Struct core::iter::Peekable

#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
pub struct Peekable<I: Iterator> { /* fields omitted */ }

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.

Methods

impl<I: Iterator> Peekable<I>

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);

Trait Implementations

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

Important traits for Peekable<I>

impl<I: Iterator> Iterator for Peekable<I> type Item = I::Item;

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

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

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

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

Formats the value using the given formatter.

impl<I: Iterator> Iterator for Peekable<I>

type Item = I::Item

The type of the elements being iterated over.

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

Advances the iterator and returns the next value.

fn count(self) -> usize

Consumes the iterator, counting the number of iterations and returning it.

fn nth(&mut self, n: usize) -> Option<I::Item>

Returns the nth element of the iterator.

fn last(self) -> Option<I::Item>

Consumes the iterator, returning the last element.

fn size_hint(&self) -> (usize, Option<usize>)

Returns the bounds on the remaining length of the iterator.

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

An iterator method that applies a function as long as it returns successfully, producing a single, final value.

fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc where     Fold: FnMut(Acc, Self::Item) -> Acc,

An iterator method that applies a function, producing a single, final value.

Important traits for StepBy<I>

impl<I> Iterator for StepBy<I> where
    I: Iterator,  type Item = I::Item;

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.

Important traits for Chain<A, B>

impl<A, B> Iterator for Chain<A, B> where
    A: Iterator,
    B: Iterator<Item = A::Item>,  type Item = A::Item;

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.

Important traits for Zip<A, B>

impl<A, B> Iterator for Zip<A, B> where
    A: Iterator,
    B: Iterator,  type Item = (A::Item, B::Item);

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.

Important traits for Map<I, F>

impl<B, I: Iterator, F> Iterator for Map<I, F> where
    F: FnMut(I::Item) -> B,  type Item = B;

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.

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

Calls a closure on each element of an iterator.

Important traits for Filter<I, P>

impl<I: Iterator, P> Iterator for Filter<I, P> where
    P: FnMut(&I::Item) -> bool,  type Item = I::Item;

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.

Important traits for FilterMap<I, F>

impl<B, I: Iterator, F> Iterator for FilterMap<I, F> where
    F: FnMut(I::Item) -> Option<B>,  type Item = B;

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.

Important traits for Enumerate<I>

impl<I> Iterator for Enumerate<I> where
    I: Iterator,  type Item = (usize, <I as Iterator>::Item);

fn enumerate(self) -> Enumerate<Self> where     Self: Sized,

Creates an iterator which gives the current iteration count as well as the next value.

Important traits for Peekable<I>

impl<I: Iterator> Iterator for Peekable<I> type Item = I::Item;

fn peekable(self) -> Peekable<Self> where     Self: Sized,

Creates an iterator which can use peek to look at the next element of the iterator without consuming it.

Important traits for SkipWhile<I, P>

impl<I: Iterator, P> Iterator for SkipWhile<I, P> where
    P: FnMut(&I::Item) -> bool,  type Item = I::Item;

fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P> where     Self: Sized,     P: FnMut(&Self::Item) -> bool,

Creates an iterator that [skip]s elements based on a predicate.

Important traits for TakeWhile<I, P>

impl<I: Iterator, P> Iterator for TakeWhile<I, P> where
    P: FnMut(&I::Item) -> bool,  type Item = I::Item;

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.

Important traits for Skip<I>

impl<I> Iterator for Skip<I> where
    I: Iterator,  type Item = <I as Iterator>::Item;

fn skip(self, n: usize) -> Skip<Self> where     Self: Sized,

Creates an iterator that skips the first n elements.

Important traits for Take<I>

impl<I> Iterator for Take<I> where
    I: Iterator,  type Item = <I as Iterator>::Item;

fn take(self, n: usize) -> Take<Self> where     Self: Sized,

Creates an iterator that yields its first n elements.

Important traits for Scan<I, St, F>

impl<B, I, St, F> Iterator for Scan<I, St, F> where
    I: Iterator,
    F: FnMut(&mut St, I::Item) -> Option<B>,  type Item = B;

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 adaptor similar to [fold] that holds internal state and produces a new iterator.

Important traits for FlatMap<I, U, F>

impl<I: Iterator, U: IntoIterator, F> Iterator for FlatMap<I, U, F> where
    F: FnMut(I::Item) -> U,  type Item = U::Item;

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.

Important traits for Flatten<I>

impl<I, U> Iterator for Flatten<I> where
    I: Iterator,
    U: Iterator,
    I::Item: IntoIterator<IntoIter = U, Item = U::Item>,  type Item = U::Item;

fn flatten(self) -> Flatten<Self> where     Self: Sized,     Self::Item: IntoIterator,

🔬 This is a nightly-only experimental API. (iterator_flatten #48213)

Creates an iterator that flattens nested structure.

Important traits for Fuse<I>

impl<I> Iterator for Fuse<I> where
    I: Iterator,  type Item = <I as Iterator>::Item;impl<I> Iterator for Fuse<I> where
    I: FusedIterator, 

fn fuse(self) -> Fuse<Self> where     Self: Sized,

Creates an iterator which ends after the first [None].

Important traits for Inspect<I, F>

impl<I: Iterator, F> Iterator for Inspect<I, F> where
    F: FnMut(&I::Item),  type Item = I::Item;

fn inspect<F>(self, f: F) -> Inspect<Self, F> where     Self: Sized,     F: FnMut(&Self::Item),

Do something with each element of an iterator, passing the value on.

fn by_ref(&mut self) -> &mut Self where     Self: Sized,

Borrows an iterator, rather than consuming it.

#[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"] fn collect<B: FromIterator<Self::Item>>(self) -> B where     Self: Sized,

Transforms an iterator into a collection.

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.

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

An iterator method that applies a fallible function to each item in the iterator, stopping at the first error and returning that error.

fn all<F>(&mut self, f: F) -> bool where     Self: Sized,     F: FnMut(Self::Item) -> bool,

Tests if every element of the iterator matches a predicate.

fn any<F>(&mut self, f: F) -> bool where     Self: Sized,     F: FnMut(Self::Item) -> bool,

Tests if any element of the iterator matches a predicate.

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.

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

🔬 This is a nightly-only experimental API. (iterator_find_map #49602)

unstable new API

Applies function to the elements of iterator and returns the first non-none result.

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.

fn rposition<P>(&mut self, predicate: P) -> Option<usize> where     P: FnMut(Self::Item) -> bool,     Self: Sized + ExactSizeIterator + DoubleEndedIterator,

Searches for an element in an iterator from the right, returning its index.

fn max(self) -> Option<Self::Item> where     Self: Sized,     Self::Item: Ord,

Returns the maximum element of an iterator.

fn min(self) -> Option<Self::Item> where     Self: Sized,     Self::Item: Ord,

Returns the minimum element of an iterator.

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.

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.

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.

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.

Important traits for Rev<I>

impl<I> Iterator for Rev<I> where
    I: DoubleEndedIterator,  type Item = <I as Iterator>::Item;

fn rev(self) -> Rev<Self> where     Self: Sized + DoubleEndedIterator,

Reverses an iterator's direction.

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.

Important traits for Cloned<I>

impl<'a, I, T: 'a> Iterator for Cloned<I> where
    I: Iterator<Item = &'a T>,
    T: Clone,  type Item = T;

fn cloned<'a, T: 'a>(self) -> Cloned<Self> where     Self: Sized + Iterator<Item = &'a T>,     T: Clone,

Creates an iterator which [clone]s all of its elements.

Important traits for Cycle<I>

impl<I> Iterator for Cycle<I> where
    I: Clone + Iterator,  type Item = <I as Iterator>::Item;

fn cycle(self) -> Cycle<Self> where     Self: Sized + Clone,

Repeats an iterator endlessly.

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

Sums the elements of an iterator.

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

Iterates over the entire iterator, multiplying all the elements

fn cmp<I>(self, other: I) -> Ordering where     I: IntoIterator<Item = Self::Item>,     Self::Item: Ord,     Self: Sized,

Lexicographically compares the elements of this Iterator with those of another.

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.

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

Determines if the elements of this Iterator are equal to those of another.

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

Determines if the elements of this Iterator are unequal to those of another.

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

Determines if the elements of this Iterator are lexicographically less than those of another.

fn le<I>(self, other: I) -> bool where     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.

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

Determines if the elements of this Iterator are lexicographically greater than those of another.

fn ge<I>(self, other: I) -> bool where     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.

impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I>

fn len(&self) -> usize

Returns the exact number of times the iterator will iterate.

fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

Returns whether the iterator is empty.

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