1.0.0[][src]Trait core::iter::DoubleEndedIterator

pub trait DoubleEndedIterator: Iterator {
    fn next_back(&mut self) -> Option<Self::Item>;

    fn nth_back(&mut self, n: usize) -> Option<Self::Item> { ... }
fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Ok = B>
, { ... }
fn rfold<B, F>(self, accum: B, f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B
, { ... }
fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        Self: Sized,
        P: FnMut(&Self::Item) -> bool
, { ... } }

An iterator able to yield elements from both ends.

Something that implements DoubleEndedIterator has one extra capability over something that implements Iterator: the ability to also take Items from the back, as well as the front.

It is important to note that both back and forth work on the same range, and do not cross: iteration is over when they meet in the middle.

In a similar fashion to the Iterator protocol, once a DoubleEndedIterator returns None from a next_back(), calling it again may or may not ever return Some again. next() and next_back() are interchangeable for this purpose.

Examples

Basic usage:

let numbers = vec![1, 2, 3, 4, 5, 6];

let mut iter = numbers.iter();

assert_eq!(Some(&1), iter.next());
assert_eq!(Some(&6), iter.next_back());
assert_eq!(Some(&5), iter.next_back());
assert_eq!(Some(&2), iter.next());
assert_eq!(Some(&3), iter.next());
assert_eq!(Some(&4), iter.next());
assert_eq!(None, iter.next());
assert_eq!(None, iter.next_back());Run

Required methods

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

Removes and returns an element from the end of the iterator.

Returns None when there are no more elements.

The trait-level docs contain more details.

Examples

Basic usage:

let numbers = vec![1, 2, 3, 4, 5, 6];

let mut iter = numbers.iter();

assert_eq!(Some(&1), iter.next());
assert_eq!(Some(&6), iter.next_back());
assert_eq!(Some(&5), iter.next_back());
assert_eq!(Some(&2), iter.next());
assert_eq!(Some(&3), iter.next());
assert_eq!(Some(&4), iter.next());
assert_eq!(None, iter.next());
assert_eq!(None, iter.next_back());Run
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Provided methods

fn nth_back(&mut self, n: usize) -> Option<Self::Item>1.37.0

Returns the nth element from the end of the iterator.

This is essentially the reversed version of nth. Although like most indexing operations, the count starts from zero, so nth_back(0) returns the first value fro the end, nth_back(1) the second, and so on.

Note that all elements between the end and the returned element will be consumed, including the returned element. This also means that calling nth_back(0) multiple times on the same iterator will return different elements.

nth_back() will return None if n is greater than or equal to the length of the iterator.

Examples

Basic usage:

let a = [1, 2, 3];
assert_eq!(a.iter().nth_back(2), Some(&1));Run

Calling nth_back() multiple times doesn't rewind the iterator:

let a = [1, 2, 3];

let mut iter = a.iter();

assert_eq!(iter.nth_back(1), Some(&2));
assert_eq!(iter.nth_back(1), None);Run

Returning None if there are less than n + 1 elements:

let a = [1, 2, 3];
assert_eq!(a.iter().nth_back(10), None);Run

fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R where
    Self: Sized,
    F: FnMut(B, Self::Item) -> R,
    R: Try<Ok = B>, 
1.27.0

This is the reverse version of try_fold(): it takes elements starting from the back of the iterator.

Examples

Basic usage:

let a = ["1", "2", "3"];
let sum = a.iter()
    .map(|&s| s.parse::<i32>())
    .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
assert_eq!(sum, Ok(6));Run

Short-circuiting:

let a = ["1", "rust", "3"];
let mut it = a.iter();
let sum = it
    .by_ref()
    .map(|&s| s.parse::<i32>())
    .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
assert!(sum.is_err());

// Because it short-circuited, the remaining elements are still
// available through the iterator.
assert_eq!(it.next_back(), Some(&"1"));Run

fn rfold<B, F>(self, accum: B, f: F) -> B where
    Self: Sized,
    F: FnMut(B, Self::Item) -> B, 
1.27.0

An iterator method that reduces the iterator's elements to a single, final value, starting from the back.

This is the reverse version of fold(): it takes elements starting from the back of the iterator.

rfold() takes two arguments: an initial value, and a closure with two arguments: an 'accumulator', and an element. The closure returns the value that the accumulator should have for the next iteration.

The initial value is the value the accumulator will have on the first call.

After applying this closure to every element of the iterator, rfold() returns the accumulator.

This operation is sometimes called 'reduce' or 'inject'.

Folding is useful whenever you have a collection of something, and want to produce a single value from it.

Examples

Basic usage:

let a = [1, 2, 3];

// the sum of all of the elements of a
let sum = a.iter()
           .rfold(0, |acc, &x| acc + x);

assert_eq!(sum, 6);Run

This example builds a string, starting with an initial value and continuing with each element from the back until the front:

let numbers = [1, 2, 3, 4, 5];

let zero = "0".to_string();

let result = numbers.iter().rfold(zero, |acc, &x| {
    format!("({} + {})", x, acc)
});

assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))");Run

fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item> where
    Self: Sized,
    P: FnMut(&Self::Item) -> bool, 
1.27.0

Searches for an element of an iterator from the back that satisfies a predicate.

rfind() takes a closure that returns true or false. It applies this closure to each element of the iterator, starting at the end, and if any of them return true, then rfind() returns Some(element). If they all return false, it returns None.

rfind() is short-circuiting; in other words, it will stop processing as soon as the closure returns true.

Because rfind() takes a reference, and many iterators iterate over references, this leads to a possibly confusing situation where the argument is a double reference. You can see this effect in the examples below, with &&x.

Examples

Basic usage:

let a = [1, 2, 3];

assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));

assert_eq!(a.iter().rfind(|&&x| x == 5), None);Run

Stopping at the first true:

let a = [1, 2, 3];

let mut iter = a.iter();

assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));

// we can still use `iter`, as there are more elements.
assert_eq!(iter.next_back(), Some(&1));Run
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Implementors

impl DoubleEndedIterator for EscapeDefault[src]

impl<'_> DoubleEndedIterator for Bytes<'_>[src]

impl<'a> DoubleEndedIterator for CharIndices<'a>[src]

impl<'a> DoubleEndedIterator for Chars<'a>[src]

impl<'a> DoubleEndedIterator for Lines<'a>[src]

impl<'a> DoubleEndedIterator for LinesAny<'a>[src]

impl<'a> DoubleEndedIterator for SplitAsciiWhitespace<'a>[src]

impl<'a> DoubleEndedIterator for SplitWhitespace<'a>[src]

impl<'a, A> DoubleEndedIterator for core::option::Iter<'a, A>[src]

impl<'a, A> DoubleEndedIterator for core::option::IterMut<'a, A>[src]

impl<'a, I, T: 'a> DoubleEndedIterator for Cloned<I> where
    I: DoubleEndedIterator<Item = &'a T>,
    T: Clone
[src]

impl<'a, I, T: 'a> DoubleEndedIterator for Copied<I> where
    I: DoubleEndedIterator<Item = &'a T>,
    T: Copy
[src]

impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I[src]

impl<'a, P> DoubleEndedIterator for MatchIndices<'a, P> where
    P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>, 
[src]

impl<'a, P> DoubleEndedIterator for Matches<'a, P> where
    P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>, 
[src]

impl<'a, P> DoubleEndedIterator for RMatchIndices<'a, P> where
    P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>, 
[src]

impl<'a, P> DoubleEndedIterator for RMatches<'a, P> where
    P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>, 
[src]

impl<'a, P> DoubleEndedIterator for core::str::RSplit<'a, P> where
    P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>, 
[src]

impl<'a, P> DoubleEndedIterator for RSplitTerminator<'a, P> where
    P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>, 
[src]

impl<'a, P> DoubleEndedIterator for core::str::Split<'a, P> where
    P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>, 
[src]

impl<'a, P> DoubleEndedIterator for SplitTerminator<'a, P> where
    P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>, 
[src]

impl<'a, T> DoubleEndedIterator for core::result::Iter<'a, T>[src]

impl<'a, T> DoubleEndedIterator for core::result::IterMut<'a, T>[src]

impl<'a, T> DoubleEndedIterator for Chunks<'a, T>[src]

impl<'a, T> DoubleEndedIterator for ChunksExact<'a, T>[src]

impl<'a, T> DoubleEndedIterator for ChunksExactMut<'a, T>[src]

impl<'a, T> DoubleEndedIterator for ChunksMut<'a, T>[src]

impl<'a, T> DoubleEndedIterator for core::slice::Iter<'a, T>[src]

impl<'a, T> DoubleEndedIterator for core::slice::IterMut<'a, T>[src]

impl<'a, T> DoubleEndedIterator for RChunks<'a, T>[src]

impl<'a, T> DoubleEndedIterator for RChunksExact<'a, T>[src]

impl<'a, T> DoubleEndedIterator for RChunksExactMut<'a, T>[src]

impl<'a, T> DoubleEndedIterator for RChunksMut<'a, T>[src]

impl<'a, T> DoubleEndedIterator for Windows<'a, T>[src]

impl<'a, T, P> DoubleEndedIterator for core::slice::RSplit<'a, T, P> where
    P: FnMut(&T) -> bool, 
[src]

impl<'a, T, P> DoubleEndedIterator for RSplitMut<'a, T, P> where
    P: FnMut(&T) -> bool, 
[src]

impl<'a, T, P> DoubleEndedIterator for core::slice::Split<'a, T, P> where
    P: FnMut(&T) -> bool, 
[src]

impl<'a, T, P> DoubleEndedIterator for SplitMut<'a, T, P> where
    P: FnMut(&T) -> bool, 
[src]

impl<A> DoubleEndedIterator for core::option::IntoIter<A>[src]

impl<A, B> DoubleEndedIterator for Chain<A, B> where
    A: DoubleEndedIterator,
    B: DoubleEndedIterator<Item = A::Item>, 
[src]

impl<A, B> DoubleEndedIterator for Zip<A, B> where
    A: DoubleEndedIterator + ExactSizeIterator,
    B: DoubleEndedIterator + ExactSizeIterator
[src]

impl<A, F: FnOnce() -> A> DoubleEndedIterator for OnceWith<F>[src]

impl<A: Clone> DoubleEndedIterator for Repeat<A>[src]

impl<A: Step> DoubleEndedIterator for Range<A>[src]

impl<A: Step> DoubleEndedIterator for RangeInclusive<A>[src]

impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for FilterMap<I, F> where
    F: FnMut(I::Item) -> Option<B>, 
[src]

impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for Map<I, F> where
    F: FnMut(I::Item) -> B, 
[src]

impl<I> DoubleEndedIterator for Enumerate<I> where
    I: ExactSizeIterator + DoubleEndedIterator
[src]

impl<I> DoubleEndedIterator for Fuse<I> where
    I: DoubleEndedIterator + FusedIterator
[src]

impl<I> DoubleEndedIterator for Fuse<I> where
    I: DoubleEndedIterator
[src]

impl<I> DoubleEndedIterator for Peekable<I> where
    I: DoubleEndedIterator
[src]

impl<I> DoubleEndedIterator for Rev<I> where
    I: DoubleEndedIterator
[src]

impl<I> DoubleEndedIterator for Skip<I> where
    I: DoubleEndedIterator + ExactSizeIterator
[src]

impl<I> DoubleEndedIterator for StepBy<I> where
    I: DoubleEndedIterator + ExactSizeIterator
[src]

impl<I> DoubleEndedIterator for Take<I> where
    I: DoubleEndedIterator + ExactSizeIterator
[src]

impl<I, U> DoubleEndedIterator for Flatten<I> where
    I: DoubleEndedIterator<Item: IntoIterator<IntoIter = U, Item = U::Item>>,
    U: DoubleEndedIterator
[src]

impl<I: DoubleEndedIterator, F> DoubleEndedIterator for Inspect<I, F> where
    F: FnMut(&I::Item), 
[src]

impl<I: DoubleEndedIterator, P> DoubleEndedIterator for Filter<I, P> where
    P: FnMut(&I::Item) -> bool, 
[src]

impl<I: DoubleEndedIterator, U, F> DoubleEndedIterator for FlatMap<I, U, F> where
    F: FnMut(I::Item) -> U,
    U: IntoIterator,
    U::IntoIter: DoubleEndedIterator
[src]

impl<T> DoubleEndedIterator for Empty<T>[src]

impl<T> DoubleEndedIterator for Once<T>[src]

impl<T> DoubleEndedIterator for core::result::IntoIter<T>[src]

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