# Struct std::iter::Map

1.0.0 · source ·
``pub struct Map<I, F> { /* private fields */ }``
Expand description

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> = [1, 2, 3].into_iter().map(|x| x + 1).rev().collect();

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

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

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

## Trait Implementations§

source§

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

source§

#### fn clone(&self) -> Map<I, F> ⓘ

Returns a copy of the value. Read more
source§

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

Performs copy-assignment from `source`. Read more
1.9.0 · source§

### impl<I, F> Debug for Map<I, F>where    I: Debug,

source§

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

Formats the value using the given formatter. Read more
source§

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

source§

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

Removes and returns an element from the end of the iterator. Read more
source§

#### fn try_rfold<Acc, G, R>(&mut self, init: Acc, g: G) -> Rwhere    Map<I, F>: Sized,    G: FnMut(Acc, <Map<I, F> as Iterator>::Item) -> R,    R: Try<Output = Acc>,

This is the reverse version of `Iterator::try_fold()`: it takes elements starting from the back of the iterator. Read more
source§

#### fn rfold<Acc, G>(self, init: Acc, g: G) -> Accwhere    G: FnMut(Acc, <Map<I, F> as Iterator>::Item) -> Acc,

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

#### 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. Read more
1.37.0 · source§

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

Returns the `n`th element from the end of the iterator. Read more
1.27.0 · source§

#### 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. Read more
source§

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

source§

#### fn len(&self) -> usize

Returns the exact remaining length of the iterator. Read more
source§

#### fn is_empty(&self) -> bool

🔬This is a nightly-only experimental API. (`exact_size_is_empty` #35428)
Returns `true` if the iterator is empty. Read more
source§

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

§

#### type Item = B

The type of the elements being iterated over.
source§

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

Advances the iterator and returns the next value. Read more
source§

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

Returns the bounds on the remaining length of the iterator. Read more
source§

#### fn try_fold<Acc, G, R>(&mut self, init: Acc, g: G) -> Rwhere    Map<I, F>: Sized,    G: FnMut(Acc, <Map<I, F> as Iterator>::Item) -> R,    R: Try<Output = Acc>,

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

#### fn fold<Acc, G>(self, init: Acc, g: G) -> Accwhere    G: FnMut(Acc, <Map<I, F> as Iterator>::Item) -> Acc,

Folds every element into an accumulator by applying an operation, returning the final result. Read more
source§

#### fn next_chunk<const N: usize>(    &mut self) -> Result<[Self::Item; N], IntoIter<Self::Item, N>>where    Self: Sized,

🔬This is a nightly-only experimental API. (`iter_next_chunk` #98326)
Advances the iterator and returns an array containing the next `N` values. Read more
source§

#### fn count(self) -> usizewhere    Self: Sized,

Consumes the iterator, counting the number of iterations and returning it. Read more
source§

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

Consumes the iterator, returning the last element. Read more
source§

#### 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. Read more
source§

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

Returns the `n`th element of the iterator. Read more
1.28.0 · source§

#### 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. Read more
source§

#### fn chain<U>(self, other: U) -> Chain<Self, <U as IntoIterator>::IntoIter> ⓘwhere    Self: Sized,    U: IntoIterator<Item = Self::Item>,

Takes two iterators and creates a new iterator over both in sequence. Read more
source§

#### fn zip<U>(self, other: U) -> Zip<Self, <U as IntoIterator>::IntoIter> ⓘwhere    Self: Sized,    U: IntoIterator,

‘Zips up’ two iterators into a single iterator of pairs. Read more
source§

#### fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G> ⓘwhere    Self: Sized,    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. Read more
source§

#### 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. Read more
1.21.0 · source§

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

Calls a closure on each element of an iterator. Read more
source§

#### 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. Read more
source§

#### 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. Read more
source§

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

Creates an iterator which gives the current iteration count as well as the next value. Read more
source§

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

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. Read more
source§

#### 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. Read more
source§

#### 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. Read more
1.57.0 · source§

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

Creates an iterator that both yields elements based on a predicate and maps. Read more
source§

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

Creates an iterator that skips the first `n` elements. Read more
source§

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

Creates an iterator that yields the first `n` elements, or fewer if the underlying iterator ends sooner. Read more
source§

#### 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 adapter which, like `fold`, holds internal state, but unlike `fold`, produces a new iterator. Read more
source§

#### 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. Read more
source§

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

Creates an iterator which ends after the first `None`. Read more
source§

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

Does something with each element of an iterator, passing the value on. Read more
source§

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

Borrows an iterator, rather than consuming it. Read more
source§

#### fn collect<B>(self) -> Bwhere    B: FromIterator<Self::Item>,    Self: Sized,

Transforms an iterator into a collection. Read more
source§

#### fn collect_into<E>(self, collection: &mut E) -> &mut Ewhere    E: Extend<Self::Item>,    Self: Sized,

🔬This is a nightly-only experimental API. (`iter_collect_into` #94780)
Collects all the items from an iterator into a collection. Read more
source§

#### 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. Read more
source§

#### fn is_partitioned<P>(self, predicate: P) -> boolwhere    Self: Sized,    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`. Read more
1.27.0 · source§

#### fn try_for_each<F, R>(&mut self, f: F) -> Rwhere    Self: Sized,    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. Read more
1.51.0 · source§

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

Reduces the elements to a single one, by repeatedly applying a reducing operation. Read more
source§

#### fn try_reduce<F, R>(    &mut self,    f: F) -> <<R as Try>::Residual as Residual<Option<<R as Try>::Output>>>::TryTypewhere    Self: Sized,    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. Read more
source§

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

Tests if every element of the iterator matches a predicate. Read more
source§

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

Tests if any element of the iterator matches a predicate. Read more
source§

#### 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. Read more
1.30.0 · source§

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

Applies function to the elements of iterator and returns the first non-none result. Read more
source§

#### fn try_find<F, R>(    &mut self,    f: F) -> <<R as Try>::Residual as Residual<Option<Self::Item>>>::TryTypewhere    Self: Sized,    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. Read more
source§

#### 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. Read more
1.6.0 · source§

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

Returns the element that gives the maximum value from the specified function. Read more
1.15.0 · source§

#### 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. Read more
1.6.0 · source§

#### fn min_by_key<B, F>(self, f: F) -> Option<Self::Item>where    B: Ord,    Self: Sized,    F: FnMut(&Self::Item) -> B,

Returns the element that gives the minimum value from the specified function. Read more
1.15.0 · source§

#### 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. Read more
source§

#### 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. Read more
1.36.0 · source§

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

Creates an iterator which copies all of its elements. Read more
source§

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

Creates an iterator which `clone`s all of its elements. Read more
source§

#### fn array_chunks<const N: usize>(self) -> ArrayChunks<Self, N> ⓘwhere    Self: Sized,

🔬This is a nightly-only experimental API. (`iter_array_chunks` #100450)
Returns an iterator over `N` elements of the iterator at a time. Read more
1.11.0 · source§

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

Sums the elements of an iterator. Read more
1.11.0 · source§

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

Iterates over the entire iterator, multiplying all the elements Read more
source§

#### fn cmp_by<I, F>(self, other: I, cmp: F) -> Orderingwhere    Self: Sized,    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. Read more
1.5.0 · source§

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

Lexicographically compares the elements of this `Iterator` with those of another. Read more
source§

#### fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>where    Self: Sized,    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. Read more
1.5.0 · source§

#### fn eq<I>(self, other: I) -> boolwhere    I: IntoIterator,    Self::Item: PartialEq<<I as IntoIterator>::Item>,    Self: Sized,

Determines if the elements of this `Iterator` are equal to those of another. Read more
source§

#### fn eq_by<I, F>(self, other: I, eq: F) -> boolwhere    Self: Sized,    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. Read more
1.5.0 · source§

#### fn ne<I>(self, other: I) -> boolwhere    I: IntoIterator,    Self::Item: PartialEq<<I as IntoIterator>::Item>,    Self: Sized,

Determines if the elements of this `Iterator` are unequal to those of another. Read more
1.5.0 · source§

#### fn lt<I>(self, other: I) -> boolwhere    I: IntoIterator,    Self::Item: PartialOrd<<I as IntoIterator>::Item>,    Self: Sized,

Determines if the elements of this `Iterator` are lexicographically less than those of another. Read more
1.5.0 · source§

#### fn le<I>(self, other: I) -> boolwhere    I: IntoIterator,    Self::Item: PartialOrd<<I as IntoIterator>::Item>,    Self: Sized,

Determines if the elements of this `Iterator` are lexicographically less or equal to those of another. Read more
1.5.0 · source§

#### fn gt<I>(self, other: I) -> boolwhere    I: IntoIterator,    Self::Item: PartialOrd<<I as IntoIterator>::Item>,    Self: Sized,

Determines if the elements of this `Iterator` are lexicographically greater than those of another. Read more
1.5.0 · source§

#### fn ge<I>(self, other: I) -> boolwhere    I: IntoIterator,    Self::Item: PartialOrd<<I as IntoIterator>::Item>,    Self: Sized,

Determines if the elements of this `Iterator` are lexicographically greater than or equal to those of another. Read more
source§

#### fn is_sorted_by<F>(self, compare: F) -> boolwhere    Self: Sized,    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. Read more
source§

#### fn is_sorted_by_key<F, K>(self, f: F) -> boolwhere    Self: Sized,    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. Read more
1.26.0 · source§

source§

§

§

§

§

§

## Blanket Implementations§

source§

### impl<T> Any for Twhere    T: 'static + ?Sized,

source§

#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
source§

### impl<T> Borrow<T> for Twhere    T: ?Sized,

const: unstable · source§

#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
source§

### impl<T> BorrowMut<T> for Twhere    T: ?Sized,

const: unstable · source§

#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
source§

### impl<T> From<T> for T

const: unstable · source§

#### fn from(t: T) -> T

Returns the argument unchanged.

source§

### impl<T, U> Into<U> for Twhere    U: From<T>,

const: unstable · source§

#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

source§

### impl<I> IntoIterator for Iwhere    I: Iterator,

§

#### type Item = <I as Iterator>::Item

The type of the elements being iterated over.
§

#### type IntoIter = I

Which kind of iterator are we turning this into?
const: unstable · source§

#### fn into_iter(self) -> I

Creates an iterator from a value. Read more
source§

### impl<T> ToOwned for Twhere    T: Clone,

§

#### type Owned = T

The resulting type after obtaining ownership.
source§

#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
source§

#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
source§

### impl<T, U> TryFrom<U> for Twhere    U: Into<T>,

§

#### type Error = Infallible

The type returned in the event of a conversion error.
const: unstable · source§

#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
source§

### impl<T, U> TryInto<U> for Twhere    U: TryFrom<T>,

§

#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
const: unstable · source§

#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.