1.0.0[−][src]Trait core::iter::ExactSizeIterator
An iterator that knows its exact length.
Many Iterator
s don't know how many times they will iterate, but some do.
If an iterator knows how many times it can iterate, providing access to
that information can be useful. For example, if you want to iterate
backwards, a good start is to know where the end is.
When implementing an ExactSizeIterator
, you must also implement
Iterator
. When doing so, the implementation of Iterator::size_hint
must return the exact size of the iterator.
The len
method has a default implementation, so you usually shouldn't
implement it. However, you may be able to provide a more performant
implementation than the default, so overriding it in this case makes sense.
Examples
Basic usage:
// a finite range knows exactly how many times it will iterate let five = 0..5; assert_eq!(5, five.len());Run
In the module-level docs, we implemented an Iterator
, Counter
.
Let's implement ExactSizeIterator
for it as well:
impl ExactSizeIterator for Counter { // We can easily calculate the remaining number of iterations. fn len(&self) -> usize { 5 - self.count } } // And now we can use it! let counter = Counter::new(); assert_eq!(5, counter.len());Run
Provided methods
fn len(&self) -> usize
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Returns the exact length of the iterator.
The implementation ensures that the iterator will return exactly len()
more times a Some(T)
value, before returning None
.
This method has a default implementation, so you usually should not
implement it directly. However, if you can provide a more efficient
implementation, you can do so. See the trait-level docs for an
example.
This function has the same safety guarantees as the
Iterator::size_hint
function.
Examples
Basic usage:
// a finite range knows exactly how many times it will iterate let five = 0..5; assert_eq!(5, five.len());Run
fn is_empty(&self) -> bool
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Returns true
if the iterator is empty.
This method has a default implementation using
ExactSizeIterator::len()
, so you don't need to implement it yourself.
Examples
Basic usage:
#![feature(exact_size_is_empty)] let mut one_element = std::iter::once(0); assert!(!one_element.is_empty()); assert_eq!(one_element.next(), Some(0)); assert!(one_element.is_empty()); assert_eq!(one_element.next(), None);Run
Implementors
impl ExactSizeIterator for core::ascii::EscapeDefault
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impl ExactSizeIterator for EscapeDebug
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impl ExactSizeIterator for core::char::EscapeDefault
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impl ExactSizeIterator for EscapeUnicode
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impl ExactSizeIterator for ToLowercase
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impl ExactSizeIterator for ToUppercase
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impl ExactSizeIterator for Range<i8>
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impl ExactSizeIterator for Range<i16>
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impl ExactSizeIterator for Range<i32>
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impl ExactSizeIterator for Range<isize>
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impl ExactSizeIterator for Range<u8>
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impl ExactSizeIterator for Range<u16>
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impl ExactSizeIterator for Range<u32>
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impl ExactSizeIterator for Range<usize>
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impl ExactSizeIterator for RangeInclusive<i8>
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impl ExactSizeIterator for RangeInclusive<i16>
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impl ExactSizeIterator for RangeInclusive<u8>
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impl ExactSizeIterator for RangeInclusive<u16>
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impl ExactSizeIterator for Bytes<'_>
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impl<'a, I, T: 'a> ExactSizeIterator for Cloned<I> where
I: ExactSizeIterator<Item = &'a T>,
T: Clone,
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I: ExactSizeIterator<Item = &'a T>,
T: Clone,
impl<'a, I, T: 'a> ExactSizeIterator for Copied<I> where
I: ExactSizeIterator<Item = &'a T>,
T: Copy,
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I: ExactSizeIterator<Item = &'a T>,
T: Copy,
impl<'a, T> ExactSizeIterator for RChunksExact<'a, T>
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impl<A> ExactSizeIterator for core::option::IntoIter<A>
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impl<A> ExactSizeIterator for core::option::Iter<'_, A>
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impl<A> ExactSizeIterator for core::option::IterMut<'_, A>
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impl<A, B> ExactSizeIterator for Zip<A, B> where
A: ExactSizeIterator,
B: ExactSizeIterator,
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A: ExactSizeIterator,
B: ExactSizeIterator,
impl<A, F: FnOnce() -> A> ExactSizeIterator for OnceWith<F>
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impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F> where
F: FnMut(I::Item) -> B,
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F: FnMut(I::Item) -> B,
impl<I> ExactSizeIterator for Enumerate<I> where
I: ExactSizeIterator,
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I: ExactSizeIterator,
impl<I> ExactSizeIterator for Fuse<I> where
I: ExactSizeIterator,
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I: ExactSizeIterator,
impl<I> ExactSizeIterator for Rev<I> where
I: ExactSizeIterator + DoubleEndedIterator,
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I: ExactSizeIterator + DoubleEndedIterator,
impl<I> ExactSizeIterator for Skip<I> where
I: ExactSizeIterator,
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I: ExactSizeIterator,
impl<I> ExactSizeIterator for StepBy<I> where
I: ExactSizeIterator,
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I: ExactSizeIterator,
impl<I> ExactSizeIterator for Take<I> where
I: ExactSizeIterator,
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I: ExactSizeIterator,
impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for &mut I
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impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I>
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impl<I: ExactSizeIterator, F> ExactSizeIterator for Inspect<I, F> where
F: FnMut(&I::Item),
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F: FnMut(&I::Item),