Struct core::iter::Peekable

1.0.0 · source ·
pub struct Peekable<I: Iterator> { /* private fields */ }
Expand description

An iterator with a peek() that returns an optional reference to the next element.

This struct is created by the peekable method on Iterator. See its documentation for more.

Implementations§

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impl<I: Iterator> Peekable<I>

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pub fn peek(&mut self) -> Option<&I::Item>

Returns a reference to the next() value without advancing the iterator.

Like next, if there is a value, it is wrapped in a Some(T). But if the iteration is over, None is returned.

Because peek() returns a reference, and many iterators iterate over references, there can be a possibly confusing situation where the return value is a double reference. You can see this effect in the examples below.

Examples

Basic usage:

let xs = [1, 2, 3];

let mut iter = xs.iter().peekable();

// peek() lets us see into the future
assert_eq!(iter.peek(), Some(&&1));
assert_eq!(iter.next(), Some(&1));

assert_eq!(iter.next(), Some(&2));

// The iterator does not advance even if we `peek` multiple times
assert_eq!(iter.peek(), Some(&&3));
assert_eq!(iter.peek(), Some(&&3));

assert_eq!(iter.next(), Some(&3));

// After the iterator is finished, so is `peek()`
assert_eq!(iter.peek(), None);
assert_eq!(iter.next(), None);
Run
1.53.0 · source

pub fn peek_mut(&mut self) -> Option<&mut I::Item>

Returns a mutable reference to the next() value without advancing the iterator.

Like next, if there is a value, it is wrapped in a Some(T). But if the iteration is over, None is returned.

Because peek_mut() returns a reference, and many iterators iterate over references, there can be a possibly confusing situation where the return value is a double reference. You can see this effect in the examples below.

Examples

Basic usage:

let mut iter = [1, 2, 3].iter().peekable();

// Like with `peek()`, we can see into the future without advancing the iterator.
assert_eq!(iter.peek_mut(), Some(&mut &1));
assert_eq!(iter.peek_mut(), Some(&mut &1));
assert_eq!(iter.next(), Some(&1));

// Peek into the iterator and set the value behind the mutable reference.
if let Some(p) = iter.peek_mut() {
    assert_eq!(*p, &2);
    *p = &5;
}

// The value we put in reappears as the iterator continues.
assert_eq!(iter.collect::<Vec<_>>(), vec![&5, &3]);
Run
1.51.0 · source

pub fn next_if(&mut self, func: impl FnOnce(&I::Item) -> bool) -> Option<I::Item>

Consume and return the next value of this iterator if a condition is true.

If func returns true for the next value of this iterator, consume and return it. Otherwise, return None.

Examples

Consume a number if it’s equal to 0.

let mut iter = (0..5).peekable();
// The first item of the iterator is 0; consume it.
assert_eq!(iter.next_if(|&x| x == 0), Some(0));
// The next item returned is now 1, so `consume` will return `false`.
assert_eq!(iter.next_if(|&x| x == 0), None);
// `next_if` saves the value of the next item if it was not equal to `expected`.
assert_eq!(iter.next(), Some(1));
Run

Consume any number less than 10.

let mut iter = (1..20).peekable();
// Consume all numbers less than 10
while iter.next_if(|&x| x < 10).is_some() {}
// The next value returned will be 10
assert_eq!(iter.next(), Some(10));
Run
1.51.0 · source

pub fn next_if_eq<T>(&mut self, expected: &T) -> Option<I::Item>where
T: ?Sized,
I::Item: PartialEq<T>,

Consume and return the next item if it is equal to expected.

Example

Consume a number if it’s equal to 0.

let mut iter = (0..5).peekable();
// The first item of the iterator is 0; consume it.
assert_eq!(iter.next_if_eq(&0), Some(0));
// The next item returned is now 1, so `consume` will return `false`.
assert_eq!(iter.next_if_eq(&0), None);
// `next_if_eq` saves the value of the next item if it was not equal to `expected`.
assert_eq!(iter.next(), Some(1));
Run

Trait Implementations§

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impl<I: Clone + Iterator> Clone for Peekable<I>where
I::Item: Clone,

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fn clone(&self) -> Peekable<I>

Returns a copy of the value. Read more
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fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source. Read more
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impl<I: Debug + Iterator> Debug for Peekable<I>where
I::Item: Debug,

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fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
1.38.0 · source§

impl<I> DoubleEndedIterator for Peekable<I>where
I: DoubleEndedIterator,

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fn next_back(&mut self) -> Option<Self::Item>

Removes and returns an element from the end of the iterator. Read more
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fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> Rwhere
Self: Sized,
F: FnMut(B, Self::Item) -> R,
R: Try<Output = B>,

This is the reverse version of Iterator::try_fold(): it takes elements starting from the back of the iterator. Read more
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fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Accwhere
Fold: FnMut(Acc, Self::Item) -> Acc,

An iterator method that reduces the iterator’s elements to a single, final value, starting from the back. Read more
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fn advance_back_by(&mut self, n: usize) -> Result<(), usize>

🔬This is a nightly-only experimental API. (iter_advance_by #77404)
Advances the iterator from the back by n elements. Read more
1.37.0 · source§

fn nth_back(&mut self, n: usize) -> Option<Self::Item>

Returns the nth element from the end of the iterator. Read more
1.27.0 · source§

fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>where
Self: Sized,
P: FnMut(&Self::Item) -> bool,

Searches for an element of an iterator from the back that satisfies a predicate. Read more
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impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I>

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fn len(&self) -> usize

Returns the exact remaining length of the iterator. Read more
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fn is_empty(&self) -> bool

🔬This is a nightly-only experimental API. (exact_size_is_empty #35428)
Returns true if the iterator is empty. Read more
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impl<I: Iterator> Iterator for Peekable<I>

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type Item = <I as Iterator>::Item

The type of the elements being iterated over.
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fn next(&mut self) -> Option<I::Item>

Advances the iterator and returns the next value. Read more
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fn count(self) -> usize

Consumes the iterator, counting the number of iterations and returning it. Read more
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fn nth(&mut self, n: usize) -> Option<I::Item>

Returns the nth element of the iterator. Read more
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fn last(self) -> Option<I::Item>

Consumes the iterator, returning the last element. Read more
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fn size_hint(&self) -> (usize, Option<usize>)

Returns the bounds on the remaining length of the iterator. Read more
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fn try_fold<B, F, R>(&mut self, init: B, f: F) -> Rwhere
Self: Sized,
F: FnMut(B, Self::Item) -> R,
R: Try<Output = B>,

An iterator method that applies a function as long as it returns successfully, producing a single, final value. Read more
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fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Accwhere
Fold: FnMut(Acc, Self::Item) -> Acc,

Folds every element into an accumulator by applying an operation, returning the final result. Read more
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fn next_chunk<const N: usize>(
&mut self
) -> Result<[Self::Item; N], IntoIter<Self::Item, N>>where
Self: Sized,

🔬This is a nightly-only experimental API. (iter_next_chunk #98326)
Advances the iterator and returns an array containing the next N values. Read more
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fn advance_by(&mut self, n: usize) -> Result<(), usize>

🔬This is a nightly-only experimental API. (iter_advance_by #77404)
Advances the iterator by n elements. Read more
1.28.0 · source§

fn step_by(self, step: usize) -> StepBy<Self> where
Self: Sized,

Creates an iterator starting at the same point, but stepping by the given amount at each iteration. Read more
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fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter> where
Self: Sized,
U: IntoIterator<Item = Self::Item>,

Takes two iterators and creates a new iterator over both in sequence. Read more
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fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter> where
Self: Sized,
U: IntoIterator,

‘Zips up’ two iterators into a single iterator of pairs. Read more
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fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G> where
Self: Sized,
G: FnMut() -> Self::Item,

🔬This is a nightly-only experimental API. (iter_intersperse #79524)
Creates a new iterator which places an item generated by separator between adjacent items of the original iterator. Read more
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fn map<B, F>(self, f: F) -> Map<Self, F> where
Self: Sized,
F: FnMut(Self::Item) -> B,

Takes a closure and creates an iterator which calls that closure on each element. Read more
1.21.0 · source§

fn for_each<F>(self, f: F)where
Self: Sized,
F: FnMut(Self::Item),

Calls a closure on each element of an iterator. Read more
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fn filter<P>(self, predicate: P) -> Filter<Self, P> where
Self: Sized,
P: FnMut(&Self::Item) -> bool,

Creates an iterator which uses a closure to determine if an element should be yielded. Read more
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fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F> where
Self: Sized,
F: FnMut(Self::Item) -> Option<B>,

Creates an iterator that both filters and maps. Read more
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fn enumerate(self) -> Enumerate<Self> where
Self: Sized,

Creates an iterator which gives the current iteration count as well as the next value. Read more
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fn peekable(self) -> Peekable<Self> where
Self: Sized,

Creates an iterator which can use the peek and peek_mut methods to look at the next element of the iterator without consuming it. See their documentation for more information. Read more
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fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P> where
Self: Sized,
P: FnMut(&Self::Item) -> bool,

Creates an iterator that skips elements based on a predicate. Read more
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fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P> where
Self: Sized,
P: FnMut(&Self::Item) -> bool,

Creates an iterator that yields elements based on a predicate. Read more
1.57.0 · source§

fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P> where
Self: Sized,
P: FnMut(Self::Item) -> Option<B>,

Creates an iterator that both yields elements based on a predicate and maps. Read more
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fn skip(self, n: usize) -> Skip<Self> where
Self: Sized,

Creates an iterator that skips the first n elements. Read more
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fn take(self, n: usize) -> Take<Self> where
Self: Sized,

Creates an iterator that yields the first n elements, or fewer if the underlying iterator ends sooner. Read more
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fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F> where
Self: Sized,
F: FnMut(&mut St, Self::Item) -> Option<B>,

An iterator adapter which, like fold, holds internal state, but unlike fold, produces a new iterator. Read more
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fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F> where
Self: Sized,
U: IntoIterator,
F: FnMut(Self::Item) -> U,

Creates an iterator that works like map, but flattens nested structure. Read more
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fn fuse(self) -> Fuse<Self> where
Self: Sized,

Creates an iterator which ends after the first None. Read more
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fn inspect<F>(self, f: F) -> Inspect<Self, F> where
Self: Sized,
F: FnMut(&Self::Item),

Does something with each element of an iterator, passing the value on. Read more
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fn by_ref(&mut self) -> &mut Selfwhere
Self: Sized,

Borrows an iterator, rather than consuming it. Read more
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fn collect<B: FromIterator<Self::Item>>(self) -> Bwhere
Self: Sized,

Transforms an iterator into a collection. Read more
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fn collect_into<E: Extend<Self::Item>>(self, collection: &mut E) -> &mut Ewhere
Self: Sized,

🔬This is a nightly-only experimental API. (iter_collect_into #94780)
Collects all the items from an iterator into a collection. Read more
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fn partition<B, F>(self, f: F) -> (B, B)where
Self: Sized,
B: Default + Extend<Self::Item>,
F: FnMut(&Self::Item) -> bool,

Consumes an iterator, creating two collections from it. Read more
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fn is_partitioned<P>(self, predicate: P) -> boolwhere
Self: Sized,
P: FnMut(Self::Item) -> bool,

🔬This is a nightly-only experimental API. (iter_is_partitioned #62544)
Checks if the elements of this iterator are partitioned according to the given predicate, such that all those that return true precede all those that return false. Read more
1.27.0 · source§

fn try_for_each<F, R>(&mut self, f: F) -> Rwhere
Self: Sized,
F: FnMut(Self::Item) -> R,
R: Try<Output = ()>,

An iterator method that applies a fallible function to each item in the iterator, stopping at the first error and returning that error. Read more
1.51.0 · source§

fn reduce<F>(self, f: F) -> Option<Self::Item>where
Self: Sized,
F: FnMut(Self::Item, Self::Item) -> Self::Item,

Reduces the elements to a single one, by repeatedly applying a reducing operation. Read more
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fn try_reduce<F, R>(
&mut self,
f: F
) -> <<R as Try>::Residual as Residual<Option<R::Output>>>::TryTypewhere
Self: Sized,
F: FnMut(Self::Item, Self::Item) -> R,
R: Try<Output = Self::Item>,
R::Residual: Residual<Option<Self::Item>>,

🔬This is a nightly-only experimental API. (iterator_try_reduce #87053)
Reduces the elements to a single one by repeatedly applying a reducing operation. If the closure returns a failure, the failure is propagated back to the caller immediately. Read more
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fn all<F>(&mut self, f: F) -> boolwhere
Self: Sized,
F: FnMut(Self::Item) -> bool,

Tests if every element of the iterator matches a predicate. Read more
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fn any<F>(&mut self, f: F) -> boolwhere
Self: Sized,
F: FnMut(Self::Item) -> bool,

Tests if any element of the iterator matches a predicate. Read more
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fn find<P>(&mut self, predicate: P) -> Option<Self::Item>where
Self: Sized,
P: FnMut(&Self::Item) -> bool,

Searches for an element of an iterator that satisfies a predicate. Read more
1.30.0 · source§

fn find_map<B, F>(&mut self, f: F) -> Option<B>where
Self: Sized,
F: FnMut(Self::Item) -> Option<B>,

Applies function to the elements of iterator and returns the first non-none result. Read more
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fn try_find<F, R>(
&mut self,
f: F
) -> <<R as Try>::Residual as Residual<Option<Self::Item>>>::TryTypewhere
Self: Sized,
F: FnMut(&Self::Item) -> R,
R: Try<Output = bool>,
R::Residual: Residual<Option<Self::Item>>,

🔬This is a nightly-only experimental API. (try_find #63178)
Applies function to the elements of iterator and returns the first true result or the first error. Read more
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fn position<P>(&mut self, predicate: P) -> Option<usize>where
Self: Sized,
P: FnMut(Self::Item) -> bool,

Searches for an element in an iterator, returning its index. Read more
1.6.0 · source§

fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>where
Self: Sized,
F: FnMut(&Self::Item) -> B,

Returns the element that gives the maximum value from the specified function. Read more
1.15.0 · source§

fn max_by<F>(self, compare: F) -> Option<Self::Item>where
Self: Sized,
F: FnMut(&Self::Item, &Self::Item) -> Ordering,

Returns the element that gives the maximum value with respect to the specified comparison function. Read more
1.6.0 · source§

fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>where
Self: Sized,
F: FnMut(&Self::Item) -> B,

Returns the element that gives the minimum value from the specified function. Read more
1.15.0 · source§

fn min_by<F>(self, compare: F) -> Option<Self::Item>where
Self: Sized,
F: FnMut(&Self::Item, &Self::Item) -> Ordering,

Returns the element that gives the minimum value with respect to the specified comparison function. Read more
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fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)where
FromA: Default + Extend<A>,
FromB: Default + Extend<B>,
Self: Sized + Iterator<Item = (A, B)>,

Converts an iterator of pairs into a pair of containers. Read more
1.36.0 · source§

fn copied<'a, T>(self) -> Copied<Self> where
Self: Sized + Iterator<Item = &'a T>,
T: Copy + 'a,

Creates an iterator which copies all of its elements. Read more
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fn cloned<'a, T>(self) -> Cloned<Self> where
Self: Sized + Iterator<Item = &'a T>,
T: Clone + 'a,

Creates an iterator which clones all of its elements. Read more
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fn array_chunks<const N: usize>(self) -> ArrayChunks<Self, N> where
Self: Sized,

🔬This is a nightly-only experimental API. (iter_array_chunks #100450)
Returns an iterator over N elements of the iterator at a time. Read more
1.11.0 · source§

fn sum<S>(self) -> Swhere
Self: Sized,
S: Sum<Self::Item>,

Sums the elements of an iterator. Read more
1.11.0 · source§

fn product<P>(self) -> Pwhere
Self: Sized,
P: Product<Self::Item>,

Iterates over the entire iterator, multiplying all the elements Read more
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fn cmp_by<I, F>(self, other: I, cmp: F) -> Orderingwhere
Self: Sized,
I: IntoIterator,
F: FnMut(Self::Item, I::Item) -> Ordering,

🔬This is a nightly-only experimental API. (iter_order_by #64295)
Lexicographically compares the elements of this Iterator with those of another with respect to the specified comparison function. Read more
1.5.0 · source§

fn partial_cmp<I>(self, other: I) -> Option<Ordering>where
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,

Lexicographically compares the elements of this Iterator with those of another. Read more
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fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>where
Self: Sized,
I: IntoIterator,
F: FnMut(Self::Item, I::Item) -> Option<Ordering>,

🔬This is a nightly-only experimental API. (iter_order_by #64295)
Lexicographically compares the elements of this Iterator with those of another with respect to the specified comparison function. Read more
1.5.0 · source§

fn eq<I>(self, other: I) -> boolwhere
I: IntoIterator,
Self::Item: PartialEq<I::Item>,
Self: Sized,

Determines if the elements of this Iterator are equal to those of another. Read more
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fn eq_by<I, F>(self, other: I, eq: F) -> boolwhere
Self: Sized,
I: IntoIterator,
F: FnMut(Self::Item, I::Item) -> bool,

🔬This is a nightly-only experimental API. (iter_order_by #64295)
Determines if the elements of this Iterator are equal to those of another with respect to the specified equality function. Read more
1.5.0 · source§

fn ne<I>(self, other: I) -> boolwhere
I: IntoIterator,
Self::Item: PartialEq<I::Item>,
Self: Sized,

Determines if the elements of this Iterator are unequal to those of another. Read more
1.5.0 · source§

fn lt<I>(self, other: I) -> boolwhere
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,

Determines if the elements of this Iterator are lexicographically less than those of another. Read more
1.5.0 · source§

fn le<I>(self, other: I) -> boolwhere
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,

Determines if the elements of this Iterator are lexicographically less or equal to those of another. Read more
1.5.0 · source§

fn gt<I>(self, other: I) -> boolwhere
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,

Determines if the elements of this Iterator are lexicographically greater than those of another. Read more
1.5.0 · source§

fn ge<I>(self, other: I) -> boolwhere
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,

Determines if the elements of this Iterator are lexicographically greater than or equal to those of another. Read more
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fn is_sorted_by<F>(self, compare: F) -> boolwhere
Self: Sized,
F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,

🔬This is a nightly-only experimental API. (is_sorted #53485)
Checks if the elements of this iterator are sorted using the given comparator function. Read more
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fn is_sorted_by_key<F, K>(self, f: F) -> boolwhere
Self: Sized,
F: FnMut(Self::Item) -> K,
K: PartialOrd,

🔬This is a nightly-only experimental API. (is_sorted #53485)
Checks if the elements of this iterator are sorted using the given key extraction function. Read more
1.26.0 · source§

impl<I: FusedIterator> FusedIterator for Peekable<I>

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impl<I> TrustedLen for Peekable<I>where
I: TrustedLen,

Auto Trait Implementations§

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impl<I> RefUnwindSafe for Peekable<I>where
I: RefUnwindSafe,
<I as Iterator>::Item: RefUnwindSafe,

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impl<I> Send for Peekable<I>where
I: Send,
<I as Iterator>::Item: Send,

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impl<I> Sync for Peekable<I>where
I: Sync,
<I as Iterator>::Item: Sync,

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impl<I> Unpin for Peekable<I>where
I: Unpin,
<I as Iterator>::Item: Unpin,

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impl<I> UnwindSafe for Peekable<I>where
I: UnwindSafe,
<I as Iterator>::Item: UnwindSafe,

Blanket Implementations§

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impl<T> Any for Twhere
T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for Twhere
T: ?Sized,

const: unstable · source§

fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for Twhere
T: ?Sized,

const: unstable · source§

fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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impl<T> From<T> for T

const: unstable · source§

fn from(t: T) -> T

Returns the argument unchanged.

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impl<T, U> Into<U> for Twhere
U: From<T>,

const: unstable · source§

fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

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impl<T, U> TryFrom<U> for Twhere
U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
const: unstable · source§

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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impl<T, U> TryInto<U> for Twhere
U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
const: unstable · source§

fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.