Struct core::iter::Peekable

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

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

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

The peek() method will return the value that a call to next() would return, but does not advance the iterator. Like next(), if there is a value, it's wrapped in a Some(T), but if the iterator is over, it will return None.

Because peek() returns reference, and many iterators iterate over references, this leads to a possibly confusing situation where the return value is a double reference. You can see this effect in the examples below, with &&i32.

Examples

Basic usage:

fn main() { 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)); // we can peek() multiple times, the iterator won't advance 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); }

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));

// we can peek() multiple times, the iterator won't advance
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);

fn is_empty(&mut self) -> bool

Unstable (peekable_is_empty #32111)

Checks if the iterator has finished iterating.

Returns true if there are no more elements in the iterator, and false if there are.

Examples

Basic usage:

#![feature(peekable_is_empty)] fn main() { let xs = [1, 2, 3]; let mut iter = xs.iter().peekable(); // there are still elements to iterate over assert_eq!(iter.is_empty(), false); // let's consume the iterator iter.next(); iter.next(); iter.next(); assert_eq!(iter.is_empty(), true); }

#![feature(peekable_is_empty)]

let xs = [1, 2, 3];

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

// there are still elements to iterate over
assert_eq!(iter.is_empty(), false);

// let's consume the iterator
iter.next();
iter.next();
iter.next();

assert_eq!(iter.is_empty(), true);

Trait Implementations

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>

Consumes the n first elements of the iterator, then returns the next() one.

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 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.

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.

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 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.

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.

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

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

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.

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.

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.

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

Creates an iterator that skips the first n elements.

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

Creates an iterator that yields its first n elements.

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.

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.

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

Creates an iterator which ends after the first None.

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.

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 fold<B, F>(self, init: B, f: F) -> B where Self: Sized, F: FnMut(B, Self::Item) -> B

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

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 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 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 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.

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.

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

Repeats an iterator endlessly.

fn sum<S>(self) -> S where S: Add<Self::Item, Output=S> + Zero, Self: Sized

Sums the elements of an iterator.

fn product<P>(self) -> P where P: Mul<Self::Item, Output=P> + One, Self: Sized

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.

Derived Implementations

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

fn fmt(&self, __arg_0: &mut Formatter) -> Result

Formats the value using the given formatter.

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

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

Returns a copy of the value.

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

Performs copy-assignment from source.