Struct core::iter::Map

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

An iterator that maps the values of iter with f.

This struct is created by the map method on Iterator. See its documentation for more.

Notes about side effects

The map iterator implements DoubleEndedIterator, meaning that you can also map backwards:

let v: Vec<i32> = vec![1, 2, 3].into_iter().map(|x| x + 1).rev().collect();

assert_eq!(v, [4, 3, 2]);

But if your closure has state, iterating backwards may act in a way you do not expect. Let's go through an example. First, in the forward direction:

let mut c = 0;

for pair in vec!['a', 'b', 'c'].into_iter()
                               .map(|letter| { c += 1; (letter, c) }) {
    println!("{:?}", pair);
}

This will print "('a', 1), ('b', 2), ('c', 3)".

Now consider this twist where we add a call to rev. This version will print ('c', 1), ('b', 2), ('a', 3). Note that the letters are reversed, but the values of the counter still go in order. This is because map() is still being called lazily on each item, but we are popping items off the back of the vector now, instead of shifting them from the front.

let mut c = 0;

for pair in vec!['a', 'b', 'c'].into_iter()
                               .map(|letter| { c += 1; (letter, c) })
                               .rev() {
    println!("{:?}", pair);
}

Trait Implementations

impl<I: Clone, F: Clone> Clone for Map<I, F>

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 clone(&self) -> Map<I, F>

Returns a copy of the value.

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

Performs copy-assignment from source.

impl<I: Debug, F> Debug for Map<I, F>

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

Formats the value using the given formatter.

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

type Item = B

The type of the elements being iterated over.

fn next(&mut self) -> Option<B>

Advances the iterator and returns the next value.

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

Returns the bounds on the remaining length of the iterator.

fn try_fold<Acc, G, R>(&mut self, init: Acc, g: G) -> R where     Self: Sized,     G: FnMut(Acc, Self::Item) -> R,     R: Try<Ok = Acc>,

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

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

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

fn count(self) -> usize where     Self: Sized,

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

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

Consumes the iterator, returning the last element.

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

Returns the nth element of the iterator.

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,

🔬 This is a nightly-only experimental API. (iterator_step_by #27741)

unstable replacement of Range::step_by

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.

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

fn next_back(&mut self) -> Option<B>

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

fn try_rfold<Acc, G, R>(&mut self, init: Acc, g: G) -> R where     Self: Sized,     G: FnMut(Acc, Self::Item) -> R,     R: Try<Ok = Acc>,

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

fn rfold<Acc, G>(self, init: Acc, g: G) -> Acc where     G: FnMut(Acc, Self::Item) -> Acc,

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

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

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

impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F> where     F: FnMut(I::Item) -> B,

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<B, I: FusedIterator, F> FusedIterator for Map<I, F> where     F: FnMut(I::Item) -> B,

impl<B, I, F> TrustedLen for Map<I, F> where     I: TrustedLen,     F: FnMut(I::Item) -> B,