Struct std::iter::Peekable

pub struct Peekable<I> where
    I: Iterator,  { /* private fields */ }

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

impl<I> Peekable<I> where
    I: Iterator, 

pub fn peek(&mut self) -> Option<&<I as Iterator>::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);

pub fn peek_mut(&mut self) -> Option<&mut <I as Iterator>::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]);

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

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));

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

Trait Implementations

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

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

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

Formats the value using the given formatter.

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

pub fn next_back(&mut self) -> Option<<Peekable<I> as Iterator>::Item>

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

pub fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R where
    F: FnMut(B, <Peekable<I> as Iterator>::Item) -> R,
    R: Try<Output = B>,
    Peekable<I>: Sized, 

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

pub fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc where
    Fold: FnMut(Acc, <Peekable<I> as Iterator>::Item) -> Acc, 

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

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.

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

Returns the nth element from the end of the iterator.

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

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

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

fn len(&self) -> usize

Returns the exact length of the iterator.

fn is_empty(&self) -> bool

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

Returns true if the iterator is empty.

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

type Item = <I as Iterator>::Item

The type of the elements being iterated over.

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

Advances the iterator and returns the next value.

pub fn count(self) -> usize

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

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

Returns the nth element of the iterator.

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

Consumes the iterator, returning the last element.

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

Returns the bounds on the remaining length of the iterator.

pub fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R where
    F: FnMut(B, <Peekable<I> as Iterator>::Item) -> R,
    R: Try<Output = B>,
    Peekable<I>: Sized, 

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

pub fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc where
    Fold: FnMut(Acc, <Peekable<I> as Iterator>::Item) -> Acc, 

Folds every element into an accumulator by applying an operation, returning the final result.

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.

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

Creates an iterator starting at the same point, but stepping by the given amount at each iteration.

fn chain<U>(self, other: U) -> Chain<Self, <U as IntoIterator>::IntoIter> where
    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 as IntoIterator>::IntoIter> where
    U: IntoIterator, 

‘Zips up’ two iterators into a single iterator of pairs.

fn intersperse(self, separator: Self::Item) -> Intersperse<Self> where
    Self::Item: Clone, 

🔬 This is a nightly-only experimental API. (iter_intersperse #79524)

Creates a new iterator which places a copy of separator between adjacent items of the original iterator.

fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G> where
    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.

fn map<B, F>(self, f: F) -> Map<Self, F> where
    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
    F: FnMut(Self::Item), 

Calls a closure on each element of an iterator.

fn filter<P>(self, predicate: P) -> Filter<Self, P> where
    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
    F: FnMut(Self::Item) -> Option<B>, 

Creates an iterator that both filters and maps.

fn enumerate(self) -> Enumerate<Self>

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

fn peekable(self) -> Peekable<Self>

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.

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

Creates an iterator that skips elements based on a predicate.

fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P> where
    P: FnMut(&Self::Item) -> bool, 

Creates an iterator that yields elements based on a predicate.

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

Creates an iterator that both yields elements based on a predicate and maps.

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

Creates an iterator that skips the first n elements.

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

Creates an iterator that yields the first n elements, or fewer if the underlying iterator ends sooner.

fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F> where
    F: FnMut(&mut St, Self::Item) -> Option<B>, 

An iterator adapter 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
    U: IntoIterator,
    F: FnMut(Self::Item) -> U, 

Creates an iterator that works like map, but flattens nested structure.

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

Creates an iterator that flattens nested structure.

fn fuse(self) -> Fuse<Self>

Creates an iterator which ends after the first None.

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

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

fn by_ref(&mut self) -> &mut Self

Borrows an iterator, rather than consuming it.

fn collect<B>(self) -> B where
    B: FromIterator<Self::Item>, 

Transforms an iterator into a collection.

fn partition<B, F>(self, f: F) -> (B, B) where
    B: Default + Extend<Self::Item>,
    F: FnMut(&Self::Item) -> bool, 

Consumes an iterator, creating two collections from it.

fn partition_in_place<'a, T, P>(self, predicate: P) -> usize where
    T: 'a,
    Self: DoubleEndedIterator<Item = &'a mut T>,
    P: FnMut(&T) -> bool, 

🔬 This is a nightly-only experimental API. (iter_partition_in_place #62543)

Reorders the elements of this iterator in-place according to the given predicate, such that all those that return true precede all those that return false. Returns the number of true elements found.

fn is_partitioned<P>(self, predicate: P) -> bool where
    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.

fn try_for_each<F, R>(&mut self, f: F) -> R where
    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.

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

Reduces the elements to a single one, by repeatedly applying a reducing operation.

fn try_reduce<F, R>(
    &mut self,
    f: F
) -> <<R as Try>::Residual as Residual<Option<<R as Try>::Output>>>::TryType where
    F: FnMut(Self::Item, Self::Item) -> R,
    R: Try<Output = Self::Item>,
    <R as Try>::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.

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

Tests if every element of the iterator matches a predicate.

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

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

fn try_find<F, R>(
    &mut self,
    f: F
) -> <<R as Try>::Residual as Residual<Option<Self::Item>>>::TryType where
    F: FnMut(&Self::Item) -> R,
    R: Try<Output = bool>,
    <R as Try>::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.

fn position<P>(&mut self, predicate: P) -> Option<usize> where
    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: ExactSizeIterator + DoubleEndedIterator, 

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

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

Returns the maximum element of an iterator.

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

Returns the minimum element of an iterator.

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

Returns the element that gives the minimum value with respect to the specified comparison function.

fn rev(self) -> Rev<Self> where
    Self: 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: Iterator<Item = (A, B)>, 

Converts an iterator of pairs into a pair of containers.

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

Creates an iterator which copies all of its elements.

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

Creates an iterator which clones all of its elements.

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

Repeats an iterator endlessly.

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

Sums the elements of an iterator.

fn product<P>(self) -> P where
    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, 

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

fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering where
    I: IntoIterator,
    F: FnMut(Self::Item, <I as IntoIterator>::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.

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

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

fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering> where
    I: IntoIterator,
    F: FnMut(Self::Item, <I as IntoIterator>::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.

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

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

fn eq_by<I, F>(self, other: I, eq: F) -> bool where
    I: IntoIterator,
    F: FnMut(Self::Item, <I as IntoIterator>::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.

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

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 as IntoIterator>::Item>, 

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 as IntoIterator>::Item>, 

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 as IntoIterator>::Item>, 

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 as IntoIterator>::Item>, 

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

fn is_sorted(self) -> bool where
    Self::Item: PartialOrd<Self::Item>, 

🔬 This is a nightly-only experimental API. (is_sorted #53485)

Checks if the elements of this iterator are sorted.

fn is_sorted_by<F>(self, compare: F) -> bool where
    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.

fn is_sorted_by_key<F, K>(self, f: F) -> bool where
    F: FnMut(Self::Item) -> K,
    K: PartialOrd<K>, 

🔬 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.

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

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