ///
1327 /// assert_eq!(iter.next(), Some(0));
1328 /// assert_eq!(iter.next(), Some(1));
1329 /// assert_eq!(iter.next(), Some(2));
1330 /// assert_eq!(iter.next(), None);
1331 /// ```
1332 ///
1333 /// If less than `n` elements are available,
1334 /// `take` will limit itself to the size of the underlying iterator:
1335 ///
1336 /// ```
1337 /// let v = [1, 2];
1338 /// let mut iter = v.into_iter().take(5);
1339 /// assert_eq!(iter.next(), Some(1));
1340 /// assert_eq!(iter.next(), Some(2));
1341 /// assert_eq!(iter.next(), None);
1342 /// ```
1343 #[inline]
1344 #[stable(feature = "rust1", since = "1.0.0")]
1345 fn take(self, n: usize) -> Take<Self>
1346 where
1347 Self: Sized,
1348 {
1349 Take::new(self, n)
1350 }
1351
1352 /// An iterator adapter which, like [`fold`], holds internal state, but
1353 /// unlike [`fold`], produces a new iterator.
1354 ///
1355 /// [`fold`]: Iterator::fold
1356 ///
1357 /// `scan()` takes two arguments: an initial value which seeds the internal
1358 /// state, and a closure with two arguments, the first being a mutable
1359 /// reference to the internal state and the second an iterator element.
1360 /// The closure can assign to the internal state to share state between
1361 /// iterations.
1362 ///
1363 /// On iteration, the closure will be applied to each element of the
1364 /// iterator and the return value from the closure, an [`Option`], is
1365 /// returned by the `next` method. Thus the closure can return
1366 /// `Some(value)` to yield `value`, or `None` to end the iteration.
1367 ///
1368 /// # Examples
1369 ///
1370 /// ```
1371 /// let a = [1, 2, 3, 4];
1372 ///
1373 /// let mut iter = a.iter().scan(1, |state, &x| {
1374 /// // each iteration, we'll multiply the state by the element ...
1375 /// *state = *state * x;
1376 ///
1377 /// // ... and terminate if the state exceeds 6
1378 /// if *state > 6 {
1379 /// return None;
1380 /// }
1381 /// // ... else yield the negation of the state
1382 /// Some(-*state)
1383 /// });
1384 ///
1385 /// assert_eq!(iter.next(), Some(-1));
1386 /// assert_eq!(iter.next(), Some(-2));
1387 /// assert_eq!(iter.next(), Some(-6));
1388 /// assert_eq!(iter.next(), None);
1389 /// ```
1390 #[inline]
1391 #[stable(feature = "rust1", since = "1.0.0")]
1392 fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1393 where
1394 Self: Sized,
1395 F: FnMut(&mut St, Self::Item) -> Option<B>,
1396 {
1397 Scan::new(self, initial_state, f)
1398 }
1399
1400 /// Creates an iterator that works like map, but flattens nested structure.
1401 ///
1402 /// The [`map`] adapter is very useful, but only when the closure
1403 /// argument produces values. If it produces an iterator instead, there's
1404 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1405 /// on its own.
1406 ///
1407 /// You can think of `flat_map(f)` as the semantic equivalent
1408 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1409 ///
1410 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1411 /// one item for each element, and `flat_map()`'s closure returns an
1412 /// iterator for each element.
1413 ///
1414 /// [`map`]: Iterator::map
1415 /// [`flatten`]: Iterator::flatten
1416 ///
1417 /// # Examples
1418 ///
1419 /// ```
1420 /// let words = ["alpha", "beta", "gamma"];
1421 ///
1422 /// // chars() returns an iterator
1423 /// let merged: String = words.iter()
1424 /// .flat_map(|s| s.chars())
1425 /// .collect();
1426 /// assert_eq!(merged, "alphabetagamma");
1427 /// ```
1428 #[inline]
1429 #[stable(feature = "rust1", since = "1.0.0")]
1430 fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1431 where
1432 Self: Sized,
1433 U: IntoIterator,
1434 F: FnMut(Self::Item) -> U,
1435 {
1436 FlatMap::new(self, f)
1437 }
1438
1439 /// Creates an iterator that flattens nested structure.
1440 ///
1441 /// This is useful when you have an iterator of iterators or an iterator of
1442 /// things that can be turned into iterators and you want to remove one
1443 /// level of indirection.
1444 ///
1445 /// # Examples
1446 ///
1447 /// Basic usage:
1448 ///
1449 /// ```
1450 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1451 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1452 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1453 /// ```
1454 ///
1455 /// Mapping and then flattening:
1456 ///
1457 /// ```
1458 /// let words = ["alpha", "beta", "gamma"];
1459 ///
1460 /// // chars() returns an iterator
1461 /// let merged: String = words.iter()
1462 /// .map(|s| s.chars())
1463 /// .flatten()
1464 /// .collect();
1465 /// assert_eq!(merged, "alphabetagamma");
1466 /// ```
1467 ///
1468 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1469 /// in this case since it conveys intent more clearly:
1470 ///
1471 /// ```
1472 /// let words = ["alpha", "beta", "gamma"];
1473 ///
1474 /// // chars() returns an iterator
1475 /// let merged: String = words.iter()
1476 /// .flat_map(|s| s.chars())
1477 /// .collect();
1478 /// assert_eq!(merged, "alphabetagamma");
1479 /// ```
1480 ///
1481 /// Flattening works on any `IntoIterator` type, including `Option` and `Result`:
1482 ///
1483 /// ```
1484 /// let options = vec![Some(123), Some(321), None, Some(231)];
1485 /// let flattened_options: Vec<_> = options.into_iter().flatten().collect();
1486 /// assert_eq!(flattened_options, vec![123, 321, 231]);
1487 ///
1488 /// let results = vec![Ok(123), Ok(321), Err(456), Ok(231)];
1489 /// let flattened_results: Vec<_> = results.into_iter().flatten().collect();
1490 /// assert_eq!(flattened_results, vec![123, 321, 231]);
1491 /// ```
1492 ///
1493 /// Flattening only removes one level of nesting at a time:
1494 ///
1495 /// ```
1496 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1497 ///
1498 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1499 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1500 ///
1501 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1502 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1503 /// ```
1504 ///
1505 /// Here we see that `flatten()` does not perform a "deep" flatten.
1506 /// Instead, only one level of nesting is removed. That is, if you
1507 /// `flatten()` a three-dimensional array, the result will be
1508 /// two-dimensional and not one-dimensional. To get a one-dimensional
1509 /// structure, you have to `flatten()` again.
1510 ///
1511 /// [`flat_map()`]: Iterator::flat_map
1512 #[inline]
1513 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1514 fn flatten(self) -> Flatten<Self>
1515 where
1516 Self: Sized,
1517 Self::Item: IntoIterator,
1518 {
1519 Flatten::new(self)
1520 }
1521
1522 /// Calls the given function `f` for each contiguous window of size `N` over
1523 /// `self` and returns an iterator over the outputs of `f`. Like [`slice::windows()`],
1524 /// the windows during mapping overlap as well.
1525 ///
1526 /// In the following example, the closure is called three times with the
1527 /// arguments `&['a', 'b']`, `&['b', 'c']` and `&['c', 'd']` respectively.
1528 ///
1529 /// ```
1530 /// #![feature(iter_map_windows)]
1531 ///
1532 /// let strings = "abcd".chars()
1533 /// .map_windows(|[x, y]| format!("{}+{}", x, y))
1534 /// .collect::<Vec<String>>();
1535 ///
1536 /// assert_eq!(strings, vec!["a+b", "b+c", "c+d"]);
1537 /// ```
1538 ///
1539 /// Note that the const parameter `N` is usually inferred by the
1540 /// destructured argument in the closure.
1541 ///
1542 /// The returned iterator yields 𝑘 − `N` + 1 items (where 𝑘 is the number of
1543 /// items yielded by `self`). If 𝑘 is less than `N`, this method yields an
1544 /// empty iterator.
1545 ///
1546 /// The returned iterator implements [`FusedIterator`], because once `self`
1547 /// returns `None`, even if it returns a `Some(T)` again in the next iterations,
1548 /// we cannot put it into a contiguous array buffer, and thus the returned iterator
1549 /// should be fused.
1550 ///
1551 /// [`slice::windows()`]: slice::windows
1552 /// [`FusedIterator`]: crate::iter::FusedIterator
1553 ///
1554 /// # Panics
1555 ///
1556 /// Panics if `N` is zero. This check will most probably get changed to a
1557 /// compile time error before this method gets stabilized.
1558 ///
1559 /// ```should_panic
1560 /// #![feature(iter_map_windows)]
1561 ///
1562 /// let iter = std::iter::repeat(0).map_windows(|&[]| ());
1563 /// ```
1564 ///
1565 /// # Examples
1566 ///
1567 /// Building the sums of neighboring numbers.
1568 ///
1569 /// ```
1570 /// #![feature(iter_map_windows)]
1571 ///
1572 /// let mut it = [1, 3, 8, 1].iter().map_windows(|&[a, b]| a + b);
1573 /// assert_eq!(it.next(), Some(4)); // 1 + 3
1574 /// assert_eq!(it.next(), Some(11)); // 3 + 8
1575 /// assert_eq!(it.next(), Some(9)); // 8 + 1
1576 /// assert_eq!(it.next(), None);
1577 /// ```
1578 ///
1579 /// Since the elements in the following example implement `Copy`, we can
1580 /// just copy the array and get an iterator over the windows.
1581 ///
1582 /// ```
1583 /// #![feature(iter_map_windows)]
1584 ///
1585 /// let mut it = "ferris".chars().map_windows(|w: &[_; 3]| *w);
1586 /// assert_eq!(it.next(), Some(['f', 'e', 'r']));
1587 /// assert_eq!(it.next(), Some(['e', 'r', 'r']));
1588 /// assert_eq!(it.next(), Some(['r', 'r', 'i']));
1589 /// assert_eq!(it.next(), Some(['r', 'i', 's']));
1590 /// assert_eq!(it.next(), None);
1591 /// ```
1592 ///
1593 /// You can also use this function to check the sortedness of an iterator.
1594 /// For the simple case, rather use [`Iterator::is_sorted`].
1595 ///
1596 /// ```
1597 /// #![feature(iter_map_windows)]
1598 ///
1599 /// let mut it = [0.5, 1.0, 3.5, 3.0, 8.5, 8.5, f32::NAN].iter()
1600 /// .map_windows(|[a, b]| a <= b);
1601 ///
1602 /// assert_eq!(it.next(), Some(true)); // 0.5 <= 1.0
1603 /// assert_eq!(it.next(), Some(true)); // 1.0 <= 3.5
1604 /// assert_eq!(it.next(), Some(false)); // 3.5 <= 3.0
1605 /// assert_eq!(it.next(), Some(true)); // 3.0 <= 8.5
1606 /// assert_eq!(it.next(), Some(true)); // 8.5 <= 8.5
1607 /// assert_eq!(it.next(), Some(false)); // 8.5 <= NAN
1608 /// assert_eq!(it.next(), None);
1609 /// ```
1610 ///
1611 /// For non-fused iterators, they are fused after `map_windows`.
1612 ///
1613 /// ```
1614 /// #![feature(iter_map_windows)]
1615 ///
1616 /// #[derive(Default)]
1617 /// struct NonFusedIterator {
1618 /// state: i32,
1619 /// }
1620 ///
1621 /// impl Iterator for NonFusedIterator {
1622 /// type Item = i32;
1623 ///
1624 /// fn next(&mut self) -> Option<i32> {
1625 /// let val = self.state;
1626 /// self.state = self.state + 1;
1627 ///
1628 /// // yields `0..5` first, then only even numbers since `6..`.
1629 /// if val < 5 || val % 2 == 0 {
1630 /// Some(val)
1631 /// } else {
1632 /// None
1633 /// }
1634 /// }
1635 /// }
1636 ///
1637 ///
1638 /// let mut iter = NonFusedIterator::default();
1639 ///
1640 /// // yields 0..5 first.
1641 /// assert_eq!(iter.next(), Some(0));
1642 /// assert_eq!(iter.next(), Some(1));
1643 /// assert_eq!(iter.next(), Some(2));
1644 /// assert_eq!(iter.next(), Some(3));
1645 /// assert_eq!(iter.next(), Some(4));
1646 /// // then we can see our iterator going back and forth
1647 /// assert_eq!(iter.next(), None);
1648 /// assert_eq!(iter.next(), Some(6));
1649 /// assert_eq!(iter.next(), None);
1650 /// assert_eq!(iter.next(), Some(8));
1651 /// assert_eq!(iter.next(), None);
1652 ///
1653 /// // however, with `.map_windows()`, it is fused.
1654 /// let mut iter = NonFusedIterator::default()
1655 /// .map_windows(|arr: &[_; 2]| *arr);
1656 ///
1657 /// assert_eq!(iter.next(), Some([0, 1]));
1658 /// assert_eq!(iter.next(), Some([1, 2]));
1659 /// assert_eq!(iter.next(), Some([2, 3]));
1660 /// assert_eq!(iter.next(), Some([3, 4]));
1661 /// assert_eq!(iter.next(), None);
1662 ///
1663 /// // it will always return `None` after the first time.
1664 /// assert_eq!(iter.next(), None);
1665 /// assert_eq!(iter.next(), None);
1666 /// assert_eq!(iter.next(), None);
1667 /// ```
1668 #[inline]
1669 #[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")]
1670 fn map_windows<F, R, const N: usize>(self, f: F) -> MapWindows<Self, F, N>
1671 where
1672 Self: Sized,
1673 F: FnMut(&[Self::Item; N]) -> R,
1674 {
1675 MapWindows::new(self, f)
1676 }
1677
1678 /// Creates an iterator which ends after the first [`None`].
1679 ///
1680 /// After an iterator returns [`None`], future calls may or may not yield
1681 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1682 /// [`None`] is given, it will always return [`None`] forever.
1683 ///
1684 /// Note that the [`Fuse`] wrapper is a no-op on iterators that implement
1685 /// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly
1686 /// if the [`FusedIterator`] trait is improperly implemented.
1687 ///
1688 /// [`Some(T)`]: Some
1689 /// [`FusedIterator`]: crate::iter::FusedIterator
1690 ///
1691 /// # Examples
1692 ///
1693 /// ```
1694 /// // an iterator which alternates between Some and None
1695 /// struct Alternate {
1696 /// state: i32,
1697 /// }
1698 ///
1699 /// impl Iterator for Alternate {
1700 /// type Item = i32;
1701 ///
1702 /// fn next(&mut self) -> Option<i32> {
1703 /// let val = self.state;
1704 /// self.state = self.state + 1;
1705 ///
1706 /// // if it's even, Some(i32), else None
1707 /// if val % 2 == 0 {
1708 /// Some(val)
1709 /// } else {
1710 /// None
1711 /// }
1712 /// }
1713 /// }
1714 ///
1715 /// let mut iter = Alternate { state: 0 };
1716 ///
1717 /// // we can see our iterator going back and forth
1718 /// assert_eq!(iter.next(), Some(0));
1719 /// assert_eq!(iter.next(), None);
1720 /// assert_eq!(iter.next(), Some(2));
1721 /// assert_eq!(iter.next(), None);
1722 ///
1723 /// // however, once we fuse it...
1724 /// let mut iter = iter.fuse();
1725 ///
1726 /// assert_eq!(iter.next(), Some(4));
1727 /// assert_eq!(iter.next(), None);
1728 ///
1729 /// // it will always return `None` after the first time.
1730 /// assert_eq!(iter.next(), None);
1731 /// assert_eq!(iter.next(), None);
1732 /// assert_eq!(iter.next(), None);
1733 /// ```
1734 #[inline]
1735 #[stable(feature = "rust1", since = "1.0.0")]
1736 fn fuse(self) -> Fuse<Self>
1737 where
1738 Self: Sized,
1739 {
1740 Fuse::new(self)
1741 }
1742
1743 /// Does something with each element of an iterator, passing the value on.
1744 ///
1745 /// When using iterators, you'll often chain several of them together.
1746 /// While working on such code, you might want to check out what's
1747 /// happening at various parts in the pipeline. To do that, insert
1748 /// a call to `inspect()`.
1749 ///
1750 /// It's more common for `inspect()` to be used as a debugging tool than to
1751 /// exist in your final code, but applications may find it useful in certain
1752 /// situations when errors need to be logged before being discarded.
1753 ///
1754 /// # Examples
1755 ///
1756 /// Basic usage:
1757 ///
1758 /// ```
1759 /// let a = [1, 4, 2, 3];
1760 ///
1761 /// // this iterator sequence is complex.
1762 /// let sum = a.iter()
1763 /// .cloned()
1764 /// .filter(|x| x % 2 == 0)
1765 /// .fold(0, |sum, i| sum + i);
1766 ///
1767 /// println!("{sum}");
1768 ///
1769 /// // let's add some inspect() calls to investigate what's happening
1770 /// let sum = a.iter()
1771 /// .cloned()
1772 /// .inspect(|x| println!("about to filter: {x}"))
1773 /// .filter(|x| x % 2 == 0)
1774 /// .inspect(|x| println!("made it through filter: {x}"))
1775 /// .fold(0, |sum, i| sum + i);
1776 ///
1777 /// println!("{sum}");
1778 /// ```
1779 ///
1780 /// This will print:
1781 ///
1782 /// ```text
1783 /// 6
1784 /// about to filter: 1
1785 /// about to filter: 4
1786 /// made it through filter: 4
1787 /// about to filter: 2
1788 /// made it through filter: 2
1789 /// about to filter: 3
1790 /// 6
1791 /// ```
1792 ///
1793 /// Logging errors before discarding them:
1794 ///
1795 /// ```
1796 /// let lines = ["1", "2", "a"];
1797 ///
1798 /// let sum: i32 = lines
1799 /// .iter()
1800 /// .map(|line| line.parse::<i32>())
1801 /// .inspect(|num| {
1802 /// if let Err(ref e) = *num {
1803 /// println!("Parsing error: {e}");
1804 /// }
1805 /// })
1806 /// .filter_map(Result::ok)
1807 /// .sum();
1808 ///
1809 /// println!("Sum: {sum}");
1810 /// ```
1811 ///
1812 /// This will print:
1813 ///
1814 /// ```text
1815 /// Parsing error: invalid digit found in string
1816 /// Sum: 3
1817 /// ```
1818 #[inline]
1819 #[stable(feature = "rust1", since = "1.0.0")]
1820 fn inspect<F>(self, f: F) -> Inspect<Self, F>
1821 where
1822 Self: Sized,
1823 F: FnMut(&Self::Item),
1824 {
1825 Inspect::new(self, f)
1826 }
1827
1828 /// Borrows an iterator, rather than consuming it.
1829 ///
1830 /// This is useful to allow applying iterator adapters while still
1831 /// retaining ownership of the original iterator.
1832 ///
1833 /// # Examples
1834 ///
1835 /// ```
1836 /// let mut words = ["hello", "world", "of", "Rust"].into_iter();
1837 ///
1838 /// // Take the first two words.
1839 /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
1840 /// assert_eq!(hello_world, vec!["hello", "world"]);
1841 ///
1842 /// // Collect the rest of the words.
1843 /// // We can only do this because we used `by_ref` earlier.
1844 /// let of_rust: Vec<_> = words.collect();
1845 /// assert_eq!(of_rust, vec!["of", "Rust"]);
1846 /// ```
1847 #[stable(feature = "rust1", since = "1.0.0")]
1848 fn by_ref(&mut self) -> &mut Self
1849 where
1850 Self: Sized,
1851 {
1852 self
1853 }
1854
1855 /// Transforms an iterator into a collection.
1856 ///
1857 /// `collect()` can take anything iterable, and turn it into a relevant
1858 /// collection. This is one of the more powerful methods in the standard
1859 /// library, used in a variety of contexts.
1860 ///
1861 /// The most basic pattern in which `collect()` is used is to turn one
1862 /// collection into another. You take a collection, call [`iter`] on it,
1863 /// do a bunch of transformations, and then `collect()` at the end.
1864 ///
1865 /// `collect()` can also create instances of types that are not typical
1866 /// collections. For example, a [`String`] can be built from [`char`]s,
1867 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1868 /// into `Result<Collection<T>, E>`. See the examples below for more.
1869 ///
1870 /// Because `collect()` is so general, it can cause problems with type
1871 /// inference. As such, `collect()` is one of the few times you'll see
1872 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1873 /// helps the inference algorithm understand specifically which collection
1874 /// you're trying to collect into.
1875 ///
1876 /// # Examples
1877 ///
1878 /// Basic usage:
1879 ///
1880 /// ```
1881 /// let a = [1, 2, 3];
1882 ///
1883 /// let doubled: Vec<i32> = a.iter()
1884 /// .map(|&x| x * 2)
1885 /// .collect();
1886 ///
1887 /// assert_eq!(vec![2, 4, 6], doubled);
1888 /// ```
1889 ///
1890 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1891 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1892 ///
1893 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1894 ///
1895 /// ```
1896 /// use std::collections::VecDeque;
1897 ///
1898 /// let a = [1, 2, 3];
1899 ///
1900 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1901 ///
1902 /// assert_eq!(2, doubled[0]);
1903 /// assert_eq!(4, doubled[1]);
1904 /// assert_eq!(6, doubled[2]);
1905 /// ```
1906 ///
1907 /// Using the 'turbofish' instead of annotating `doubled`:
1908 ///
1909 /// ```
1910 /// let a = [1, 2, 3];
1911 ///
1912 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1913 ///
1914 /// assert_eq!(vec![2, 4, 6], doubled);
1915 /// ```
1916 ///
1917 /// Because `collect()` only cares about what you're collecting into, you can
1918 /// still use a partial type hint, `_`, with the turbofish:
1919 ///
1920 /// ```
1921 /// let a = [1, 2, 3];
1922 ///
1923 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1924 ///
1925 /// assert_eq!(vec![2, 4, 6], doubled);
1926 /// ```
1927 ///
1928 /// Using `collect()` to make a [`String`]:
1929 ///
1930 /// ```
1931 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1932 ///
1933 /// let hello: String = chars.iter()
1934 /// .map(|&x| x as u8)
1935 /// .map(|x| (x + 1) as char)
1936 /// .collect();
1937 ///
1938 /// assert_eq!("hello", hello);
1939 /// ```
1940 ///
1941 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1942 /// see if any of them failed:
1943 ///
1944 /// ```
1945 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1946 ///
1947 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1948 ///
1949 /// // gives us the first error
1950 /// assert_eq!(Err("nope"), result);
1951 ///
1952 /// let results = [Ok(1), Ok(3)];
1953 ///
1954 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1955 ///
1956 /// // gives us the list of answers
1957 /// assert_eq!(Ok(vec![1, 3]), result);
1958 /// ```
1959 ///
1960 /// [`iter`]: Iterator::next
1961 /// [`String`]: ../../std/string/struct.String.html
1962 /// [`char`]: type@char
1963 #[inline]
1964 #[stable(feature = "rust1", since = "1.0.0")]
1965 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1966 #[cfg_attr(not(test), rustc_diagnostic_item = "iterator_collect_fn")]
1967 fn collect<B: FromIterator<Self::Item>>(self) -> B
1968 where
1969 Self: Sized,
1970 {
1971 FromIterator::from_iter(self)
1972 }
1973
1974 /// Fallibly transforms an iterator into a collection, short circuiting if
1975 /// a failure is encountered.
1976 ///
1977 /// `try_collect()` is a variation of [`collect()`][`collect`] that allows fallible
1978 /// conversions during collection. Its main use case is simplifying conversions from
1979 /// iterators yielding [`Option<T>`][`Option`] into `Option<Collection<T>>`, or similarly for other [`Try`]
1980 /// types (e.g. [`Result`]).
1981 ///
1982 /// Importantly, `try_collect()` doesn't require that the outer [`Try`] type also implements [`FromIterator`];
1983 /// only the inner type produced on `Try::Output` must implement it. Concretely,
1984 /// this means that collecting into `ControlFlow<_, Vec<i32>>` is valid because `Vec<i32>` implements
1985 /// [`FromIterator`], even though [`ControlFlow`] doesn't.
1986 ///
1987 /// Also, if a failure is encountered during `try_collect()`, the iterator is still valid and
1988 /// may continue to be used, in which case it will continue iterating starting after the element that
1989 /// triggered the failure. See the last example below for an example of how this works.
1990 ///
1991 /// # Examples
1992 /// Successfully collecting an iterator of `Option<i32>` into `Option<Vec<i32>>`:
1993 /// ```
1994 /// #![feature(iterator_try_collect)]
1995 ///
1996 /// let u = vec![Some(1), Some(2), Some(3)];
1997 /// let v = u.into_iter().try_collect::<Vec<i32>>();
1998 /// assert_eq!(v, Some(vec![1, 2, 3]));
1999 /// ```
2000 ///
2001 /// Failing to collect in the same way:
2002 /// ```
2003 /// #![feature(iterator_try_collect)]
2004 ///
2005 /// let u = vec![Some(1), Some(2), None, Some(3)];
2006 /// let v = u.into_iter().try_collect::<Vec<i32>>();
2007 /// assert_eq!(v, None);
2008 /// ```
2009 ///
2010 /// A similar example, but with `Result`:
2011 /// ```
2012 /// #![feature(iterator_try_collect)]
2013 ///
2014 /// let u: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2), Ok(3)];
2015 /// let v = u.into_iter().try_collect::<Vec<i32>>();
2016 /// assert_eq!(v, Ok(vec![1, 2, 3]));
2017 ///
2018 /// let u = vec![Ok(1), Ok(2), Err(()), Ok(3)];
2019 /// let v = u.into_iter().try_collect::<Vec<i32>>();
2020 /// assert_eq!(v, Err(()));
2021 /// ```
2022 ///
2023 /// Finally, even [`ControlFlow`] works, despite the fact that it
2024 /// doesn't implement [`FromIterator`]. Note also that the iterator can
2025 /// continue to be used, even if a failure is encountered:
2026 ///
2027 /// ```
2028 /// #![feature(iterator_try_collect)]
2029 ///
2030 /// use core::ops::ControlFlow::{Break, Continue};
2031 ///
2032 /// let u = [Continue(1), Continue(2), Break(3), Continue(4), Continue(5)];
2033 /// let mut it = u.into_iter();
2034 ///
2035 /// let v = it.try_collect::<Vec<_>>();
2036 /// assert_eq!(v, Break(3));
2037 ///
2038 /// let v = it.try_collect::<Vec<_>>();
2039 /// assert_eq!(v, Continue(vec![4, 5]));
2040 /// ```
2041 ///
2042 /// [`collect`]: Iterator::collect
2043 #[inline]
2044 #[unstable(feature = "iterator_try_collect", issue = "94047")]
2045 fn try_collect<B>(&mut self) -> ChangeOutputType<Self::Item, B>
2046 where
2047 Self: Sized,
2048 Self::Item: Try<Residual: Residual<B>>,
2049 B: FromIterator<<Self::Item as Try>::Output>,
2050 {
2051 try_process(ByRefSized(self), |i| i.collect())
2052 }
2053
2054 /// Collects all the items from an iterator into a collection.
2055 ///
2056 /// This method consumes the iterator and adds all its items to the
2057 /// passed collection. The collection is then returned, so the call chain
2058 /// can be continued.
2059 ///
2060 /// This is useful when you already have a collection and want to add
2061 /// the iterator items to it.
2062 ///
2063 /// This method is a convenience method to call [Extend::extend](trait.Extend.html),
2064 /// but instead of being called on a collection, it's called on an iterator.
2065 ///
2066 /// # Examples
2067 ///
2068 /// Basic usage:
2069 ///
2070 /// ```
2071 /// #![feature(iter_collect_into)]
2072 ///
2073 /// let a = [1, 2, 3];
2074 /// let mut vec: Vec::<i32> = vec![0, 1];
2075 ///
2076 /// a.iter().map(|&x| x * 2).collect_into(&mut vec);
2077 /// a.iter().map(|&x| x * 10).collect_into(&mut vec);
2078 ///
2079 /// assert_eq!(vec, vec![0, 1, 2, 4, 6, 10, 20, 30]);
2080 /// ```
2081 ///
2082 /// `Vec` can have a manual set capacity to avoid reallocating it:
2083 ///
2084 /// ```
2085 /// #![feature(iter_collect_into)]
2086 ///
2087 /// let a = [1, 2, 3];
2088 /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
2089 ///
2090 /// a.iter().map(|&x| x * 2).collect_into(&mut vec);
2091 /// a.iter().map(|&x| x * 10).collect_into(&mut vec);
2092 ///
2093 /// assert_eq!(6, vec.capacity());
2094 /// assert_eq!(vec, vec![2, 4, 6, 10, 20, 30]);
2095 /// ```
2096 ///
2097 /// The returned mutable reference can be used to continue the call chain:
2098 ///
2099 /// ```
2100 /// #![feature(iter_collect_into)]
2101 ///
2102 /// let a = [1, 2, 3];
2103 /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
2104 ///
2105 /// let count = a.iter().collect_into(&mut vec).iter().count();
2106 ///
2107 /// assert_eq!(count, vec.len());
2108 /// assert_eq!(vec, vec![1, 2, 3]);
2109 ///
2110 /// let count = a.iter().collect_into(&mut vec).iter().count();
2111 ///
2112 /// assert_eq!(count, vec.len());
2113 /// assert_eq!(vec, vec![1, 2, 3, 1, 2, 3]);
2114 /// ```
2115 #[inline]
2116 #[unstable(feature = "iter_collect_into", reason = "new API", issue = "94780")]
2117 fn collect_into<E: Extend<Self::Item>>(self, collection: &mut E) -> &mut E
2118 where
2119 Self: Sized,
2120 {
2121 collection.extend(self);
2122 collection
2123 }
2124
2125 /// Consumes an iterator, creating two collections from it.
2126 ///
2127 /// The predicate passed to `partition()` can return `true`, or `false`.
2128 /// `partition()` returns a pair, all of the elements for which it returned
2129 /// `true`, and all of the elements for which it returned `false`.
2130 ///
2131 /// See also [`is_partitioned()`] and [`partition_in_place()`].
2132 ///
2133 /// [`is_partitioned()`]: Iterator::is_partitioned
2134 /// [`partition_in_place()`]: Iterator::partition_in_place
2135 ///
2136 /// # Examples
2137 ///
2138 /// ```
2139 /// let a = [1, 2, 3];
2140 ///
2141 /// let (even, odd): (Vec<_>, Vec<_>) = a
2142 /// .into_iter()
2143 /// .partition(|n| n % 2 == 0);
2144 ///
2145 /// assert_eq!(even, vec![2]);
2146 /// assert_eq!(odd, vec![1, 3]);
2147 /// ```
2148 #[stable(feature = "rust1", since = "1.0.0")]
2149 fn partition<B, F>(self, f: F) -> (B, B)
2150 where
2151 Self: Sized,
2152 B: Default + Extend<Self::Item>,
2153 F: FnMut(&Self::Item) -> bool,
2154 {
2155 #[inline]
2156 fn extend<'a, T, B: Extend<T>>(
2157 mut f: impl FnMut(&T) -> bool + 'a,
2158 left: &'a mut B,
2159 right: &'a mut B,
2160 ) -> impl FnMut((), T) + 'a {
2161 move |(), x| {
2162 if f(&x) {
2163 left.extend_one(x);
2164 } else {
2165 right.extend_one(x);
2166 }
2167 }
2168 }
2169
2170 let mut left: B = Default::default();
2171 let mut right: B = Default::default();
2172
2173 self.fold((), extend(f, &mut left, &mut right));
2174
2175 (left, right)
2176 }
2177
2178 /// Reorders the elements of this iterator *in-place* according to the given predicate,
2179 /// such that all those that return `true` precede all those that return `false`.
2180 /// Returns the number of `true` elements found.
2181 ///
2182 /// The relative order of partitioned items is not maintained.
2183 ///
2184 /// # Current implementation
2185 ///
2186 /// The current algorithm tries to find the first element for which the predicate evaluates
2187 /// to false and the last element for which it evaluates to true, and repeatedly swaps them.
2188 ///
2189 /// Time complexity: *O*(*n*)
2190 ///
2191 /// See also [`is_partitioned()`] and [`partition()`].
2192 ///
2193 /// [`is_partitioned()`]: Iterator::is_partitioned
2194 /// [`partition()`]: Iterator::partition
2195 ///
2196 /// # Examples
2197 ///
2198 /// ```
2199 /// #![feature(iter_partition_in_place)]
2200 ///
2201 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
2202 ///
2203 /// // Partition in-place between evens and odds
2204 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
2205 ///
2206 /// assert_eq!(i, 3);
2207 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
2208 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
2209 /// ```
2210 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
2211 fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
2212 where
2213 Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
2214 P: FnMut(&T) -> bool,
2215 {
2216 // FIXME: should we worry about the count overflowing? The only way to have more than
2217 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
2218
2219 // These closure "factory" functions exist to avoid genericity in `Self`.
2220
2221 #[inline]
2222 fn is_false<'a, T>(
2223 predicate: &'a mut impl FnMut(&T) -> bool,
2224 true_count: &'a mut usize,
2225 ) -> impl FnMut(&&mut T) -> bool + 'a {
2226 move |x| {
2227 let p = predicate(&**x);
2228 *true_count += p as usize;
2229 !p
2230 }
2231 }
2232
2233 #[inline]
2234 fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
2235 move |x| predicate(&**x)
2236 }
2237
2238 // Repeatedly find the first `false` and swap it with the last `true`.
2239 let mut true_count = 0;
2240 while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
2241 if let Some(tail) = self.rfind(is_true(predicate)) {
2242 crate::mem::swap(head, tail);
2243 true_count += 1;
2244 } else {
2245 break;
2246 }
2247 }
2248 true_count
2249 }
2250
2251 /// Checks if the elements of this iterator are partitioned according to the given predicate,
2252 /// such that all those that return `true` precede all those that return `false`.
2253 ///
2254 /// See also [`partition()`] and [`partition_in_place()`].
2255 ///
2256 /// [`partition()`]: Iterator::partition
2257 /// [`partition_in_place()`]: Iterator::partition_in_place
2258 ///
2259 /// # Examples
2260 ///
2261 /// ```
2262 /// #![feature(iter_is_partitioned)]
2263 ///
2264 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
2265 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
2266 /// ```
2267 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
2268 fn is_partitioned<P>(mut self, mut predicate: P) -> bool
2269 where
2270 Self: Sized,
2271 P: FnMut(Self::Item) -> bool,
2272 {
2273 // Either all items test `true`, or the first clause stops at `false`
2274 // and we check that there are no more `true` items after that.
2275 self.all(&mut predicate) || !self.any(predicate)
2276 }
2277
2278 /// An iterator method that applies a function as long as it returns
2279 /// successfully, producing a single, final value.
2280 ///
2281 /// `try_fold()` takes two arguments: an initial value, and a closure with
2282 /// two arguments: an 'accumulator', and an element. The closure either
2283 /// returns successfully, with the value that the accumulator should have
2284 /// for the next iteration, or it returns failure, with an error value that
2285 /// is propagated back to the caller immediately (short-circuiting).
2286 ///
2287 /// The initial value is the value the accumulator will have on the first
2288 /// call. If applying the closure succeeded against every element of the
2289 /// iterator, `try_fold()` returns the final accumulator as success.
2290 ///
2291 /// Folding is useful whenever you have a collection of something, and want
2292 /// to produce a single value from it.
2293 ///
2294 /// # Note to Implementors
2295 ///
2296 /// Several of the other (forward) methods have default implementations in
2297 /// terms of this one, so try to implement this explicitly if it can
2298 /// do something better than the default `for` loop implementation.
2299 ///
2300 /// In particular, try to have this call `try_fold()` on the internal parts
2301 /// from which this iterator is composed. If multiple calls are needed,
2302 /// the `?` operator may be convenient for chaining the accumulator value
2303 /// along, but beware any invariants that need to be upheld before those
2304 /// early returns. This is a `&mut self` method, so iteration needs to be
2305 /// resumable after hitting an error here.
2306 ///
2307 /// # Examples
2308 ///
2309 /// Basic usage:
2310 ///
2311 /// ```
2312 /// let a = [1, 2, 3];
2313 ///
2314 /// // the checked sum of all of the elements of the array
2315 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
2316 ///
2317 /// assert_eq!(sum, Some(6));
2318 /// ```
2319 ///
2320 /// Short-circuiting:
2321 ///
2322 /// ```
2323 /// let a = [10, 20, 30, 100, 40, 50];
2324 /// let mut it = a.iter();
2325 ///
2326 /// // This sum overflows when adding the 100 element
2327 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
2328 /// assert_eq!(sum, None);
2329 ///
2330 /// // Because it short-circuited, the remaining elements are still
2331 /// // available through the iterator.
2332 /// assert_eq!(it.len(), 2);
2333 /// assert_eq!(it.next(), Some(&40));
2334 /// ```
2335 ///
2336 /// While you cannot `break` from a closure, the [`ControlFlow`] type allows
2337 /// a similar idea:
2338 ///
2339 /// ```
2340 /// use std::ops::ControlFlow;
2341 ///
2342 /// let triangular = (1..30).try_fold(0_i8, |prev, x| {
2343 /// if let Some(next) = prev.checked_add(x) {
2344 /// ControlFlow::Continue(next)
2345 /// } else {
2346 /// ControlFlow::Break(prev)
2347 /// }
2348 /// });
2349 /// assert_eq!(triangular, ControlFlow::Break(120));
2350 ///
2351 /// let triangular = (1..30).try_fold(0_u64, |prev, x| {
2352 /// if let Some(next) = prev.checked_add(x) {
2353 /// ControlFlow::Continue(next)
2354 /// } else {
2355 /// ControlFlow::Break(prev)
2356 /// }
2357 /// });
2358 /// assert_eq!(triangular, ControlFlow::Continue(435));
2359 /// ```
2360 #[inline]
2361 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2362 fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
2363 where
2364 Self: Sized,
2365 F: FnMut(B, Self::Item) -> R,
2366 R: Try<Output = B>,
2367 {
2368 let mut accum = init;
2369 while let Some(x) = self.next() {
2370 accum = f(accum, x)?;
2371 }
2372 try { accum }
2373 }
2374
2375 /// An iterator method that applies a fallible function to each item in the
2376 /// iterator, stopping at the first error and returning that error.
2377 ///
2378 /// This can also be thought of as the fallible form of [`for_each()`]
2379 /// or as the stateless version of [`try_fold()`].
2380 ///
2381 /// [`for_each()`]: Iterator::for_each
2382 /// [`try_fold()`]: Iterator::try_fold
2383 ///
2384 /// # Examples
2385 ///
2386 /// ```
2387 /// use std::fs::rename;
2388 /// use std::io::{stdout, Write};
2389 /// use std::path::Path;
2390 ///
2391 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
2392 ///
2393 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{x}"));
2394 /// assert!(res.is_ok());
2395 ///
2396 /// let mut it = data.iter().cloned();
2397 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
2398 /// assert!(res.is_err());
2399 /// // It short-circuited, so the remaining items are still in the iterator:
2400 /// assert_eq!(it.next(), Some("stale_bread.json"));
2401 /// ```
2402 ///
2403 /// The [`ControlFlow`] type can be used with this method for the situations
2404 /// in which you'd use `break` and `continue` in a normal loop:
2405 ///
2406 /// ```
2407 /// use std::ops::ControlFlow;
2408 ///
2409 /// let r = (2..100).try_for_each(|x| {
2410 /// if 323 % x == 0 {
2411 /// return ControlFlow::Break(x)
2412 /// }
2413 ///
2414 /// ControlFlow::Continue(())
2415 /// });
2416 /// assert_eq!(r, ControlFlow::Break(17));
2417 /// ```
2418 #[inline]
2419 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2420 fn try_for_each<F, R>(&mut self, f: F) -> R
2421 where
2422 Self: Sized,
2423 F: FnMut(Self::Item) -> R,
2424 R: Try<Output = ()>,
2425 {
2426 #[inline]
2427 fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
2428 move |(), x| f(x)
2429 }
2430
2431 self.try_fold((), call(f))
2432 }
2433
2434 /// Folds every element into an accumulator by applying an operation,
2435 /// returning the final result.
2436 ///
2437 /// `fold()` takes two arguments: an initial value, and a closure with two
2438 /// arguments: an 'accumulator', and an element. The closure returns the value that
2439 /// the accumulator should have for the next iteration.
2440 ///
2441 /// The initial value is the value the accumulator will have on the first
2442 /// call.
2443 ///
2444 /// After applying this closure to every element of the iterator, `fold()`
2445 /// returns the accumulator.
2446 ///
2447 /// This operation is sometimes called 'reduce' or 'inject'.
2448 ///
2449 /// Folding is useful whenever you have a collection of something, and want
2450 /// to produce a single value from it.
2451 ///
2452 /// Note: `fold()`, and similar methods that traverse the entire iterator,
2453 /// might not terminate for infinite iterators, even on traits for which a
2454 /// result is determinable in finite time.
2455 ///
2456 /// Note: [`reduce()`] can be used to use the first element as the initial
2457 /// value, if the accumulator type and item type is the same.
2458 ///
2459 /// Note: `fold()` combines elements in a *left-associative* fashion. For associative
2460 /// operators like `+`, the order the elements are combined in is not important, but for non-associative
2461 /// operators like `-` the order will affect the final result.
2462 /// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`].
2463 ///
2464 /// # Note to Implementors
2465 ///
2466 /// Several of the other (forward) methods have default implementations in
2467 /// terms of this one, so try to implement this explicitly if it can
2468 /// do something better than the default `for` loop implementation.
2469 ///
2470 /// In particular, try to have this call `fold()` on the internal parts
2471 /// from which this iterator is composed.
2472 ///
2473 /// # Examples
2474 ///
2475 /// Basic usage:
2476 ///
2477 /// ```
2478 /// let a = [1, 2, 3];
2479 ///
2480 /// // the sum of all of the elements of the array
2481 /// let sum = a.iter().fold(0, |acc, x| acc + x);
2482 ///
2483 /// assert_eq!(sum, 6);
2484 /// ```
2485 ///
2486 /// Let's walk through each step of the iteration here:
2487 ///
2488 /// | element | acc | x | result |
2489 /// |---------|-----|---|--------|
2490 /// | | 0 | | |
2491 /// | 1 | 0 | 1 | 1 |
2492 /// | 2 | 1 | 2 | 3 |
2493 /// | 3 | 3 | 3 | 6 |
2494 ///
2495 /// And so, our final result, `6`.
2496 ///
2497 /// This example demonstrates the left-associative nature of `fold()`:
2498 /// it builds a string, starting with an initial value
2499 /// and continuing with each element from the front until the back:
2500 ///
2501 /// ```
2502 /// let numbers = [1, 2, 3, 4, 5];
2503 ///
2504 /// let zero = "0".to_string();
2505 ///
2506 /// let result = numbers.iter().fold(zero, |acc, &x| {
2507 /// format!("({acc} + {x})")
2508 /// });
2509 ///
2510 /// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");
2511 /// ```
2512 /// It's common for people who haven't used iterators a lot to
2513 /// use a `for` loop with a list of things to build up a result. Those
2514 /// can be turned into `fold()`s:
2515 ///
2516 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
2517 ///
2518 /// ```
2519 /// let numbers = [1, 2, 3, 4, 5];
2520 ///
2521 /// let mut result = 0;
2522 ///
2523 /// // for loop:
2524 /// for i in &numbers {
2525 /// result = result + i;
2526 /// }
2527 ///
2528 /// // fold:
2529 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
2530 ///
2531 /// // they're the same
2532 /// assert_eq!(result, result2);
2533 /// ```
2534 ///
2535 /// [`reduce()`]: Iterator::reduce
2536 #[doc(alias = "inject", alias = "foldl")]
2537 #[inline]
2538 #[stable(feature = "rust1", since = "1.0.0")]
2539 fn fold<B, F>(mut self, init: B, mut f: F) -> B
2540 where
2541 Self: Sized,
2542 F: FnMut(B, Self::Item) -> B,
2543 {
2544 let mut accum = init;
2545 while let Some(x) = self.next() {
2546 accum = f(accum, x);
2547 }
2548 accum
2549 }
2550
2551 /// Reduces the elements to a single one, by repeatedly applying a reducing
2552 /// operation.
2553 ///
2554 /// If the iterator is empty, returns [`None`]; otherwise, returns the
2555 /// result of the reduction.
2556 ///
2557 /// The reducing function is a closure with two arguments: an 'accumulator', and an element.
2558 /// For iterators with at least one element, this is the same as [`fold()`]
2559 /// with the first element of the iterator as the initial accumulator value, folding
2560 /// every subsequent element into it.
2561 ///
2562 /// [`fold()`]: Iterator::fold
2563 ///
2564 /// # Example
2565 ///
2566 /// ```
2567 /// let reduced: i32 = (1..10).reduce(|acc, e| acc + e).unwrap_or(0);
2568 /// assert_eq!(reduced, 45);
2569 ///
2570 /// // Which is equivalent to doing it with `fold`:
2571 /// let folded: i32 = (1..10).fold(0, |acc, e| acc + e);
2572 /// assert_eq!(reduced, folded);
2573 /// ```
2574 #[inline]
2575 #[stable(feature = "iterator_fold_self", since = "1.51.0")]
2576 fn reduce<F>(mut self, f: F) -> Option<Self::Item>
2577 where
2578 Self: Sized,
2579 F: FnMut(Self::Item, Self::Item) -> Self::Item,
2580 {
2581 let first = self.next()?;
2582 Some(self.fold(first, f))
2583 }
2584
2585 /// Reduces the elements to a single one by repeatedly applying a reducing operation. If the
2586 /// closure returns a failure, the failure is propagated back to the caller immediately.
2587 ///
2588 /// The return type of this method depends on the return type of the closure. If the closure
2589 /// returns `Result<Self::Item, E>`, then this function will return `Result<Option<Self::Item>,
2590 /// E>`. If the closure returns `Option<Self::Item>`, then this function will return
2591 /// `Option<Option<Self::Item>>`.
2592 ///
2593 /// When called on an empty iterator, this function will return either `Some(None)` or
2594 /// `Ok(None)` depending on the type of the provided closure.
2595 ///
2596 /// For iterators with at least one element, this is essentially the same as calling
2597 /// [`try_fold()`] with the first element of the iterator as the initial accumulator value.
2598 ///
2599 /// [`try_fold()`]: Iterator::try_fold
2600 ///
2601 /// # Examples
2602 ///
2603 /// Safely calculate the sum of a series of numbers:
2604 ///
2605 /// ```
2606 /// #![feature(iterator_try_reduce)]
2607 ///
2608 /// let numbers: Vec<usize> = vec![10, 20, 5, 23, 0];
2609 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2610 /// assert_eq!(sum, Some(Some(58)));
2611 /// ```
2612 ///
2613 /// Determine when a reduction short circuited:
2614 ///
2615 /// ```
2616 /// #![feature(iterator_try_reduce)]
2617 ///
2618 /// let numbers = vec![1, 2, 3, usize::MAX, 4, 5];
2619 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2620 /// assert_eq!(sum, None);
2621 /// ```
2622 ///
2623 /// Determine when a reduction was not performed because there are no elements:
2624 ///
2625 /// ```
2626 /// #![feature(iterator_try_reduce)]
2627 ///
2628 /// let numbers: Vec<usize> = Vec::new();
2629 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2630 /// assert_eq!(sum, Some(None));
2631 /// ```
2632 ///
2633 /// Use a [`Result`] instead of an [`Option`]:
2634 ///
2635 /// ```
2636 /// #![feature(iterator_try_reduce)]
2637 ///
2638 /// let numbers = vec!["1", "2", "3", "4", "5"];
2639 /// let max: Result<Option<_>, <usize as std::str::FromStr>::Err> =
2640 /// numbers.into_iter().try_reduce(|x, y| {
2641 /// if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) }
2642 /// });
2643 /// assert_eq!(max, Ok(Some("5")));
2644 /// ```
2645 #[inline]
2646 #[unstable(feature = "iterator_try_reduce", reason = "new API", issue = "87053")]
2647 fn try_reduce<R>(
2648 &mut self,
2649 f: impl FnMut(Self::Item, Self::Item) -> R,
2650 ) -> ChangeOutputType<R, Option<R::Output>>
2651 where
2652 Self: Sized,
2653 R: Try<Output = Self::Item, Residual: Residual<Option<Self::Item>>>,
2654 {
2655 let first = match self.next() {
2656 Some(i) => i,
2657 None => return Try::from_output(None),
2658 };
2659
2660 match self.try_fold(first, f).branch() {
2661 ControlFlow::Break(r) => FromResidual::from_residual(r),
2662 ControlFlow::Continue(i) => Try::from_output(Some(i)),
2663 }
2664 }
2665
2666 /// Tests if every element of the iterator matches a predicate.
2667 ///
2668 /// `all()` takes a closure that returns `true` or `false`. It applies
2669 /// this closure to each element of the iterator, and if they all return
2670 /// `true`, then so does `all()`. If any of them return `false`, it
2671 /// returns `false`.
2672 ///
2673 /// `all()` is short-circuiting; in other words, it will stop processing
2674 /// as soon as it finds a `false`, given that no matter what else happens,
2675 /// the result will also be `false`.
2676 ///
2677 /// An empty iterator returns `true`.
2678 ///
2679 /// # Examples
2680 ///
2681 /// Basic usage:
2682 ///
2683 /// ```
2684 /// let a = [1, 2, 3];
2685 ///
2686 /// assert!(a.iter().all(|&x| x > 0));
2687 ///
2688 /// assert!(!a.iter().all(|&x| x > 2));
2689 /// ```
2690 ///
2691 /// Stopping at the first `false`:
2692 ///
2693 /// ```
2694 /// let a = [1, 2, 3];
2695 ///
2696 /// let mut iter = a.iter();
2697 ///
2698 /// assert!(!iter.all(|&x| x != 2));
2699 ///
2700 /// // we can still use `iter`, as there are more elements.
2701 /// assert_eq!(iter.next(), Some(&3));
2702 /// ```
2703 #[inline]
2704 #[stable(feature = "rust1", since = "1.0.0")]
2705 fn all<F>(&mut self, f: F) -> bool
2706 where
2707 Self: Sized,
2708 F: FnMut(Self::Item) -> bool,
2709 {
2710 #[inline]
2711 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2712 move |(), x| {
2713 if f(x) { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
2714 }
2715 }
2716 self.try_fold((), check(f)) == ControlFlow::Continue(())
2717 }
2718
2719 /// Tests if any element of the iterator matches a predicate.
2720 ///
2721 /// `any()` takes a closure that returns `true` or `false`. It applies
2722 /// this closure to each element of the iterator, and if any of them return
2723 /// `true`, then so does `any()`. If they all return `false`, it
2724 /// returns `false`.
2725 ///
2726 /// `any()` is short-circuiting; in other words, it will stop processing
2727 /// as soon as it finds a `true`, given that no matter what else happens,
2728 /// the result will also be `true`.
2729 ///
2730 /// An empty iterator returns `false`.
2731 ///
2732 /// # Examples
2733 ///
2734 /// Basic usage:
2735 ///
2736 /// ```
2737 /// let a = [1, 2, 3];
2738 ///
2739 /// assert!(a.iter().any(|&x| x > 0));
2740 ///
2741 /// assert!(!a.iter().any(|&x| x > 5));
2742 /// ```
2743 ///
2744 /// Stopping at the first `true`:
2745 ///
2746 /// ```
2747 /// let a = [1, 2, 3];
2748 ///
2749 /// let mut iter = a.iter();
2750 ///
2751 /// assert!(iter.any(|&x| x != 2));
2752 ///
2753 /// // we can still use `iter`, as there are more elements.
2754 /// assert_eq!(iter.next(), Some(&2));
2755 /// ```
2756 #[inline]
2757 #[stable(feature = "rust1", since = "1.0.0")]
2758 fn any<F>(&mut self, f: F) -> bool
2759 where
2760 Self: Sized,
2761 F: FnMut(Self::Item) -> bool,
2762 {
2763 #[inline]
2764 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2765 move |(), x| {
2766 if f(x) { ControlFlow::Break(()) } else { ControlFlow::Continue(()) }
2767 }
2768 }
2769
2770 self.try_fold((), check(f)) == ControlFlow::Break(())
2771 }
2772
2773 /// Searches for an element of an iterator that satisfies a predicate.
2774 ///
2775 /// `find()` takes a closure that returns `true` or `false`. It applies
2776 /// this closure to each element of the iterator, and if any of them return
2777 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2778 /// `false`, it returns [`None`].
2779 ///
2780 /// `find()` is short-circuiting; in other words, it will stop processing
2781 /// as soon as the closure returns `true`.
2782 ///
2783 /// Because `find()` takes a reference, and many iterators iterate over
2784 /// references, this leads to a possibly confusing situation where the
2785 /// argument is a double reference. You can see this effect in the
2786 /// examples below, with `&&x`.
2787 ///
2788 /// If you need the index of the element, see [`position()`].
2789 ///
2790 /// [`Some(element)`]: Some
2791 /// [`position()`]: Iterator::position
2792 ///
2793 /// # Examples
2794 ///
2795 /// Basic usage:
2796 ///
2797 /// ```
2798 /// let a = [1, 2, 3];
2799 ///
2800 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2801 ///
2802 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2803 /// ```
2804 ///
2805 /// Stopping at the first `true`:
2806 ///
2807 /// ```
2808 /// let a = [1, 2, 3];
2809 ///
2810 /// let mut iter = a.iter();
2811 ///
2812 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2813 ///
2814 /// // we can still use `iter`, as there are more elements.
2815 /// assert_eq!(iter.next(), Some(&3));
2816 /// ```
2817 ///
2818 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2819 #[inline]
2820 #[stable(feature = "rust1", since = "1.0.0")]
2821 fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2822 where
2823 Self: Sized,
2824 P: FnMut(&Self::Item) -> bool,
2825 {
2826 #[inline]
2827 fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
2828 move |(), x| {
2829 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::Continue(()) }
2830 }
2831 }
2832
2833 self.try_fold((), check(predicate)).break_value()
2834 }
2835
2836 /// Applies function to the elements of iterator and returns
2837 /// the first non-none result.
2838 ///
2839 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2840 ///
2841 /// # Examples
2842 ///
2843 /// ```
2844 /// let a = ["lol", "NaN", "2", "5"];
2845 ///
2846 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2847 ///
2848 /// assert_eq!(first_number, Some(2));
2849 /// ```
2850 #[inline]
2851 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2852 fn find_map<B, F>(&mut self, f: F) -> Option<B>
2853 where
2854 Self: Sized,
2855 F: FnMut(Self::Item) -> Option<B>,
2856 {
2857 #[inline]
2858 fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
2859 move |(), x| match f(x) {
2860 Some(x) => ControlFlow::Break(x),
2861 None => ControlFlow::Continue(()),
2862 }
2863 }
2864
2865 self.try_fold((), check(f)).break_value()
2866 }
2867
2868 /// Applies function to the elements of iterator and returns
2869 /// the first true result or the first error.
2870 ///
2871 /// The return type of this method depends on the return type of the closure.
2872 /// If you return `Result<bool, E>` from the closure, you'll get a `Result<Option<Self::Item>, E>`.
2873 /// If you return `Option<bool>` from the closure, you'll get an `Option<Option<Self::Item>>`.
2874 ///
2875 /// # Examples
2876 ///
2877 /// ```
2878 /// #![feature(try_find)]
2879 ///
2880 /// let a = ["1", "2", "lol", "NaN", "5"];
2881 ///
2882 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2883 /// Ok(s.parse::<i32>()? == search)
2884 /// };
2885 ///
2886 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2887 /// assert_eq!(result, Ok(Some(&"2")));
2888 ///
2889 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2890 /// assert!(result.is_err());
2891 /// ```
2892 ///
2893 /// This also supports other types which implement [`Try`], not just [`Result`].
2894 ///
2895 /// ```
2896 /// #![feature(try_find)]
2897 ///
2898 /// use std::num::NonZero;
2899 ///
2900 /// let a = [3, 5, 7, 4, 9, 0, 11u32];
2901 /// let result = a.iter().try_find(|&&x| NonZero::new(x).map(|y| y.is_power_of_two()));
2902 /// assert_eq!(result, Some(Some(&4)));
2903 /// let result = a.iter().take(3).try_find(|&&x| NonZero::new(x).map(|y| y.is_power_of_two()));
2904 /// assert_eq!(result, Some(None));
2905 /// let result = a.iter().rev().try_find(|&&x| NonZero::new(x).map(|y| y.is_power_of_two()));
2906 /// assert_eq!(result, None);
2907 /// ```
2908 #[inline]
2909 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2910 fn try_find<R>(
2911 &mut self,
2912 f: impl FnMut(&Self::Item) -> R,
2913 ) -> ChangeOutputType<R, Option<Self::Item>>
2914 where
2915 Self: Sized,
2916 R: Try<Output = bool, Residual: Residual<Option<Self::Item>>>,
2917 {
2918 #[inline]
2919 fn check<I, V, R>(
2920 mut f: impl FnMut(&I) -> V,
2921 ) -> impl FnMut((), I) -> ControlFlow<R::TryType>
2922 where
2923 V: Try<Output = bool, Residual = R>,
2924 R: Residual<Option<I>>,
2925 {
2926 move |(), x| match f(&x).branch() {
2927 ControlFlow::Continue(false) => ControlFlow::Continue(()),
2928 ControlFlow::Continue(true) => ControlFlow::Break(Try::from_output(Some(x))),
2929 ControlFlow::Break(r) => ControlFlow::Break(FromResidual::from_residual(r)),
2930 }
2931 }
2932
2933 match self.try_fold((), check(f)) {
2934 ControlFlow::Break(x) => x,
2935 ControlFlow::Continue(()) => Try::from_output(None),
2936 }
2937 }
2938
2939 /// Searches for an element in an iterator, returning its index.
2940 ///
2941 /// `position()` takes a closure that returns `true` or `false`. It applies
2942 /// this closure to each element of the iterator, and if one of them
2943 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2944 /// them return `false`, it returns [`None`].
2945 ///
2946 /// `position()` is short-circuiting; in other words, it will stop
2947 /// processing as soon as it finds a `true`.
2948 ///
2949 /// # Overflow Behavior
2950 ///
2951 /// The method does no guarding against overflows, so if there are more
2952 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2953 /// result or panics. If debug assertions are enabled, a panic is
2954 /// guaranteed.
2955 ///
2956 /// # Panics
2957 ///
2958 /// This function might panic if the iterator has more than `usize::MAX`
2959 /// non-matching elements.
2960 ///
2961 /// [`Some(index)`]: Some
2962 ///
2963 /// # Examples
2964 ///
2965 /// Basic usage:
2966 ///
2967 /// ```
2968 /// let a = [1, 2, 3];
2969 ///
2970 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2971 ///
2972 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2973 /// ```
2974 ///
2975 /// Stopping at the first `true`:
2976 ///
2977 /// ```
2978 /// let a = [1, 2, 3, 4];
2979 ///
2980 /// let mut iter = a.iter();
2981 ///
2982 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2983 ///
2984 /// // we can still use `iter`, as there are more elements.
2985 /// assert_eq!(iter.next(), Some(&3));
2986 ///
2987 /// // The returned index depends on iterator state
2988 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
2989 ///
2990 /// ```
2991 #[inline]
2992 #[stable(feature = "rust1", since = "1.0.0")]
2993 fn position<P>(&mut self, predicate: P) -> Option<usize>
2994 where
2995 Self: Sized,
2996 P: FnMut(Self::Item) -> bool,
2997 {
2998 #[inline]
2999 fn check<'a, T>(
3000 mut predicate: impl FnMut(T) -> bool + 'a,
3001 acc: &'a mut usize,
3002 ) -> impl FnMut((), T) -> ControlFlow<usize, ()> + 'a {
3003 #[rustc_inherit_overflow_checks]
3004 move |_, x| {
3005 if predicate(x) {
3006 ControlFlow::Break(*acc)
3007 } else {
3008 *acc += 1;
3009 ControlFlow::Continue(())
3010 }
3011 }
3012 }
3013
3014 let mut acc = 0;
3015 self.try_fold((), check(predicate, &mut acc)).break_value()
3016 }
3017
3018 /// Searches for an element in an iterator from the right, returning its
3019 /// index.
3020 ///
3021 /// `rposition()` takes a closure that returns `true` or `false`. It applies
3022 /// this closure to each element of the iterator, starting from the end,
3023 /// and if one of them returns `true`, then `rposition()` returns
3024 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
3025 ///
3026 /// `rposition()` is short-circuiting; in other words, it will stop
3027 /// processing as soon as it finds a `true`.
3028 ///
3029 /// [`Some(index)`]: Some
3030 ///
3031 /// # Examples
3032 ///
3033 /// Basic usage:
3034 ///
3035 /// ```
3036 /// let a = [1, 2, 3];
3037 ///
3038 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
3039 ///
3040 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
3041 /// ```
3042 ///
3043 /// Stopping at the first `true`:
3044 ///
3045 /// ```
3046 /// let a = [-1, 2, 3, 4];
3047 ///
3048 /// let mut iter = a.iter();
3049 ///
3050 /// assert_eq!(iter.rposition(|&x| x >= 2), Some(3));
3051 ///
3052 /// // we can still use `iter`, as there are more elements.
3053 /// assert_eq!(iter.next(), Some(&-1));
3054 /// assert_eq!(iter.next_back(), Some(&3));
3055 /// ```
3056 #[inline]
3057 #[stable(feature = "rust1", since = "1.0.0")]
3058 fn rposition<P>(&mut self, predicate: P) -> Option<usize>
3059 where
3060 P: FnMut(Self::Item) -> bool,
3061 Self: Sized + ExactSizeIterator + DoubleEndedIterator,
3062 {
3063 // No need for an overflow check here, because `ExactSizeIterator`
3064 // implies that the number of elements fits into a `usize`.
3065 #[inline]
3066 fn check<T>(
3067 mut predicate: impl FnMut(T) -> bool,
3068 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
3069 move |i, x| {
3070 let i = i - 1;
3071 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
3072 }
3073 }
3074
3075 let n = self.len();
3076 self.try_rfold(n, check(predicate)).break_value()
3077 }
3078
3079 /// Returns the maximum element of an iterator.
3080 ///
3081 /// If several elements are equally maximum, the last element is
3082 /// returned. If the iterator is empty, [`None`] is returned.
3083 ///
3084 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
3085 /// incomparable. You can work around this by using [`Iterator::reduce`]:
3086 /// ```
3087 /// assert_eq!(
3088 /// [2.4, f32::NAN, 1.3]
3089 /// .into_iter()
3090 /// .reduce(f32::max)
3091 /// .unwrap_or(0.),
3092 /// 2.4
3093 /// );
3094 /// ```
3095 ///
3096 /// # Examples
3097 ///
3098 /// ```
3099 /// let a = [1, 2, 3];
3100 /// let b: Vec<u32> = Vec::new();
3101 ///
3102 /// assert_eq!(a.iter().max(), Some(&3));
3103 /// assert_eq!(b.iter().max(), None);
3104 /// ```
3105 #[inline]
3106 #[stable(feature = "rust1", since = "1.0.0")]
3107 fn max(self) -> Option<Self::Item>
3108 where
3109 Self: Sized,
3110 Self::Item: Ord,
3111 {
3112 self.max_by(Ord::cmp)
3113 }
3114
3115 /// Returns the minimum element of an iterator.
3116 ///
3117 /// If several elements are equally minimum, the first element is returned.
3118 /// If the iterator is empty, [`None`] is returned.
3119 ///
3120 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
3121 /// incomparable. You can work around this by using [`Iterator::reduce`]:
3122 /// ```
3123 /// assert_eq!(
3124 /// [2.4, f32::NAN, 1.3]
3125 /// .into_iter()
3126 /// .reduce(f32::min)
3127 /// .unwrap_or(0.),
3128 /// 1.3
3129 /// );
3130 /// ```
3131 ///
3132 /// # Examples
3133 ///
3134 /// ```
3135 /// let a = [1, 2, 3];
3136 /// let b: Vec<u32> = Vec::new();
3137 ///
3138 /// assert_eq!(a.iter().min(), Some(&1));
3139 /// assert_eq!(b.iter().min(), None);
3140 /// ```
3141 #[inline]
3142 #[stable(feature = "rust1", since = "1.0.0")]
3143 fn min(self) -> Option<Self::Item>
3144 where
3145 Self: Sized,
3146 Self::Item: Ord,
3147 {
3148 self.min_by(Ord::cmp)
3149 }
3150
3151 /// Returns the element that gives the maximum value from the
3152 /// specified function.
3153 ///
3154 /// If several elements are equally maximum, the last element is
3155 /// returned. If the iterator is empty, [`None`] is returned.
3156 ///
3157 /// # Examples
3158 ///
3159 /// ```
3160 /// let a = [-3_i32, 0, 1, 5, -10];
3161 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
3162 /// ```
3163 #[inline]
3164 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
3165 fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
3166 where
3167 Self: Sized,
3168 F: FnMut(&Self::Item) -> B,
3169 {
3170 #[inline]
3171 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
3172 move |x| (f(&x), x)
3173 }
3174
3175 #[inline]
3176 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
3177 x_p.cmp(y_p)
3178 }
3179
3180 let (_, x) = self.map(key(f)).max_by(compare)?;
3181 Some(x)
3182 }
3183
3184 /// Returns the element that gives the maximum value with respect to the
3185 /// specified comparison function.
3186 ///
3187 /// If several elements are equally maximum, the last element is
3188 /// returned. If the iterator is empty, [`None`] is returned.
3189 ///
3190 /// # Examples
3191 ///
3192 /// ```
3193 /// let a = [-3_i32, 0, 1, 5, -10];
3194 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
3195 /// ```
3196 #[inline]
3197 #[stable(feature = "iter_max_by", since = "1.15.0")]
3198 fn max_by<F>(self, compare: F) -> Option<Self::Item>
3199 where
3200 Self: Sized,
3201 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
3202 {
3203 #[inline]
3204 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
3205 move |x, y| cmp::max_by(x, y, &mut compare)
3206 }
3207
3208 self.reduce(fold(compare))
3209 }
3210
3211 /// Returns the element that gives the minimum value from the
3212 /// specified function.
3213 ///
3214 /// If several elements are equally minimum, the first element is
3215 /// returned. If the iterator is empty, [`None`] is returned.
3216 ///
3217 /// # Examples
3218 ///
3219 /// ```
3220 /// let a = [-3_i32, 0, 1, 5, -10];
3221 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
3222 /// ```
3223 #[inline]
3224 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
3225 fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
3226 where
3227 Self: Sized,
3228 F: FnMut(&Self::Item) -> B,
3229 {
3230 #[inline]
3231 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
3232 move |x| (f(&x), x)
3233 }
3234
3235 #[inline]
3236 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
3237 x_p.cmp(y_p)
3238 }
3239
3240 let (_, x) = self.map(key(f)).min_by(compare)?;
3241 Some(x)
3242 }
3243
3244 /// Returns the element that gives the minimum value with respect to the
3245 /// specified comparison function.
3246 ///
3247 /// If several elements are equally minimum, the first element is
3248 /// returned. If the iterator is empty, [`None`] is returned.
3249 ///
3250 /// # Examples
3251 ///
3252 /// ```
3253 /// let a = [-3_i32, 0, 1, 5, -10];
3254 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
3255 /// ```
3256 #[inline]
3257 #[stable(feature = "iter_min_by", since = "1.15.0")]
3258 fn min_by<F>(self, compare: F) -> Option<Self::Item>
3259 where
3260 Self: Sized,
3261 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
3262 {
3263 #[inline]
3264 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
3265 move |x, y| cmp::min_by(x, y, &mut compare)
3266 }
3267
3268 self.reduce(fold(compare))
3269 }
3270
3271 /// Reverses an iterator's direction.
3272 ///
3273 /// Usually, iterators iterate from left to right. After using `rev()`,
3274 /// an iterator will instead iterate from right to left.
3275 ///
3276 /// This is only possible if the iterator has an end, so `rev()` only
3277 /// works on [`DoubleEndedIterator`]s.
3278 ///
3279 /// # Examples
3280 ///
3281 /// ```
3282 /// let a = [1, 2, 3];
3283 ///
3284 /// let mut iter = a.iter().rev();
3285 ///
3286 /// assert_eq!(iter.next(), Some(&3));
3287 /// assert_eq!(iter.next(), Some(&2));
3288 /// assert_eq!(iter.next(), Some(&1));
3289 ///
3290 /// assert_eq!(iter.next(), None);
3291 /// ```
3292 #[inline]
3293 #[doc(alias = "reverse")]
3294 #[stable(feature = "rust1", since = "1.0.0")]
3295 fn rev(self) -> Rev<Self>
3296 where
3297 Self: Sized + DoubleEndedIterator,
3298 {
3299 Rev::new(self)
3300 }
3301
3302 /// Converts an iterator of pairs into a pair of containers.
3303 ///
3304 /// `unzip()` consumes an entire iterator of pairs, producing two
3305 /// collections: one from the left elements of the pairs, and one
3306 /// from the right elements.
3307 ///
3308 /// This function is, in some sense, the opposite of [`zip`].
3309 ///
3310 /// [`zip`]: Iterator::zip
3311 ///
3312 /// # Examples
3313 ///
3314 /// ```
3315 /// let a = [(1, 2), (3, 4), (5, 6)];
3316 ///
3317 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
3318 ///
3319 /// assert_eq!(left, [1, 3, 5]);
3320 /// assert_eq!(right, [2, 4, 6]);
3321 ///
3322 /// // you can also unzip multiple nested tuples at once
3323 /// let a = [(1, (2, 3)), (4, (5, 6))];
3324 ///
3325 /// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.iter().cloned().unzip();
3326 /// assert_eq!(x, [1, 4]);
3327 /// assert_eq!(y, [2, 5]);
3328 /// assert_eq!(z, [3, 6]);
3329 /// ```
3330 #[stable(feature = "rust1", since = "1.0.0")]
3331 fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
3332 where
3333 FromA: Default + Extend<A>,
3334 FromB: Default + Extend<B>,
3335 Self: Sized + Iterator<Item = (A, B)>,
3336 {
3337 let mut unzipped: (FromA, FromB) = Default::default();
3338 unzipped.extend(self);
3339 unzipped
3340 }
3341
3342 /// Creates an iterator which copies all of its elements.
3343 ///
3344 /// This is useful when you have an iterator over `&T`, but you need an
3345 /// iterator over `T`.
3346 ///
3347 /// # Examples
3348 ///
3349 /// ```
3350 /// let a = [1, 2, 3];
3351 ///
3352 /// let v_copied: Vec<_> = a.iter().copied().collect();
3353 ///
3354 /// // copied is the same as .map(|&x| x)
3355 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3356 ///
3357 /// assert_eq!(v_copied, vec![1, 2, 3]);
3358 /// assert_eq!(v_map, vec![1, 2, 3]);
3359 /// ```
3360 #[stable(feature = "iter_copied", since = "1.36.0")]
3361 #[cfg_attr(not(test), rustc_diagnostic_item = "iter_copied")]
3362 fn copied<'a, T: 'a>(self) -> Copied<Self>
3363 where
3364 Self: Sized + Iterator<Item = &'a T>,
3365 T: Copy,
3366 {
3367 Copied::new(self)
3368 }
3369
3370 /// Creates an iterator which [`clone`]s all of its elements.
3371 ///
3372 /// This is useful when you have an iterator over `&T`, but you need an
3373 /// iterator over `T`.
3374 ///
3375 /// There is no guarantee whatsoever about the `clone` method actually
3376 /// being called *or* optimized away. So code should not depend on
3377 /// either.
3378 ///
3379 /// [`clone`]: Clone::clone
3380 ///
3381 /// # Examples
3382 ///
3383 /// Basic usage:
3384 ///
3385 /// ```
3386 /// let a = [1, 2, 3];
3387 ///
3388 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
3389 ///
3390 /// // cloned is the same as .map(|&x| x), for integers
3391 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3392 ///
3393 /// assert_eq!(v_cloned, vec![1, 2, 3]);
3394 /// assert_eq!(v_map, vec![1, 2, 3]);
3395 /// ```
3396 ///
3397 /// To get the best performance, try to clone late:
3398 ///
3399 /// ```
3400 /// let a = [vec![0_u8, 1, 2], vec![3, 4], vec![23]];
3401 /// // don't do this:
3402 /// let slower: Vec<_> = a.iter().cloned().filter(|s| s.len() == 1).collect();
3403 /// assert_eq!(&[vec![23]], &slower[..]);
3404 /// // instead call `cloned` late
3405 /// let faster: Vec<_> = a.iter().filter(|s| s.len() == 1).cloned().collect();
3406 /// assert_eq!(&[vec![23]], &faster[..]);
3407 /// ```
3408 #[stable(feature = "rust1", since = "1.0.0")]
3409 #[cfg_attr(not(test), rustc_diagnostic_item = "iter_cloned")]
3410 fn cloned<'a, T: 'a>(self) -> Cloned<Self>
3411 where
3412 Self: Sized + Iterator<Item = &'a T>,
3413 T: Clone,
3414 {
3415 Cloned::new(self)
3416 }
3417
3418 /// Repeats an iterator endlessly.
3419 ///
3420 /// Instead of stopping at [`None`], the iterator will instead start again,
3421 /// from the beginning. After iterating again, it will start at the
3422 /// beginning again. And again. And again. Forever. Note that in case the
3423 /// original iterator is empty, the resulting iterator will also be empty.
3424 ///
3425 /// # Examples
3426 ///
3427 /// ```
3428 /// let a = [1, 2, 3];
3429 ///
3430 /// let mut it = a.iter().cycle();
3431 ///
3432 /// assert_eq!(it.next(), Some(&1));
3433 /// assert_eq!(it.next(), Some(&2));
3434 /// assert_eq!(it.next(), Some(&3));
3435 /// assert_eq!(it.next(), Some(&1));
3436 /// assert_eq!(it.next(), Some(&2));
3437 /// assert_eq!(it.next(), Some(&3));
3438 /// assert_eq!(it.next(), Some(&1));
3439 /// ```
3440 #[stable(feature = "rust1", since = "1.0.0")]
3441 #[inline]
3442 fn cycle(self) -> Cycle<Self>
3443 where
3444 Self: Sized + Clone,
3445 {
3446 Cycle::new(self)
3447 }
3448
3449 /// Returns an iterator over `N` elements of the iterator at a time.
3450 ///
3451 /// The chunks do not overlap. If `N` does not divide the length of the
3452 /// iterator, then the last up to `N-1` elements will be omitted and can be
3453 /// retrieved from the [`.into_remainder()`][ArrayChunks::into_remainder]
3454 /// function of the iterator.
3455 ///
3456 /// # Panics
3457 ///
3458 /// Panics if `N` is zero.
3459 ///
3460 /// # Examples
3461 ///
3462 /// Basic usage:
3463 ///
3464 /// ```
3465 /// #![feature(iter_array_chunks)]
3466 ///
3467 /// let mut iter = "lorem".chars().array_chunks();
3468 /// assert_eq!(iter.next(), Some(['l', 'o']));
3469 /// assert_eq!(iter.next(), Some(['r', 'e']));
3470 /// assert_eq!(iter.next(), None);
3471 /// assert_eq!(iter.into_remainder().unwrap().as_slice(), &['m']);
3472 /// ```
3473 ///
3474 /// ```
3475 /// #![feature(iter_array_chunks)]
3476 ///
3477 /// let data = [1, 1, 2, -2, 6, 0, 3, 1];
3478 /// // ^-----^ ^------^
3479 /// for [x, y, z] in data.iter().array_chunks() {
3480 /// assert_eq!(x + y + z, 4);
3481 /// }
3482 /// ```
3483 #[track_caller]
3484 #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
3485 fn array_chunks<const N: usize>(self) -> ArrayChunks<Self, N>
3486 where
3487 Self: Sized,
3488 {
3489 ArrayChunks::new(self)
3490 }
3491
3492 /// Sums the elements of an iterator.
3493 ///
3494 /// Takes each element, adds them together, and returns the result.
3495 ///
3496 /// An empty iterator returns the *additive identity* ("zero") of the type,
3497 /// which is `0` for integers and `-0.0` for floats.
3498 ///
3499 /// `sum()` can be used to sum any type implementing [`Sum`][`core::iter::Sum`],
3500 /// including [`Option`][`Option::sum`] and [`Result`][`Result::sum`].
3501 ///
3502 /// # Panics
3503 ///
3504 /// When calling `sum()` and a primitive integer type is being returned, this
3505 /// method will panic if the computation overflows and debug assertions are
3506 /// enabled.
3507 ///
3508 /// # Examples
3509 ///
3510 /// ```
3511 /// let a = [1, 2, 3];
3512 /// let sum: i32 = a.iter().sum();
3513 ///
3514 /// assert_eq!(sum, 6);
3515 ///
3516 /// let b: Vec<f32> = vec![];
3517 /// let sum: f32 = b.iter().sum();
3518 /// assert_eq!(sum, -0.0_f32);
3519 /// ```
3520 #[stable(feature = "iter_arith", since = "1.11.0")]
3521 fn sum<S>(self) -> S
3522 where
3523 Self: Sized,
3524 S: Sum<Self::Item>,
3525 {
3526 Sum::sum(self)
3527 }
3528
3529 /// Iterates over the entire iterator, multiplying all the elements
3530 ///
3531 /// An empty iterator returns the one value of the type.
3532 ///
3533 /// `product()` can be used to multiply any type implementing [`Product`][`core::iter::Product`],
3534 /// including [`Option`][`Option::product`] and [`Result`][`Result::product`].
3535 ///
3536 /// # Panics
3537 ///
3538 /// When calling `product()` and a primitive integer type is being returned,
3539 /// method will panic if the computation overflows and debug assertions are
3540 /// enabled.
3541 ///
3542 /// # Examples
3543 ///
3544 /// ```
3545 /// fn factorial(n: u32) -> u32 {
3546 /// (1..=n).product()
3547 /// }
3548 /// assert_eq!(factorial(0), 1);
3549 /// assert_eq!(factorial(1), 1);
3550 /// assert_eq!(factorial(5), 120);
3551 /// ```
3552 #[stable(feature = "iter_arith", since = "1.11.0")]
3553 fn product<P>(self) -> P
3554 where
3555 Self: Sized,
3556 P: Product<Self::Item>,
3557 {
3558 Product::product(self)
3559 }
3560
3561 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3562 /// of another.
3563 ///
3564 /// # Examples
3565 ///
3566 /// ```
3567 /// use std::cmp::Ordering;
3568 ///
3569 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
3570 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
3571 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
3572 /// ```
3573 #[stable(feature = "iter_order", since = "1.5.0")]
3574 fn cmp<I>(self, other: I) -> Ordering
3575 where
3576 I: IntoIterator<Item = Self::Item>,
3577 Self::Item: Ord,
3578 Self: Sized,
3579 {
3580 self.cmp_by(other, |x, y| x.cmp(&y))
3581 }
3582
3583 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3584 /// of another with respect to the specified comparison function.
3585 ///
3586 /// # Examples
3587 ///
3588 /// ```
3589 /// #![feature(iter_order_by)]
3590 ///
3591 /// use std::cmp::Ordering;
3592 ///
3593 /// let xs = [1, 2, 3, 4];
3594 /// let ys = [1, 4, 9, 16];
3595 ///
3596 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
3597 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
3598 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
3599 /// ```
3600 #[unstable(feature = "iter_order_by", issue = "64295")]
3601 fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering
3602 where
3603 Self: Sized,
3604 I: IntoIterator,
3605 F: FnMut(Self::Item, I::Item) -> Ordering,
3606 {
3607 #[inline]
3608 fn compare<X, Y, F>(mut cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Ordering>
3609 where
3610 F: FnMut(X, Y) -> Ordering,
3611 {
3612 move |x, y| match cmp(x, y) {
3613 Ordering::Equal => ControlFlow::Continue(()),
3614 non_eq => ControlFlow::Break(non_eq),
3615 }
3616 }
3617
3618 match iter_compare(self, other.into_iter(), compare(cmp)) {
3619 ControlFlow::Continue(ord) => ord,
3620 ControlFlow::Break(ord) => ord,
3621 }
3622 }
3623
3624 /// [Lexicographically](Ord#lexicographical-comparison) compares the [`PartialOrd`] elements of
3625 /// this [`Iterator`] with those of another. The comparison works like short-circuit
3626 /// evaluation, returning a result without comparing the remaining elements.
3627 /// As soon as an order can be determined, the evaluation stops and a result is returned.
3628 ///
3629 /// # Examples
3630 ///
3631 /// ```
3632 /// use std::cmp::Ordering;
3633 ///
3634 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
3635 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
3636 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
3637 /// ```
3638 ///
3639 /// For floating-point numbers, NaN does not have a total order and will result
3640 /// in `None` when compared:
3641 ///
3642 /// ```
3643 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
3644 /// ```
3645 ///
3646 /// The results are determined by the order of evaluation.
3647 ///
3648 /// ```
3649 /// use std::cmp::Ordering;
3650 ///
3651 /// assert_eq!([1.0, f64::NAN].iter().partial_cmp([2.0, f64::NAN].iter()), Some(Ordering::Less));
3652 /// assert_eq!([2.0, f64::NAN].iter().partial_cmp([1.0, f64::NAN].iter()), Some(Ordering::Greater));
3653 /// assert_eq!([f64::NAN, 1.0].iter().partial_cmp([f64::NAN, 2.0].iter()), None);
3654 /// ```
3655 ///
3656 #[stable(feature = "iter_order", since = "1.5.0")]
3657 fn partial_cmp<I>(self, other: I) -> Option<Ordering>
3658 where
3659 I: IntoIterator,
3660 Self::Item: PartialOrd<I::Item>,
3661 Self: Sized,
3662 {
3663 self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
3664 }
3665
3666 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3667 /// of another with respect to the specified comparison function.
3668 ///
3669 /// # Examples
3670 ///
3671 /// ```
3672 /// #![feature(iter_order_by)]
3673 ///
3674 /// use std::cmp::Ordering;
3675 ///
3676 /// let xs = [1.0, 2.0, 3.0, 4.0];
3677 /// let ys = [1.0, 4.0, 9.0, 16.0];
3678 ///
3679 /// assert_eq!(
3680 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
3681 /// Some(Ordering::Less)
3682 /// );
3683 /// assert_eq!(
3684 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
3685 /// Some(Ordering::Equal)
3686 /// );
3687 /// assert_eq!(
3688 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
3689 /// Some(Ordering::Greater)
3690 /// );
3691 /// ```
3692 #[unstable(feature = "iter_order_by", issue = "64295")]
3693 fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>
3694 where
3695 Self: Sized,
3696 I: IntoIterator,
3697 F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
3698 {
3699 #[inline]
3700 fn compare<X, Y, F>(mut partial_cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Option<Ordering>>
3701 where
3702 F: FnMut(X, Y) -> Option<Ordering>,
3703 {
3704 move |x, y| match partial_cmp(x, y) {
3705 Some(Ordering::Equal) => ControlFlow::Continue(()),
3706 non_eq => ControlFlow::Break(non_eq),
3707 }
3708 }
3709
3710 match iter_compare(self, other.into_iter(), compare(partial_cmp)) {
3711 ControlFlow::Continue(ord) => Some(ord),
3712 ControlFlow::Break(ord) => ord,
3713 }
3714 }
3715
3716 /// Determines if the elements of this [`Iterator`] are equal to those of
3717 /// another.
3718 ///
3719 /// # Examples
3720 ///
3721 /// ```
3722 /// assert_eq!([1].iter().eq([1].iter()), true);
3723 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3724 /// ```
3725 #[stable(feature = "iter_order", since = "1.5.0")]
3726 fn eq<I>(self, other: I) -> bool
3727 where
3728 I: IntoIterator,
3729 Self::Item: PartialEq<I::Item>,
3730 Self: Sized,
3731 {
3732 self.eq_by(other, |x, y| x == y)
3733 }
3734
3735 /// Determines if the elements of this [`Iterator`] are equal to those of
3736 /// another with respect to the specified equality function.
3737 ///
3738 /// # Examples
3739 ///
3740 /// ```
3741 /// #![feature(iter_order_by)]
3742 ///
3743 /// let xs = [1, 2, 3, 4];
3744 /// let ys = [1, 4, 9, 16];
3745 ///
3746 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3747 /// ```
3748 #[unstable(feature = "iter_order_by", issue = "64295")]
3749 fn eq_by<I, F>(self, other: I, eq: F) -> bool
3750 where
3751 Self: Sized,
3752 I: IntoIterator,
3753 F: FnMut(Self::Item, I::Item) -> bool,
3754 {
3755 #[inline]
3756 fn compare<X, Y, F>(mut eq: F) -> impl FnMut(X, Y) -> ControlFlow<()>
3757 where
3758 F: FnMut(X, Y) -> bool,
3759 {
3760 move |x, y| {
3761 if eq(x, y) { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
3762 }
3763 }
3764
3765 match iter_compare(self, other.into_iter(), compare(eq)) {
3766 ControlFlow::Continue(ord) => ord == Ordering::Equal,
3767 ControlFlow::Break(()) => false,
3768 }
3769 }
3770
3771 /// Determines if the elements of this [`Iterator`] are not equal to those of
3772 /// another.
3773 ///
3774 /// # Examples
3775 ///
3776 /// ```
3777 /// assert_eq!([1].iter().ne([1].iter()), false);
3778 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3779 /// ```
3780 #[stable(feature = "iter_order", since = "1.5.0")]
3781 fn ne<I>(self, other: I) -> bool
3782 where
3783 I: IntoIterator,
3784 Self::Item: PartialEq<I::Item>,
3785 Self: Sized,
3786 {
3787 !self.eq(other)
3788 }
3789
3790 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3791 /// less than those of another.
3792 ///
3793 /// # Examples
3794 ///
3795 /// ```
3796 /// assert_eq!([1].iter().lt([1].iter()), false);
3797 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3798 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3799 /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3800 /// ```
3801 #[stable(feature = "iter_order", since = "1.5.0")]
3802 fn lt<I>(self, other: I) -> bool
3803 where
3804 I: IntoIterator,
3805 Self::Item: PartialOrd<I::Item>,
3806 Self: Sized,
3807 {
3808 self.partial_cmp(other) == Some(Ordering::Less)
3809 }
3810
3811 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3812 /// less or equal to those of another.
3813 ///
3814 /// # Examples
3815 ///
3816 /// ```
3817 /// assert_eq!([1].iter().le([1].iter()), true);
3818 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3819 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3820 /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3821 /// ```
3822 #[stable(feature = "iter_order", since = "1.5.0")]
3823 fn le<I>(self, other: I) -> bool
3824 where
3825 I: IntoIterator,
3826 Self::Item: PartialOrd<I::Item>,
3827 Self: Sized,
3828 {
3829 matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3830 }
3831
3832 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3833 /// greater than those of another.
3834 ///
3835 /// # Examples
3836 ///
3837 /// ```
3838 /// assert_eq!([1].iter().gt([1].iter()), false);
3839 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3840 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3841 /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3842 /// ```
3843 #[stable(feature = "iter_order", since = "1.5.0")]
3844 fn gt<I>(self, other: I) -> bool
3845 where
3846 I: IntoIterator,
3847 Self::Item: PartialOrd<I::Item>,
3848 Self: Sized,
3849 {
3850 self.partial_cmp(other) == Some(Ordering::Greater)
3851 }
3852
3853 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3854 /// greater than or equal to those of another.
3855 ///
3856 /// # Examples
3857 ///
3858 /// ```
3859 /// assert_eq!([1].iter().ge([1].iter()), true);
3860 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3861 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3862 /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3863 /// ```
3864 #[stable(feature = "iter_order", since = "1.5.0")]
3865 fn ge<I>(self, other: I) -> bool
3866 where
3867 I: IntoIterator,
3868 Self::Item: PartialOrd<I::Item>,
3869 Self: Sized,
3870 {
3871 matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3872 }
3873
3874 /// Checks if the elements of this iterator are sorted.
3875 ///
3876 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3877 /// iterator yields exactly zero or one element, `true` is returned.
3878 ///
3879 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3880 /// implies that this function returns `false` if any two consecutive items are not
3881 /// comparable.
3882 ///
3883 /// # Examples
3884 ///
3885 /// ```
3886 /// assert!([1, 2, 2, 9].iter().is_sorted());
3887 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3888 /// assert!([0].iter().is_sorted());
3889 /// assert!(std::iter::empty::<i32>().is_sorted());
3890 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3891 /// ```
3892 #[inline]
3893 #[stable(feature = "is_sorted", since = "1.82.0")]
3894 fn is_sorted(self) -> bool
3895 where
3896 Self: Sized,
3897 Self::Item: PartialOrd,
3898 {
3899 self.is_sorted_by(|a, b| a <= b)
3900 }
3901
3902 /// Checks if the elements of this iterator are sorted using the given comparator function.
3903 ///
3904 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3905 /// function to determine whether two elements are to be considered in sorted order.
3906 ///
3907 /// # Examples
3908 ///
3909 /// ```
3910 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a <= b));
3911 /// assert!(![1, 2, 2, 9].iter().is_sorted_by(|a, b| a < b));
3912 ///
3913 /// assert!([0].iter().is_sorted_by(|a, b| true));
3914 /// assert!([0].iter().is_sorted_by(|a, b| false));
3915 ///
3916 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| false));
3917 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| true));
3918 /// ```
3919 #[stable(feature = "is_sorted", since = "1.82.0")]
3920 fn is_sorted_by<F>(mut self, compare: F) -> bool
3921 where
3922 Self: Sized,
3923 F: FnMut(&Self::Item, &Self::Item) -> bool,
3924 {
3925 #[inline]
3926 fn check<'a, T>(
3927 last: &'a mut T,
3928 mut compare: impl FnMut(&T, &T) -> bool + 'a,
3929 ) -> impl FnMut(T) -> bool + 'a {
3930 move |curr| {
3931 if !compare(&last, &curr) {
3932 return false;
3933 }
3934 *last = curr;
3935 true
3936 }
3937 }
3938
3939 let mut last = match self.next() {
3940 Some(e) => e,
3941 None => return true,
3942 };
3943
3944 self.all(check(&mut last, compare))
3945 }
3946
3947 /// Checks if the elements of this iterator are sorted using the given key extraction
3948 /// function.
3949 ///
3950 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3951 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3952 /// its documentation for more information.
3953 ///
3954 /// [`is_sorted`]: Iterator::is_sorted
3955 ///
3956 /// # Examples
3957 ///
3958 /// ```
3959 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3960 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3961 /// ```
3962 #[inline]
3963 #[stable(feature = "is_sorted", since = "1.82.0")]
3964 fn is_sorted_by_key<F, K>(self, f: F) -> bool
3965 where
3966 Self: Sized,
3967 F: FnMut(Self::Item) -> K,
3968 K: PartialOrd,
3969 {
3970 self.map(f).is_sorted()
3971 }
3972
3973 /// See [TrustedRandomAccess][super::super::TrustedRandomAccess]
3974 // The unusual name is to avoid name collisions in method resolution
3975 // see #76479.
3976 #[inline]
3977 #[doc(hidden)]
3978 #[unstable(feature = "trusted_random_access", issue = "none")]
3979 unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
3980 where
3981 Self: TrustedRandomAccessNoCoerce,
3982 {
3983 unreachable!("Always specialized");
3984 }
3985}
3986
3987/// Compares two iterators element-wise using the given function.
3988///
3989/// If `ControlFlow::Continue(())` is returned from the function, the comparison moves on to the next
3990/// elements of both iterators. Returning `ControlFlow::Break(x)` short-circuits the iteration and
3991/// returns `ControlFlow::Break(x)`. If one of the iterators runs out of elements,
3992/// `ControlFlow::Continue(ord)` is returned where `ord` is the result of comparing the lengths of
3993/// the iterators.
3994///
3995/// Isolates the logic shared by ['cmp_by'](Iterator::cmp_by),
3996/// ['partial_cmp_by'](Iterator::partial_cmp_by), and ['eq_by'](Iterator::eq_by).
3997#[inline]
3998fn iter_compare<A, B, F, T>(mut a: A, mut b: B, f: F) -> ControlFlow<T, Ordering>
3999where
4000 A: Iterator,
4001 B: Iterator,
4002 F: FnMut(A::Item, B::Item) -> ControlFlow<T>,
4003{
4004 #[inline]
4005 fn compare<'a, B, X, T>(
4006 b: &'a mut B,
4007 mut f: impl FnMut(X, B::Item) -> ControlFlow<T> + 'a,
4008 ) -> impl FnMut(X) -> ControlFlow<ControlFlow<T, Ordering>> + 'a
4009 where
4010 B: Iterator,
4011 {
4012 move |x| match b.next() {
4013 None => ControlFlow::Break(ControlFlow::Continue(Ordering::Greater)),
4014 Some(y) => f(x, y).map_break(ControlFlow::Break),
4015 }
4016 }
4017
4018 match a.try_for_each(compare(&mut b, f)) {
4019 ControlFlow::Continue(()) => ControlFlow::Continue(match b.next() {
4020 None => Ordering::Equal,
4021 Some(_) => Ordering::Less,
4022 }),
4023 ControlFlow::Break(x) => x,
4024 }
4025}
4026
4027#[stable(feature = "rust1", since = "1.0.0")]
4028impl<I: Iterator + ?Sized> Iterator for &mut I {
4029 type Item = I::Item;
4030 #[inline]
4031 fn next(&mut self) -> Option<I::Item> {
4032 (**self).next()
4033 }
4034 fn size_hint(&self) -> (usize, Option<usize>) {
4035 (**self).size_hint()
4036 }
4037 fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
4038 (**self).advance_by(n)
4039 }
4040 fn nth(&mut self, n: usize) -> Option<Self::Item> {
4041 (**self).nth(n)
4042 }
4043 fn fold<B, F>(self, init: B, f: F) -> B
4044 where
4045 F: FnMut(B, Self::Item) -> B,
4046 {
4047 self.spec_fold(init, f)
4048 }
4049 fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
4050 where
4051 F: FnMut(B, Self::Item) -> R,
4052 R: Try<Output = B>,
4053 {
4054 self.spec_try_fold(init, f)
4055 }
4056}
4057
4058/// Helper trait to specialize `fold` and `try_fold` for `&mut I where I: Sized`
4059trait IteratorRefSpec: Iterator {
4060 fn spec_fold<B, F>(self, init: B, f: F) -> B
4061 where
4062 F: FnMut(B, Self::Item) -> B;
4063
4064 fn spec_try_fold<B, F, R>(&mut self, init: B, f: F) -> R
4065 where
4066 F: FnMut(B, Self::Item) -> R,
4067 R: Try<Output = B>;
4068}
4069
4070impl<I: Iterator + ?Sized> IteratorRefSpec for &mut I {
4071 default fn spec_fold<B, F>(self, init: B, mut f: F) -> B
4072 where
4073 F: FnMut(B, Self::Item) -> B,
4074 {
4075 let mut accum = init;
4076 while let Some(x) = self.next() {
4077 accum = f(accum, x);
4078 }
4079 accum
4080 }
4081
4082 default fn spec_try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
4083 where
4084 F: FnMut(B, Self::Item) -> R,
4085 R: Try<Output = B>,
4086 {
4087 let mut accum = init;
4088 while let Some(x) = self.next() {
4089 accum = f(accum, x)?;
4090 }
4091 try { accum }
4092 }
4093}
4094
4095impl<I: Iterator> IteratorRefSpec for &mut I {
4096 impl_fold_via_try_fold! { spec_fold -> spec_try_fold }
4097
4098 fn spec_try_fold<B, F, R>(&mut self, init: B, f: F) -> R
4099 where
4100 F: FnMut(B, Self::Item) -> R,
4101 R: Try<Output = B>,
4102 {
4103 (**self).try_fold(init, f)
4104 }
4105}