Trait std::iter::Iterator

pub trait Iterator {
    type Item;
    fn next(&mut self) -> Option<Self::Item>;

    fn size_hint(&self) -> (usize, Option<usize>) { ... }
    fn count(self) -> usize { ... }
    fn last(self) -> Option<Self::Item> { ... }
    fn advance_by(&mut self, n: usize) -> Result<(), usize> { ... }
    fn nth(&mut self, n: usize) -> Option<Self::Item> { ... }
    fn step_by(self, step: usize) -> StepBy<Self> { ... }
    fn chain<U>(self, other: U) -> Chain<Self, <U as IntoIterator>::IntoIter>
    where
        U: IntoIterator<Item = Self::Item>,
    { ... }
    fn zip<U>(self, other: U) -> Zip<Self, <U as IntoIterator>::IntoIter>
    where
        U: IntoIterator,
    { ... }
    fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
    where
        Self::Item: Clone,
    { ... }
    fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
    where
        G: FnMut() -> Self::Item,
    { ... }
    fn map<B, F>(self, f: F) -> Map<Self, F>
    where
        F: FnMut(Self::Item) -> B,
    { ... }
    fn for_each<F>(self, f: F)
    where
        F: FnMut(Self::Item),
    { ... }
    fn filter<P>(self, predicate: P) -> Filter<Self, P>
    where
        P: FnMut(&Self::Item) -> bool,
    { ... }
    fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
    where
        F: FnMut(Self::Item) -> Option<B>,
    { ... }
    fn enumerate(self) -> Enumerate<Self> { ... }
    fn peekable(self) -> Peekable<Self> { ... }
    fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
    where
        P: FnMut(&Self::Item) -> bool,
    { ... }
    fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
    where
        P: FnMut(&Self::Item) -> bool,
    { ... }
    fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
    where
        P: FnMut(Self::Item) -> Option<B>,
    { ... }
    fn skip(self, n: usize) -> Skip<Self> { ... }
    fn take(self, n: usize) -> Take<Self> { ... }
    fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
    where
        F: FnMut(&mut St, Self::Item) -> Option<B>,
    { ... }
    fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
    where
        F: FnMut(Self::Item) -> U,
        U: IntoIterator,
    { ... }
    fn flatten(self) -> Flatten<Self>
    where
        Self::Item: IntoIterator,
    { ... }
    fn fuse(self) -> Fuse<Self> { ... }
    fn inspect<F>(self, f: F) -> Inspect<Self, F>
    where
        F: FnMut(&Self::Item),
    { ... }
    fn by_ref(&mut self) -> &mut Self { ... }
    fn collect<B>(self) -> B
    where
        B: FromIterator<Self::Item>,
    { ... }
    fn partition<B, F>(self, f: F) -> (B, B)
    where
        F: FnMut(&Self::Item) -> bool,
        B: Default + Extend<Self::Item>,
    { ... }
    fn partition_in_place<'a, T, P>(self, predicate: P) -> usize
    where
        Self: DoubleEndedIterator<Item = &'a mut T>,
        T: 'a,
        P: FnMut(&T) -> bool,
    { ... }
    fn is_partitioned<P>(self, predicate: P) -> bool
    where
        P: FnMut(Self::Item) -> bool,
    { ... }
    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>,
    { ... }
    fn try_for_each<F, R>(&mut self, f: F) -> R
    where
        F: FnMut(Self::Item) -> R,
        R: Try<Output = ()>,
    { ... }
    fn fold<B, F>(self, init: B, f: F) -> B
    where
        F: FnMut(B, Self::Item) -> B,
    { ... }
    fn reduce<F>(self, f: F) -> Option<Self::Item>
    where
        F: FnMut(Self::Item, Self::Item) -> Self::Item,
    { ... }
    fn all<F>(&mut self, f: F) -> bool
    where
        F: FnMut(Self::Item) -> bool,
    { ... }
    fn any<F>(&mut self, f: F) -> bool
    where
        F: FnMut(Self::Item) -> bool,
    { ... }
    fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        P: FnMut(&Self::Item) -> bool,
    { ... }
    fn find_map<B, F>(&mut self, f: F) -> Option<B>
    where
        F: FnMut(Self::Item) -> Option<B>,
    { ... }
    fn try_find<F, R, E>(&mut self, f: F) -> Result<Option<Self::Item>, E>
    where
        F: FnMut(&Self::Item) -> R,
        R: Try<Output = bool, Residual = Result<Infallible, E>> + Try,
    { ... }
    fn position<P>(&mut self, predicate: P) -> Option<usize>
    where
        P: FnMut(Self::Item) -> bool,
    { ... }
    fn rposition<P>(&mut self, predicate: P) -> Option<usize>
    where
        Self: ExactSizeIterator + DoubleEndedIterator,
        P: FnMut(Self::Item) -> bool,
    { ... }
    fn max(self) -> Option<Self::Item>
    where
        Self::Item: Ord,
    { ... }
    fn min(self) -> Option<Self::Item>
    where
        Self::Item: Ord,
    { ... }
    fn max_by_key<B, F>(self, f: F) -> Option<Self::Item>
    where
        F: FnMut(&Self::Item) -> B,
        B: Ord,
    { ... }
    fn max_by<F>(self, compare: F) -> Option<Self::Item>
    where
        F: FnMut(&Self::Item, &Self::Item) -> Ordering,
    { ... }
    fn min_by_key<B, F>(self, f: F) -> Option<Self::Item>
    where
        F: FnMut(&Self::Item) -> B,
        B: Ord,
    { ... }
    fn min_by<F>(self, compare: F) -> Option<Self::Item>
    where
        F: FnMut(&Self::Item, &Self::Item) -> Ordering,
    { ... }
    fn rev(self) -> Rev<Self>
    where
        Self: DoubleEndedIterator,
    { ... }
    fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
    where
        Self: Iterator<Item = (A, B)>,
        FromA: Default + Extend<A>,
        FromB: Default + Extend<B>,
    { ... }
    fn copied<'a, T>(self) -> Copied<Self>
    where
        Self: Iterator<Item = &'a T>,
        T: 'a + Copy,
    { ... }
    fn cloned<'a, T>(self) -> Cloned<Self>
    where
        Self: Iterator<Item = &'a T>,
        T: 'a + Clone,
    { ... }
    fn cycle(self) -> Cycle<Self>
    where
        Self: Clone,
    { ... }
    fn sum<S>(self) -> S
    where
        S: Sum<Self::Item>,
    { ... }
    fn product<P>(self) -> P
    where
        P: Product<Self::Item>,
    { ... }
    fn cmp<I>(self, other: I) -> Ordering
    where
        I: IntoIterator<Item = Self::Item>,
        Self::Item: Ord,
    { ... }
    fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering
    where
        F: FnMut(Self::Item, <I as IntoIterator>::Item) -> Ordering,
        I: IntoIterator,
    { ... }
    fn partial_cmp<I>(self, other: I) -> Option<Ordering>
    where
        I: IntoIterator,
        Self::Item: PartialOrd<<I as IntoIterator>::Item>,
    { ... }
    fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>
    where
        F: FnMut(Self::Item, <I as IntoIterator>::Item) -> Option<Ordering>,
        I: IntoIterator,
    { ... }
    fn eq<I>(self, other: I) -> bool
    where
        I: IntoIterator,
        Self::Item: PartialEq<<I as IntoIterator>::Item>,
    { ... }
    fn eq_by<I, F>(self, other: I, eq: F) -> bool
    where
        F: FnMut(Self::Item, <I as IntoIterator>::Item) -> bool,
        I: IntoIterator,
    { ... }
    fn ne<I>(self, other: I) -> bool
    where
        I: IntoIterator,
        Self::Item: PartialEq<<I as IntoIterator>::Item>,
    { ... }
    fn lt<I>(self, other: I) -> bool
    where
        I: IntoIterator,
        Self::Item: PartialOrd<<I as IntoIterator>::Item>,
    { ... }
    fn le<I>(self, other: I) -> bool
    where
        I: IntoIterator,
        Self::Item: PartialOrd<<I as IntoIterator>::Item>,
    { ... }
    fn gt<I>(self, other: I) -> bool
    where
        I: IntoIterator,
        Self::Item: PartialOrd<<I as IntoIterator>::Item>,
    { ... }
    fn ge<I>(self, other: I) -> bool
    where
        I: IntoIterator,
        Self::Item: PartialOrd<<I as IntoIterator>::Item>,
    { ... }
    fn is_sorted(self) -> bool
    where
        Self::Item: PartialOrd<Self::Item>,
    { ... }
    fn is_sorted_by<F>(self, compare: F) -> bool
    where
        F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
    { ... }
    fn is_sorted_by_key<F, K>(self, f: F) -> bool
    where
        F: FnMut(Self::Item) -> K,
        K: PartialOrd<K>,
    { ... }
}

An interface for dealing with iterators.

This is the main iterator trait. For more about the concept of iterators generally, please see the module-level documentation. In particular, you may want to know how to implement Iterator.

Associated Types

type Item

The type of the elements being iterated over.

Required methods

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

Advances the iterator and returns the next value.

Returns None when iteration is finished. Individual iterator implementations may choose to resume iteration, and so calling next() again may or may not eventually start returning Some(Item) again at some point.

Examples

Basic usage:

let a = [1, 2, 3];

let mut iter = a.iter();

// A call to next() returns the next value...
assert_eq!(Some(&1), iter.next());
assert_eq!(Some(&2), iter.next());
assert_eq!(Some(&3), iter.next());

// ... and then None once it's over.
assert_eq!(None, iter.next());

// More calls may or may not return `None`. Here, they always will.
assert_eq!(None, iter.next());
assert_eq!(None, iter.next());

Provided methods

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

Returns the bounds on the remaining length of the iterator.

Specifically, size_hint() returns a tuple where the first element is the lower bound, and the second element is the upper bound.

The second half of the tuple that is returned is an Option<usize>. A None here means that either there is no known upper bound, or the upper bound is larger than usize.

Implementation notes

It is not enforced that an iterator implementation yields the declared number of elements. A buggy iterator may yield less than the lower bound or more than the upper bound of elements.

size_hint() is primarily intended to be used for optimizations such as reserving space for the elements of the iterator, but must not be trusted to e.g., omit bounds checks in unsafe code. An incorrect implementation of size_hint() should not lead to memory safety violations.

That said, the implementation should provide a correct estimation, because otherwise it would be a violation of the trait’s protocol.

The default implementation returns (0, None) which is correct for any iterator.

Examples

Basic usage:

let a = [1, 2, 3];
let iter = a.iter();

assert_eq!((3, Some(3)), iter.size_hint());

A more complex example:

// The even numbers in the range of zero to nine.
let iter = (0..10).filter(|x| x % 2 == 0);

// We might iterate from zero to ten times. Knowing that it's five
// exactly wouldn't be possible without executing filter().
assert_eq!((0, Some(10)), iter.size_hint());

// Let's add five more numbers with chain()
let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);

// now both bounds are increased by five
assert_eq!((5, Some(15)), iter.size_hint());

Returning None for an upper bound:

// an infinite iterator has no upper bound
// and the maximum possible lower bound
let iter = 0..;

assert_eq!((usize::MAX, None), iter.size_hint());

fn count(self) -> usize

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

This method will call next repeatedly until None is encountered, returning the number of times it saw Some. Note that next has to be called at least once even if the iterator does not have any elements.

Overflow Behavior

The method does no guarding against overflows, so counting elements of an iterator with more than usize::MAX elements either produces the wrong result or panics. If debug assertions are enabled, a panic is guaranteed.

Panics

This function might panic if the iterator has more than usize::MAX elements.

Examples

Basic usage:

let a = [1, 2, 3];
assert_eq!(a.iter().count(), 3);

let a = [1, 2, 3, 4, 5];
assert_eq!(a.iter().count(), 5);

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

Consumes the iterator, returning the last element.

This method will evaluate the iterator until it returns None. While doing so, it keeps track of the current element. After None is returned, last() will then return the last element it saw.

Examples

Basic usage:

let a = [1, 2, 3];
assert_eq!(a.iter().last(), Some(&3));

let a = [1, 2, 3, 4, 5];
assert_eq!(a.iter().last(), Some(&5));

fn advance_by(&mut self, n: usize) -> Result<(), usize>

🔬 This is a nightly-only experimental API. (iter_advance_by #77404)

recently added

Advances the iterator by n elements.

This method will eagerly skip n elements by calling next up to n times until None is encountered.

advance_by(n) will return Ok(()) if the iterator successfully advances by n elements, or Err(k) if None is encountered, where k is the number of elements the iterator is advanced by before running out of elements (i.e. the length of the iterator). Note that k is always less than n.

Calling advance_by(0) does not consume any elements and always returns Ok(()).

Examples

Basic usage:

#![feature(iter_advance_by)]

let a = [1, 2, 3, 4];
let mut iter = a.iter();

assert_eq!(iter.advance_by(2), Ok(()));
assert_eq!(iter.next(), Some(&3));
assert_eq!(iter.advance_by(0), Ok(()));
assert_eq!(iter.advance_by(100), Err(1)); // only `&4` was skipped

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

Returns the nth element of the iterator.

Like most indexing operations, the count starts from zero, so nth(0) returns the first value, nth(1) the second, and so on.

Note that all preceding elements, as well as the returned element, will be consumed from the iterator. That means that the preceding elements will be discarded, and also that calling nth(0) multiple times on the same iterator will return different elements.

nth() will return None if n is greater than or equal to the length of the iterator.

Examples

Basic usage:

let a = [1, 2, 3];
assert_eq!(a.iter().nth(1), Some(&2));

Calling nth() multiple times doesn’t rewind the iterator:

let a = [1, 2, 3];

let mut iter = a.iter();

assert_eq!(iter.nth(1), Some(&2));
assert_eq!(iter.nth(1), None);

Returning None if there are less than n + 1 elements:

let a = [1, 2, 3];
assert_eq!(a.iter().nth(10), None);

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.

Note 1: The first element of the iterator will always be returned, regardless of the step given.

Note 2: The time at which ignored elements are pulled is not fixed. StepBy behaves like the sequence self.next(), self.nth(step-1), self.nth(step-1), …, but is also free to behave like the sequence advance_n_and_return_first(&mut self, step), advance_n_and_return_first(&mut self, step), … Which way is used may change for some iterators for performance reasons. The second way will advance the iterator earlier and may consume more items.

advance_n_and_return_first is the equivalent of:

fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
where
    I: Iterator,
{
    let next = iter.next();
    if n > 1 {
        iter.nth(n - 2);
    }
    next
}

Panics

The method will panic if the given step is 0.

Examples

Basic usage:

let a = [0, 1, 2, 3, 4, 5];
let mut iter = a.iter().step_by(2);

assert_eq!(iter.next(), Some(&0));
assert_eq!(iter.next(), Some(&2));
assert_eq!(iter.next(), Some(&4));
assert_eq!(iter.next(), None);

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.

chain() will return a new iterator which will first iterate over values from the first iterator and then over values from the second iterator.

In other words, it links two iterators together, in a chain. 🔗

once is commonly used to adapt a single value into a chain of other kinds of iteration.

Examples

Basic usage:

let a1 = [1, 2, 3];
let a2 = [4, 5, 6];

let mut iter = a1.iter().chain(a2.iter());

assert_eq!(iter.next(), Some(&1));
assert_eq!(iter.next(), Some(&2));
assert_eq!(iter.next(), Some(&3));
assert_eq!(iter.next(), Some(&4));
assert_eq!(iter.next(), Some(&5));
assert_eq!(iter.next(), Some(&6));
assert_eq!(iter.next(), None);

Since the argument to chain() uses IntoIterator, we can pass anything that can be converted into an Iterator, not just an Iterator itself. For example, slices (&[T]) implement IntoIterator, and so can be passed to chain() directly:

let s1 = &[1, 2, 3];
let s2 = &[4, 5, 6];

let mut iter = s1.iter().chain(s2);

assert_eq!(iter.next(), Some(&1));
assert_eq!(iter.next(), Some(&2));
assert_eq!(iter.next(), Some(&3));
assert_eq!(iter.next(), Some(&4));
assert_eq!(iter.next(), Some(&5));
assert_eq!(iter.next(), Some(&6));
assert_eq!(iter.next(), None);

If you work with Windows API, you may wish to convert OsStr to Vec<u16>:

#[cfg(windows)]
fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
    use std::os::windows::ffi::OsStrExt;
    s.encode_wide().chain(std::iter::once(0)).collect()
}

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.

zip() returns a new iterator that will iterate over two other iterators, returning a tuple where the first element comes from the first iterator, and the second element comes from the second iterator.

In other words, it zips two iterators together, into a single one.

If either iterator returns None, next from the zipped iterator will return None. If the first iterator returns None, zip will short-circuit and next will not be called on the second iterator.

Examples

Basic usage:

let a1 = [1, 2, 3];
let a2 = [4, 5, 6];

let mut iter = a1.iter().zip(a2.iter());

assert_eq!(iter.next(), Some((&1, &4)));
assert_eq!(iter.next(), Some((&2, &5)));
assert_eq!(iter.next(), Some((&3, &6)));
assert_eq!(iter.next(), None);

Since the argument to zip() uses IntoIterator, we can pass anything that can be converted into an Iterator, not just an Iterator itself. For example, slices (&[T]) implement IntoIterator, and so can be passed to zip() directly:

let s1 = &[1, 2, 3];
let s2 = &[4, 5, 6];

let mut iter = s1.iter().zip(s2);

assert_eq!(iter.next(), Some((&1, &4)));
assert_eq!(iter.next(), Some((&2, &5)));
assert_eq!(iter.next(), Some((&3, &6)));
assert_eq!(iter.next(), None);

zip() is often used to zip an infinite iterator to a finite one. This works because the finite iterator will eventually return None, ending the zipper. Zipping with (0..) can look a lot like enumerate:

let enumerate: Vec<_> = "foo".chars().enumerate().collect();

let zipper: Vec<_> = (0..).zip("foo".chars()).collect();

assert_eq!((0, 'f'), enumerate[0]);
assert_eq!((0, 'f'), zipper[0]);

assert_eq!((1, 'o'), enumerate[1]);
assert_eq!((1, 'o'), zipper[1]);

assert_eq!((2, 'o'), enumerate[2]);
assert_eq!((2, 'o'), zipper[2]);

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

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

recently added

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

In case separator does not implement Clone or needs to be computed every time, use intersperse_with.

Examples

Basic usage:

#![feature(iter_intersperse)]

let mut a = [0, 1, 2].iter().intersperse(&100);
assert_eq!(a.next(), Some(&0));   // The first element from `a`.
assert_eq!(a.next(), Some(&100)); // The separator.
assert_eq!(a.next(), Some(&1));   // The next element from `a`.
assert_eq!(a.next(), Some(&100)); // The separator.
assert_eq!(a.next(), Some(&2));   // The last element from `a`.
assert_eq!(a.next(), None);       // The iterator is finished.

intersperse can be very useful to join an iterator’s items using a common element:

#![feature(iter_intersperse)]

let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
assert_eq!(hello, "Hello World !");

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)

recently added

Creates a new iterator which places an item generated by separator between adjacent items of the original iterator.

The closure will be called exactly once each time an item is placed between two adjacent items from the underlying iterator; specifically, the closure is not called if the underlying iterator yields less than two items and after the last item is yielded.

If the iterator’s item implements Clone, it may be easier to use intersperse.

Examples

Basic usage:

#![feature(iter_intersperse)]

#[derive(PartialEq, Debug)]
struct NotClone(usize);

let v = vec![NotClone(0), NotClone(1), NotClone(2)];
let mut it = v.into_iter().intersperse_with(|| NotClone(99));

assert_eq!(it.next(), Some(NotClone(0)));  // The first element from `v`.
assert_eq!(it.next(), Some(NotClone(99))); // The separator.
assert_eq!(it.next(), Some(NotClone(1)));  // The next element from `v`.
assert_eq!(it.next(), Some(NotClone(99))); // The separator.
assert_eq!(it.next(), Some(NotClone(2)));  // The last element from from `v`.
assert_eq!(it.next(), None);               // The iterator is finished.

intersperse_with can be used in situations where the separator needs to be computed:

#![feature(iter_intersperse)]

let src = ["Hello", "to", "all", "people", "!!"].iter().copied();

// The closure mutably borrows its context to generate an item.
let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
let separator = || happy_emojis.next().unwrap_or(" 🦀 ");

let result = src.intersperse_with(separator).collect::<String>();
assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");

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.

map() transforms one iterator into another, by means of its argument: something that implements FnMut. It produces a new iterator which calls this closure on each element of the original iterator.

If you are good at thinking in types, you can think of map() like this: If you have an iterator that gives you elements of some type A, and you want an iterator of some other type B, you can use map(), passing a closure that takes an A and returns a B.

map() is conceptually similar to a for loop. However, as map() is lazy, it is best used when you’re already working with other iterators. If you’re doing some sort of looping for a side effect, it’s considered more idiomatic to use for than map().

Examples

Basic usage:

let a = [1, 2, 3];

let mut iter = a.iter().map(|x| 2 * x);

assert_eq!(iter.next(), Some(2));
assert_eq!(iter.next(), Some(4));
assert_eq!(iter.next(), Some(6));
assert_eq!(iter.next(), None);

If you’re doing some sort of side effect, prefer for to map():

// don't do this:
(0..5).map(|x| println!("{}", x));

// it won't even execute, as it is lazy. Rust will warn you about this.

// Instead, use for:
for x in 0..5 {
    println!("{}", x);
}

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

Calls a closure on each element of an iterator.

This is equivalent to using a for loop on the iterator, although break and continue are not possible from a closure. It’s generally more idiomatic to use a for loop, but for_each may be more legible when processing items at the end of longer iterator chains. In some cases for_each may also be faster than a loop, because it will use internal iteration on adapters like Chain.

Examples

Basic usage:

use std::sync::mpsc::channel;

let (tx, rx) = channel();
(0..5).map(|x| x * 2 + 1)
      .for_each(move |x| tx.send(x).unwrap());

let v: Vec<_> =  rx.iter().collect();
assert_eq!(v, vec![1, 3, 5, 7, 9]);

For such a small example, a for loop may be cleaner, but for_each might be preferable to keep a functional style with longer iterators:

(0..5).flat_map(|x| x * 100 .. x * 110)
      .enumerate()
      .filter(|&(i, x)| (i + x) % 3 == 0)
      .for_each(|(i, x)| println!("{}:{}", i, x));

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.

Given an element the closure must return true or false. The returned iterator will yield only the elements for which the closure returns true.

Examples

Basic usage:

let a = [0i32, 1, 2];

let mut iter = a.iter().filter(|x| x.is_positive());

assert_eq!(iter.next(), Some(&1));
assert_eq!(iter.next(), Some(&2));
assert_eq!(iter.next(), None);

Because the closure passed to filter() takes a reference, and many iterators iterate over references, this leads to a possibly confusing situation, where the type of the closure is a double reference:

let a = [0, 1, 2];

let mut iter = a.iter().filter(|x| **x > 1); // need two *s!

assert_eq!(iter.next(), Some(&2));
assert_eq!(iter.next(), None);

It’s common to instead use destructuring on the argument to strip away one:

let a = [0, 1, 2];

let mut iter = a.iter().filter(|&x| *x > 1); // both & and *

assert_eq!(iter.next(), Some(&2));
assert_eq!(iter.next(), None);

or both:

let a = [0, 1, 2];

let mut iter = a.iter().filter(|&&x| x > 1); // two &s

assert_eq!(iter.next(), Some(&2));
assert_eq!(iter.next(), None);

of these layers.

Note that iter.filter(f).next() is equivalent to iter.find(f).

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.

The returned iterator yields only the values for which the supplied closure returns Some(value).

filter_map can be used to make chains of filter and map more concise. The example below shows how a map().filter().map() can be shortened to a single call to filter_map.

Examples

Basic usage:

let a = ["1", "two", "NaN", "four", "5"];

let mut iter = a.iter().filter_map(|s| s.parse().ok());

assert_eq!(iter.next(), Some(1));
assert_eq!(iter.next(), Some(5));
assert_eq!(iter.next(), None);

Here’s the same example, but with filter and map:

let a = ["1", "two", "NaN", "four", "5"];
let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
assert_eq!(iter.next(), Some(1));
assert_eq!(iter.next(), Some(5));
assert_eq!(iter.next(), None);

fn enumerate(self) -> Enumerate<Self>

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

The iterator returned yields pairs (i, val), where i is the current index of iteration and val is the value returned by the iterator.

enumerate() keeps its count as a usize. If you want to count by a different sized integer, the zip function provides similar functionality.

Overflow Behavior

The method does no guarding against overflows, so enumerating more than usize::MAX elements either produces the wrong result or panics. If debug assertions are enabled, a panic is guaranteed.

Panics

The returned iterator might panic if the to-be-returned index would overflow a usize.

Examples

let a = ['a', 'b', 'c'];

let mut iter = a.iter().enumerate();

assert_eq!(iter.next(), Some((0, &'a')));
assert_eq!(iter.next(), Some((1, &'b')));
assert_eq!(iter.next(), Some((2, &'c')));
assert_eq!(iter.next(), None);

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.

Note that the underlying iterator is still advanced when peek or peek_mut are called for the first time: In order to retrieve the next element, next is called on the underlying iterator, hence any side effects (i.e. anything other than fetching the next value) of the next method will occur.

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));

// 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);

Using peek_mut to mutate the next item without advancing the iterator:

let xs = [1, 2, 3];

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

// `peek_mut()` lets us see into the future
assert_eq!(iter.peek_mut(), Some(&mut &1));
assert_eq!(iter.peek_mut(), Some(&mut &1));
assert_eq!(iter.next(), Some(&1));

if let Some(mut p) = iter.peek_mut() {
    assert_eq!(*p, &2);
    // put a value into the iterator
    *p = &1000;
}

// The value reappears as the iterator continues
assert_eq!(iter.collect::<Vec<_>>(), vec![&1000, &3]);

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.

skip_while() takes a closure as an argument. It will call this closure on each element of the iterator, and ignore elements until it returns false.

After false is returned, skip_while()’s job is over, and the rest of the elements are yielded.

Examples

Basic usage:

let a = [-1i32, 0, 1];

let mut iter = a.iter().skip_while(|x| x.is_negative());

assert_eq!(iter.next(), Some(&0));
assert_eq!(iter.next(), Some(&1));
assert_eq!(iter.next(), None);

Because the closure passed to skip_while() takes a reference, and many iterators iterate over references, this leads to a possibly confusing situation, where the type of the closure argument is a double reference:

let a = [-1, 0, 1];

let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!

assert_eq!(iter.next(), Some(&0));
assert_eq!(iter.next(), Some(&1));
assert_eq!(iter.next(), None);

Stopping after an initial false:

let a = [-1, 0, 1, -2];

let mut iter = a.iter().skip_while(|x| **x < 0);

assert_eq!(iter.next(), Some(&0));
assert_eq!(iter.next(), Some(&1));

// while this would have been false, since we already got a false,
// skip_while() isn't used any more
assert_eq!(iter.next(), Some(&-2));

assert_eq!(iter.next(), None);

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.

take_while() takes a closure as an argument. It will call this closure on each element of the iterator, and yield elements while it returns true.

After false is returned, take_while()’s job is over, and the rest of the elements are ignored.

Examples

Basic usage:

let a = [-1i32, 0, 1];

let mut iter = a.iter().take_while(|x| x.is_negative());

assert_eq!(iter.next(), Some(&-1));
assert_eq!(iter.next(), None);

Because the closure passed to take_while() takes a reference, and many iterators iterate over references, this leads to a possibly confusing situation, where the type of the closure is a double reference:

let a = [-1, 0, 1];

let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!

assert_eq!(iter.next(), Some(&-1));
assert_eq!(iter.next(), None);

Stopping after an initial false:

let a = [-1, 0, 1, -2];

let mut iter = a.iter().take_while(|x| **x < 0);

assert_eq!(iter.next(), Some(&-1));

// We have more elements that are less than zero, but since we already
// got a false, take_while() isn't used any more
assert_eq!(iter.next(), None);

Because take_while() needs to look at the value in order to see if it should be included or not, consuming iterators will see that it is removed:

let a = [1, 2, 3, 4];
let mut iter = a.iter();

let result: Vec<i32> = iter.by_ref()
                           .take_while(|n| **n != 3)
                           .cloned()
                           .collect();

assert_eq!(result, &[1, 2]);

let result: Vec<i32> = iter.cloned().collect();

assert_eq!(result, &[4]);

The 3 is no longer there, because it was consumed in order to see if the iteration should stop, but wasn’t placed back into the iterator.

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

🔬 This is a nightly-only experimental API. (iter_map_while #68537)

recently added

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

map_while() takes a closure as an argument. It will call this closure on each element of the iterator, and yield elements while it returns Some(_).

Examples

Basic usage:

#![feature(iter_map_while)]
let a = [-1i32, 4, 0, 1];

let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));

assert_eq!(iter.next(), Some(-16));
assert_eq!(iter.next(), Some(4));
assert_eq!(iter.next(), None);

Here’s the same example, but with take_while and map:

let a = [-1i32, 4, 0, 1];

let mut iter = a.iter()
                .map(|x| 16i32.checked_div(*x))
                .take_while(|x| x.is_some())
                .map(|x| x.unwrap());

assert_eq!(iter.next(), Some(-16));
assert_eq!(iter.next(), Some(4));
assert_eq!(iter.next(), None);

Stopping after an initial None:

#![feature(iter_map_while)]
use std::convert::TryFrom;

let a = [0, 1, 2, -3, 4, 5, -6];

let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
let vec = iter.collect::<Vec<_>>();

// We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
// (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
assert_eq!(vec, vec![0, 1, 2]);

Because map_while() needs to look at the value in order to see if it should be included or not, consuming iterators will see that it is removed:

#![feature(iter_map_while)]
use std::convert::TryFrom;

let a = [1, 2, -3, 4];
let mut iter = a.iter();

let result: Vec<u32> = iter.by_ref()
                           .map_while(|n| u32::try_from(*n).ok())
                           .collect();

assert_eq!(result, &[1, 2]);

let result: Vec<i32> = iter.cloned().collect();

assert_eq!(result, &[4]);

The -3 is no longer there, because it was consumed in order to see if the iteration should stop, but wasn’t placed back into the iterator.

Note that unlike take_while this iterator is not fused. It is also not specified what this iterator returns after the first None is returned. If you need fused iterator, use fuse.

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

Creates an iterator that skips the first n elements.

skip(n) skips elements until n elements are skipped or the end of the iterator is reached (whichever happens first). After that, all the remaining elements are yielded. In particular, if the original iterator is too short, then the returned iterator is empty.

Rather than overriding this method directly, instead override the nth method.

Examples

Basic usage:

let a = [1, 2, 3];

let mut iter = a.iter().skip(2);

assert_eq!(iter.next(), Some(&3));
assert_eq!(iter.next(), None);

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

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

take(n) yields elements until n elements are yielded or the end of the iterator is reached (whichever happens first). The returned iterator is a prefix of length n if the original iterator contains at least n elements, otherwise it contains all of the (fewer than n) elements of the original iterator.

Examples

Basic usage:

let a = [1, 2, 3];

let mut iter = a.iter().take(2);

assert_eq!(iter.next(), Some(&1));
assert_eq!(iter.next(), Some(&2));
assert_eq!(iter.next(), None);

take() is often used with an infinite iterator, to make it finite:

let mut iter = (0..).take(3);

assert_eq!(iter.next(), Some(0));
assert_eq!(iter.next(), Some(1));
assert_eq!(iter.next(), Some(2));
assert_eq!(iter.next(), None);

If less than n elements are available, take will limit itself to the size of the underlying iterator:

let v = vec![1, 2];
let mut iter = v.into_iter().take(5);
assert_eq!(iter.next(), Some(1));
assert_eq!(iter.next(), Some(2));
assert_eq!(iter.next(), None);

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.

scan() takes two arguments: an initial value which seeds the internal state, and a closure with two arguments, the first being a mutable reference to the internal state and the second an iterator element. The closure can assign to the internal state to share state between iterations.

On iteration, the closure will be applied to each element of the iterator and the return value from the closure, an Option, is yielded by the iterator.

Examples

Basic usage:

let a = [1, 2, 3];

let mut iter = a.iter().scan(1, |state, &x| {
    // each iteration, we'll multiply the state by the element
    *state = *state * x;

    // then, we'll yield the negation of the state
    Some(-*state)
});

assert_eq!(iter.next(), Some(-1));
assert_eq!(iter.next(), Some(-2));
assert_eq!(iter.next(), Some(-6));
assert_eq!(iter.next(), None);

fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F> where
    F: FnMut(Self::Item) -> U,
    U: IntoIterator, 

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

The map adapter is very useful, but only when the closure argument produces values. If it produces an iterator instead, there’s an extra layer of indirection. flat_map() will remove this extra layer on its own.

You can think of flat_map(f) as the semantic equivalent of mapping, and then flattening as in map(f).flatten().

Another way of thinking about flat_map(): map’s closure returns one item for each element, and flat_map()’s closure returns an iterator for each element.

Examples

Basic usage:

let words = ["alpha", "beta", "gamma"];

// chars() returns an iterator
let merged: String = words.iter()
                          .flat_map(|s| s.chars())
                          .collect();
assert_eq!(merged, "alphabetagamma");

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

Creates an iterator that flattens nested structure.

This is useful when you have an iterator of iterators or an iterator of things that can be turned into iterators and you want to remove one level of indirection.

Examples

Basic usage:

let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);

Mapping and then flattening:

let words = ["alpha", "beta", "gamma"];

// chars() returns an iterator
let merged: String = words.iter()
                          .map(|s| s.chars())
                          .flatten()
                          .collect();
assert_eq!(merged, "alphabetagamma");

You can also rewrite this in terms of flat_map(), which is preferable in this case since it conveys intent more clearly:

let words = ["alpha", "beta", "gamma"];

// chars() returns an iterator
let merged: String = words.iter()
                          .flat_map(|s| s.chars())
                          .collect();
assert_eq!(merged, "alphabetagamma");

Flattening only removes one level of nesting at a time:

let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];

let d2 = d3.iter().flatten().collect::<Vec<_>>();
assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);

let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);

Here we see that flatten() does not perform a “deep” flatten. Instead, only one level of nesting is removed. That is, if you flatten() a three-dimensional array, the result will be two-dimensional and not one-dimensional. To get a one-dimensional structure, you have to flatten() again.

fn fuse(self) -> Fuse<Self>

Creates an iterator which ends after the first None.

After an iterator returns None, future calls may or may not yield Some(T) again. fuse() adapts an iterator, ensuring that after a None is given, it will always return None forever.

Note that the Fuse wrapper is a no-op on iterators that implement the FusedIterator trait. fuse() may therefore behave incorrectly if the FusedIterator trait is improperly implemented.

Examples

Basic usage:

// an iterator which alternates between Some and None
struct Alternate {
    state: i32,
}

impl Iterator for Alternate {
    type Item = i32;

    fn next(&mut self) -> Option<i32> {
        let val = self.state;
        self.state = self.state + 1;

        // if it's even, Some(i32), else None
        if val % 2 == 0 {
            Some(val)
        } else {
            None
        }
    }
}

let mut iter = Alternate { state: 0 };

// we can see our iterator going back and forth
assert_eq!(iter.next(), Some(0));
assert_eq!(iter.next(), None);
assert_eq!(iter.next(), Some(2));
assert_eq!(iter.next(), None);

// however, once we fuse it...
let mut iter = iter.fuse();

assert_eq!(iter.next(), Some(4));
assert_eq!(iter.next(), None);

// it will always return `None` after the first time.
assert_eq!(iter.next(), None);
assert_eq!(iter.next(), None);
assert_eq!(iter.next(), 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.

When using iterators, you’ll often chain several of them together. While working on such code, you might want to check out what’s happening at various parts in the pipeline. To do that, insert a call to inspect().

It’s more common for inspect() to be used as a debugging tool than to exist in your final code, but applications may find it useful in certain situations when errors need to be logged before being discarded.

Examples

Basic usage:

let a = [1, 4, 2, 3];

// this iterator sequence is complex.
let sum = a.iter()
    .cloned()
    .filter(|x| x % 2 == 0)
    .fold(0, |sum, i| sum + i);

println!("{}", sum);

// let's add some inspect() calls to investigate what's happening
let sum = a.iter()
    .cloned()
    .inspect(|x| println!("about to filter: {}", x))
    .filter(|x| x % 2 == 0)
    .inspect(|x| println!("made it through filter: {}", x))
    .fold(0, |sum, i| sum + i);

println!("{}", sum);

This will print:

6
about to filter: 1
about to filter: 4
made it through filter: 4
about to filter: 2
made it through filter: 2
about to filter: 3
6

Logging errors before discarding them:

let lines = ["1", "2", "a"];

let sum: i32 = lines
    .iter()
    .map(|line| line.parse::<i32>())
    .inspect(|num| {
        if let Err(ref e) = *num {
            println!("Parsing error: {}", e);
        }
    })
    .filter_map(Result::ok)
    .sum();

println!("Sum: {}", sum);

This will print:

Parsing error: invalid digit found in string
Sum: 3

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

Borrows an iterator, rather than consuming it.

This is useful to allow applying iterator adapters while still retaining ownership of the original iterator.

Examples

Basic usage:

let mut words = vec!["hello", "world", "of", "Rust"].into_iter();

// Take the first two words.
let hello_world: Vec<_> = words.by_ref().take(2).collect();
assert_eq!(hello_world, vec!["hello", "world"]);

// Collect the rest of the words.
// We can only do this because we used `by_ref` earlier.
let of_rust: Vec<_> = words.collect();
assert_eq!(of_rust, vec!["of", "Rust"]);

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

Transforms an iterator into a collection.

collect() can take anything iterable, and turn it into a relevant collection. This is one of the more powerful methods in the standard library, used in a variety of contexts.

The most basic pattern in which collect() is used is to turn one collection into another. You take a collection, call iter on it, do a bunch of transformations, and then collect() at the end.

collect() can also create instances of types that are not typical collections. For example, a String can be built from chars, and an iterator of Result<T, E> items can be collected into Result<Collection<T>, E>. See the examples below for more.

Because collect() is so general, it can cause problems with type inference. As such, collect() is one of the few times you’ll see the syntax affectionately known as the ‘turbofish’: ::<>. This helps the inference algorithm understand specifically which collection you’re trying to collect into.

Examples

Basic usage:

let a = [1, 2, 3];

let doubled: Vec<i32> = a.iter()
                         .map(|&x| x * 2)
                         .collect();

assert_eq!(vec![2, 4, 6], doubled);

Note that we needed the : Vec<i32> on the left-hand side. This is because we could collect into, for example, a VecDeque<T> instead:

use std::collections::VecDeque;

let a = [1, 2, 3];

let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();

assert_eq!(2, doubled[0]);
assert_eq!(4, doubled[1]);
assert_eq!(6, doubled[2]);

Using the ‘turbofish’ instead of annotating doubled:

let a = [1, 2, 3];

let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();

assert_eq!(vec![2, 4, 6], doubled);

Because collect() only cares about what you’re collecting into, you can still use a partial type hint, _, with the turbofish:

let a = [1, 2, 3];

let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();

assert_eq!(vec![2, 4, 6], doubled);

Using collect() to make a String:

let chars = ['g', 'd', 'k', 'k', 'n'];

let hello: String = chars.iter()
    .map(|&x| x as u8)
    .map(|x| (x + 1) as char)
    .collect();

assert_eq!("hello", hello);

If you have a list of Result<T, E>s, you can use collect() to see if any of them failed:

let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];

let result: Result<Vec<_>, &str> = results.iter().cloned().collect();

// gives us the first error
assert_eq!(Err("nope"), result);

let results = [Ok(1), Ok(3)];

let result: Result<Vec<_>, &str> = results.iter().cloned().collect();

// gives us the list of answers
assert_eq!(Ok(vec![1, 3]), result);

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

Consumes an iterator, creating two collections from it.

The predicate passed to partition() can return true, or false. partition() returns a pair, all of the elements for which it returned true, and all of the elements for which it returned false.

See also is_partitioned() and partition_in_place().

Examples

Basic usage:

let a = [1, 2, 3];

let (even, odd): (Vec<i32>, Vec<i32>) = a
    .iter()
    .partition(|&n| n % 2 == 0);

assert_eq!(even, vec![2]);
assert_eq!(odd, vec![1, 3]);

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

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

new API

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.

The relative order of partitioned items is not maintained.

Current implementation

Current algorithms tries finding the first element for which the predicate evaluates to false, and the last element for which it evaluates to true and repeatedly swaps them.

Time Complexity: O(N)

See also is_partitioned() and partition().

Examples

#![feature(iter_partition_in_place)]

let mut a = [1, 2, 3, 4, 5, 6, 7];

// Partition in-place between evens and odds
let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);

assert_eq!(i, 3);
assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds

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)

new API

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.

See also partition() and partition_in_place().

Examples

#![feature(iter_is_partitioned)]

assert!("Iterator".chars().is_partitioned(char::is_uppercase));
assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));

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

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

try_fold() takes two arguments: an initial value, and a closure with two arguments: an ‘accumulator’, and an element. The closure either returns successfully, with the value that the accumulator should have for the next iteration, or it returns failure, with an error value that is propagated back to the caller immediately (short-circuiting).

The initial value is the value the accumulator will have on the first call. If applying the closure succeeded against every element of the iterator, try_fold() returns the final accumulator as success.

Folding is useful whenever you have a collection of something, and want to produce a single value from it.

Note to Implementors

Several of the other (forward) methods have default implementations in terms of this one, so try to implement this explicitly if it can do something better than the default for loop implementation.

In particular, try to have this call try_fold() on the internal parts from which this iterator is composed. If multiple calls are needed, the ? operator may be convenient for chaining the accumulator value along, but beware any invariants that need to be upheld before those early returns. This is a &mut self method, so iteration needs to be resumable after hitting an error here.

Examples

Basic usage:

let a = [1, 2, 3];

// the checked sum of all of the elements of the array
let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));

assert_eq!(sum, Some(6));

Short-circuiting:

let a = [10, 20, 30, 100, 40, 50];
let mut it = a.iter();

// This sum overflows when adding the 100 element
let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
assert_eq!(sum, None);

// Because it short-circuited, the remaining elements are still
// available through the iterator.
assert_eq!(it.len(), 2);
assert_eq!(it.next(), Some(&40));

While you cannot break from a closure, the ControlFlow type allows a similar idea:

use std::ops::ControlFlow;

let triangular = (1..30).try_fold(0_i8, |prev, x| {
    if let Some(next) = prev.checked_add(x) {
        ControlFlow::Continue(next)
    } else {
        ControlFlow::Break(prev)
    }
});
assert_eq!(triangular, ControlFlow::Break(120));

let triangular = (1..30).try_fold(0_u64, |prev, x| {
    if let Some(next) = prev.checked_add(x) {
        ControlFlow::Continue(next)
    } else {
        ControlFlow::Break(prev)
    }
});
assert_eq!(triangular, ControlFlow::Continue(435));

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.

This can also be thought of as the fallible form of for_each() or as the stateless version of try_fold().

Examples

use std::fs::rename;
use std::io::{stdout, Write};
use std::path::Path;

let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];

let res = data.iter().try_for_each(|x| writeln!(stdout(), "{}", x));
assert!(res.is_ok());

let mut it = data.iter().cloned();
let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
assert!(res.is_err());
// It short-circuited, so the remaining items are still in the iterator:
assert_eq!(it.next(), Some("stale_bread.json"));

The ControlFlow type can be used with this method for the situations in which you’d use break and continue in a normal loop:

use std::ops::ControlFlow;

let r = (2..100).try_for_each(|x| {
    if 323 % x == 0 {
        return ControlFlow::Break(x)
    }

    ControlFlow::Continue(())
});
assert_eq!(r, ControlFlow::Break(17));

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

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

fold() takes two arguments: an initial value, and a closure with two arguments: an ‘accumulator’, and an element. The closure returns the value that the accumulator should have for the next iteration.

The initial value is the value the accumulator will have on the first call.

After applying this closure to every element of the iterator, fold() returns the accumulator.

This operation is sometimes called ‘reduce’ or ‘inject’.

Folding is useful whenever you have a collection of something, and want to produce a single value from it.

Note: fold(), and similar methods that traverse the entire iterator, might not terminate for infinite iterators, even on traits for which a result is determinable in finite time.

Note: reduce() can be used to use the first element as the initial value, if the accumulator type and item type is the same.

Note: fold() combines elements in a left-associative fashion. For associative operators like +, the order the elements are combined in is not important, but for non-associative operators like - the order will affect the final result. For a right-associative version of fold(), see DoubleEndedIterator::rfold().

Note to Implementors

Several of the other (forward) methods have default implementations in terms of this one, so try to implement this explicitly if it can do something better than the default for loop implementation.

In particular, try to have this call fold() on the internal parts from which this iterator is composed.

Examples

Basic usage:

let a = [1, 2, 3];

// the sum of all of the elements of the array
let sum = a.iter().fold(0, |acc, x| acc + x);

assert_eq!(sum, 6);

Let’s walk through each step of the iteration here:

element	acc	x	result
	0
1	0	1	1
2	1	2	3
3	3	3	6

And so, our final result, 6.

This example demonstrates the left-associative nature of fold(): it builds a string, starting with an initial value and continuing with each element from the front until the back:

let numbers = [1, 2, 3, 4, 5];

let zero = "0".to_string();

let result = numbers.iter().fold(zero, |acc, &x| {
    format!("({} + {})", acc, x)
});

assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");

It’s common for people who haven’t used iterators a lot to use a for loop with a list of things to build up a result. Those can be turned into fold()s:

let numbers = [1, 2, 3, 4, 5];

let mut result = 0;

// for loop:
for i in &numbers {
    result = result + i;
}

// fold:
let result2 = numbers.iter().fold(0, |acc, &x| acc + x);

// they're the same
assert_eq!(result, result2);

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.

If the iterator is empty, returns None; otherwise, returns the result of the reduction.

For iterators with at least one element, this is the same as fold() with the first element of the iterator as the initial value, folding every subsequent element into it.

Example

Find the maximum value:

fn find_max<I>(iter: I) -> Option<I::Item>
    where I: Iterator,
          I::Item: Ord,
{
    iter.reduce(|a, b| {
        if a >= b { a } else { b }
    })
}
let a = [10, 20, 5, -23, 0];
let b: [u32; 0] = [];

assert_eq!(find_max(a.iter()), Some(&20));
assert_eq!(find_max(b.iter()), None);

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

Tests if every element of the iterator matches a predicate.

all() takes a closure that returns true or false. It applies this closure to each element of the iterator, and if they all return true, then so does all(). If any of them return false, it returns false.

all() is short-circuiting; in other words, it will stop processing as soon as it finds a false, given that no matter what else happens, the result will also be false.

An empty iterator returns true.

Examples

Basic usage:

let a = [1, 2, 3];

assert!(a.iter().all(|&x| x > 0));

assert!(!a.iter().all(|&x| x > 2));

Stopping at the first false:

let a = [1, 2, 3];

let mut iter = a.iter();

assert!(!iter.all(|&x| x != 2));

// we can still use `iter`, as there are more elements.
assert_eq!(iter.next(), Some(&3));

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

Tests if any element of the iterator matches a predicate.

any() takes a closure that returns true or false. It applies this closure to each element of the iterator, and if any of them return true, then so does any(). If they all return false, it returns false.

any() is short-circuiting; in other words, it will stop processing as soon as it finds a true, given that no matter what else happens, the result will also be true.

An empty iterator returns false.

Examples

Basic usage:

let a = [1, 2, 3];

assert!(a.iter().any(|&x| x > 0));

assert!(!a.iter().any(|&x| x > 5));

Stopping at the first true:

let a = [1, 2, 3];

let mut iter = a.iter();

assert!(iter.any(|&x| x != 2));

// we can still use `iter`, as there are more elements.
assert_eq!(iter.next(), Some(&2));

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.

find() takes a closure that returns true or false. It applies this closure to each element of the iterator, and if any of them return true, then find() returns Some(element). If they all return false, it returns None.

find() is short-circuiting; in other words, it will stop processing as soon as the closure returns true.

Because find() takes a reference, and many iterators iterate over references, this leads to a possibly confusing situation where the argument is a double reference. You can see this effect in the examples below, with &&x.

Examples

Basic usage:

let a = [1, 2, 3];

assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));

assert_eq!(a.iter().find(|&&x| x == 5), None);

Stopping at the first true:

let a = [1, 2, 3];

let mut iter = a.iter();

assert_eq!(iter.find(|&&x| x == 2), Some(&2));

// we can still use `iter`, as there are more elements.
assert_eq!(iter.next(), Some(&3));

Note that iter.find(f) is equivalent to iter.filter(f).next().

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.

iter.find_map(f) is equivalent to iter.filter_map(f).next().

Examples

let a = ["lol", "NaN", "2", "5"];

let first_number = a.iter().find_map(|s| s.parse().ok());

assert_eq!(first_number, Some(2));

fn try_find<F, R, E>(&mut self, f: F) -> Result<Option<Self::Item>, E> where
    F: FnMut(&Self::Item) -> R,
    R: Try<Output = bool, Residual = Result<Infallible, E>> + Try, 

🔬 This is a nightly-only experimental API. (try_find #63178)

new API

Applies function to the elements of iterator and returns the first true result or the first error.

Examples

#![feature(try_find)]

let a = ["1", "2", "lol", "NaN", "5"];

let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
    Ok(s.parse::<i32>()?  == search)
};

let result = a.iter().try_find(|&&s| is_my_num(s, 2));
assert_eq!(result, Ok(Some(&"2")));

let result = a.iter().try_find(|&&s| is_my_num(s, 5));
assert!(result.is_err());

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.

position() takes a closure that returns true or false. It applies this closure to each element of the iterator, and if one of them returns true, then position() returns Some(index). If all of them return false, it returns None.

position() is short-circuiting; in other words, it will stop processing as soon as it finds a true.

Overflow Behavior

The method does no guarding against overflows, so if there are more than usize::MAX non-matching elements, it either produces the wrong result or panics. If debug assertions are enabled, a panic is guaranteed.

Panics

This function might panic if the iterator has more than usize::MAX non-matching elements.

Examples

Basic usage:

let a = [1, 2, 3];

assert_eq!(a.iter().position(|&x| x == 2), Some(1));

assert_eq!(a.iter().position(|&x| x == 5), None);

Stopping at the first true:

let a = [1, 2, 3, 4];

let mut iter = a.iter();

assert_eq!(iter.position(|&x| x >= 2), Some(1));

// we can still use `iter`, as there are more elements.
assert_eq!(iter.next(), Some(&3));

// The returned index depends on iterator state
assert_eq!(iter.position(|&x| x == 4), Some(0));

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

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

rposition() takes a closure that returns true or false. It applies this closure to each element of the iterator, starting from the end, and if one of them returns true, then rposition() returns Some(index). If all of them return false, it returns None.

rposition() is short-circuiting; in other words, it will stop processing as soon as it finds a true.

Examples

Basic usage:

let a = [1, 2, 3];

assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));

assert_eq!(a.iter().rposition(|&x| x == 5), None);

Stopping at the first true:

let a = [1, 2, 3];

let mut iter = a.iter();

assert_eq!(iter.rposition(|&x| x == 2), Some(1));

// we can still use `iter`, as there are more elements.
assert_eq!(iter.next(), Some(&1));

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

Returns the maximum element of an iterator.

If several elements are equally maximum, the last element is returned. If the iterator is empty, None is returned.

Note that f32/f64 doesn’t implement Ord due to NaN being incomparable. You can work around this by using Iterator::reduce:

assert_eq!(
    vec![2.4, f32::NAN, 1.3]
        .into_iter()
        .reduce(f32::max)
        .unwrap(),
    2.4
);

Examples

Basic usage:

let a = [1, 2, 3];
let b: Vec<u32> = Vec::new();

assert_eq!(a.iter().max(), Some(&3));
assert_eq!(b.iter().max(), None);

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

Returns the minimum element of an iterator.

If several elements are equally minimum, the first element is returned. If the iterator is empty, None is returned.

Note that f32/f64 doesn’t implement Ord due to NaN being incomparable. You can work around this by using Iterator::reduce:

assert_eq!(
    vec![2.4, f32::NAN, 1.3]
        .into_iter()
        .reduce(f32::min)
        .unwrap(),
    1.3
);

Examples

Basic usage:

let a = [1, 2, 3];
let b: Vec<u32> = Vec::new();

assert_eq!(a.iter().min(), Some(&1));
assert_eq!(b.iter().min(), None);

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

Returns the element that gives the maximum value from the specified function.

If several elements are equally maximum, the last element is returned. If the iterator is empty, None is returned.

Examples

let a = [-3_i32, 0, 1, 5, -10];
assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);

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.

If several elements are equally maximum, the last element is returned. If the iterator is empty, None is returned.

Examples

let a = [-3_i32, 0, 1, 5, -10];
assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);

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

Returns the element that gives the minimum value from the specified function.

If several elements are equally minimum, the first element is returned. If the iterator is empty, None is returned.

Examples

let a = [-3_i32, 0, 1, 5, -10];
assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);

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.

If several elements are equally minimum, the first element is returned. If the iterator is empty, None is returned.

Examples

let a = [-3_i32, 0, 1, 5, -10];
assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);

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

Reverses an iterator’s direction.

Usually, iterators iterate from left to right. After using rev(), an iterator will instead iterate from right to left.

This is only possible if the iterator has an end, so rev() only works on DoubleEndedIterators.

Examples

let a = [1, 2, 3];

let mut iter = a.iter().rev();

assert_eq!(iter.next(), Some(&3));
assert_eq!(iter.next(), Some(&2));
assert_eq!(iter.next(), Some(&1));

assert_eq!(iter.next(), None);

fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB) where
    Self: Iterator<Item = (A, B)>,
    FromA: Default + Extend<A>,
    FromB: Default + Extend<B>, 

Converts an iterator of pairs into a pair of containers.

unzip() consumes an entire iterator of pairs, producing two collections: one from the left elements of the pairs, and one from the right elements.

This function is, in some sense, the opposite of zip.

Examples

Basic usage:

let a = [(1, 2), (3, 4)];

let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();

assert_eq!(left, [1, 3]);
assert_eq!(right, [2, 4]);

// you can also unzip multiple nested tuples at once
let a = [(1, (2, 3)), (4, (5, 6))];

let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.iter().cloned().unzip();
assert_eq!(x, [1, 4]);
assert_eq!(y, [2, 5]);
assert_eq!(z, [3, 6]);

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

Creates an iterator which copies all of its elements.

This is useful when you have an iterator over &T, but you need an iterator over T.

Examples

Basic usage:

let a = [1, 2, 3];

let v_copied: Vec<_> = a.iter().copied().collect();

// copied is the same as .map(|&x| x)
let v_map: Vec<_> = a.iter().map(|&x| x).collect();

assert_eq!(v_copied, vec![1, 2, 3]);
assert_eq!(v_map, vec![1, 2, 3]);

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

Creates an iterator which clones all of its elements.

This is useful when you have an iterator over &T, but you need an iterator over T.

Examples

Basic usage:

let a = [1, 2, 3];

let v_cloned: Vec<_> = a.iter().cloned().collect();

// cloned is the same as .map(|&x| x), for integers
let v_map: Vec<_> = a.iter().map(|&x| x).collect();

assert_eq!(v_cloned, vec![1, 2, 3]);
assert_eq!(v_map, vec![1, 2, 3]);

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

Repeats an iterator endlessly.

Instead of stopping at None, the iterator will instead start again, from the beginning. After iterating again, it will start at the beginning again. And again. And again. Forever.

Examples

Basic usage:

let a = [1, 2, 3];

let mut it = a.iter().cycle();

assert_eq!(it.next(), Some(&1));
assert_eq!(it.next(), Some(&2));
assert_eq!(it.next(), Some(&3));
assert_eq!(it.next(), Some(&1));
assert_eq!(it.next(), Some(&2));
assert_eq!(it.next(), Some(&3));
assert_eq!(it.next(), Some(&1));

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

Sums the elements of an iterator.

Takes each element, adds them together, and returns the result.

An empty iterator returns the zero value of the type.

Panics

When calling sum() and a primitive integer type is being returned, this method will panic if the computation overflows and debug assertions are enabled.

Examples

Basic usage:

let a = [1, 2, 3];
let sum: i32 = a.iter().sum();

assert_eq!(sum, 6);

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

Iterates over the entire iterator, multiplying all the elements

An empty iterator returns the one value of the type.

Panics

When calling product() and a primitive integer type is being returned, method will panic if the computation overflows and debug assertions are enabled.

Examples

fn factorial(n: u32) -> u32 {
    (1..=n).product()
}
assert_eq!(factorial(0), 1);
assert_eq!(factorial(1), 1);
assert_eq!(factorial(5), 120);

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.

Examples

use std::cmp::Ordering;

assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);

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

🔬 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.

Examples

Basic usage:

#![feature(iter_order_by)]

use std::cmp::Ordering;

let xs = [1, 2, 3, 4];
let ys = [1, 4, 9, 16];

assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);

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.

Examples

use std::cmp::Ordering;

assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));

assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);

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

🔬 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.

Examples

Basic usage:

#![feature(iter_order_by)]

use std::cmp::Ordering;

let xs = [1.0, 2.0, 3.0, 4.0];
let ys = [1.0, 4.0, 9.0, 16.0];

assert_eq!(
    xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
    Some(Ordering::Less)
);
assert_eq!(
    xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
    Some(Ordering::Equal)
);
assert_eq!(
    xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
    Some(Ordering::Greater)
);

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.

Examples

assert_eq!([1].iter().eq([1].iter()), true);
assert_eq!([1].iter().eq([1, 2].iter()), false);

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

🔬 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.

Examples

Basic usage:

#![feature(iter_order_by)]

let xs = [1, 2, 3, 4];
let ys = [1, 4, 9, 16];

assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));

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.

Examples

assert_eq!([1].iter().ne([1].iter()), false);
assert_eq!([1].iter().ne([1, 2].iter()), true);

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.

Examples

assert_eq!([1].iter().lt([1].iter()), false);
assert_eq!([1].iter().lt([1, 2].iter()), true);
assert_eq!([1, 2].iter().lt([1].iter()), false);
assert_eq!([1, 2].iter().lt([1, 2].iter()), false);

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.

Examples

assert_eq!([1].iter().le([1].iter()), true);
assert_eq!([1].iter().le([1, 2].iter()), true);
assert_eq!([1, 2].iter().le([1].iter()), false);
assert_eq!([1, 2].iter().le([1, 2].iter()), true);

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.

Examples

assert_eq!([1].iter().gt([1].iter()), false);
assert_eq!([1].iter().gt([1, 2].iter()), false);
assert_eq!([1, 2].iter().gt([1].iter()), true);
assert_eq!([1, 2].iter().gt([1, 2].iter()), false);

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.

Examples

assert_eq!([1].iter().ge([1].iter()), true);
assert_eq!([1].iter().ge([1, 2].iter()), false);
assert_eq!([1, 2].iter().ge([1].iter()), true);
assert_eq!([1, 2].iter().ge([1, 2].iter()), true);

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

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

new API

Checks if the elements of this iterator are sorted.

That is, for each element a and its following element b, a <= b must hold. If the iterator yields exactly zero or one element, true is returned.

Note that if Self::Item is only PartialOrd, but not Ord, the above definition implies that this function returns false if any two consecutive items are not comparable.

Examples

#![feature(is_sorted)]

assert!([1, 2, 2, 9].iter().is_sorted());
assert!(![1, 3, 2, 4].iter().is_sorted());
assert!([0].iter().is_sorted());
assert!(std::iter::empty::<i32>().is_sorted());
assert!(![0.0, 1.0, f32::NAN].iter().is_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)

new API

Checks if the elements of this iterator are sorted using the given comparator function.

Instead of using PartialOrd::partial_cmp, this function uses the given compare function to determine the ordering of two elements. Apart from that, it’s equivalent to is_sorted; see its documentation for more information.

Examples

#![feature(is_sorted)]

assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));

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)

new API

Checks if the elements of this iterator are sorted using the given key extraction function.

Instead of comparing the iterator’s elements directly, this function compares the keys of the elements, as determined by f. Apart from that, it’s equivalent to is_sorted; see its documentation for more information.

Examples

#![feature(is_sorted)]

assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));

Implementations on Foreign Types

impl<'a, T, P> Iterator for SplitInclusive<'a, T, P> where
    P: FnMut(&T) -> bool, 

type Item = &'a [T]

pub fn next(&mut self) -> Option<&'a [T]>

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

impl<'a> Iterator for EscapeAscii<'a>

type Item = u8

pub fn next(&mut self) -> Option<u8>

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

pub fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
    R: Try<Output = Acc>,
    Fold: FnMut(Acc, <EscapeAscii<'a> as Iterator>::Item) -> R, 

pub fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc where
    Fold: FnMut(Acc, <EscapeAscii<'a> as Iterator>::Item) -> Acc, 

pub fn last(self) -> Option<u8>

impl<'a> Iterator for Utf8LossyChunksIter<'a>

type Item = Utf8LossyChunk<'a>

pub fn next(&mut self) -> Option<Utf8LossyChunk<'a>>

impl<'a, T, P> Iterator for SplitInclusiveMut<'a, T, P> where
    P: FnMut(&T) -> bool, 

type Item = &'a mut [T]

pub fn next(&mut self) -> Option<&'a mut [T]>

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

impl<'a, T, S, A> Iterator for Union<'a, T, S, A> where
    T: Eq + Hash,
    S: BuildHasher,
    A: Allocator + Clone, 

type Item = &'a T

pub fn next(&mut self) -> Option<&'a T>

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

impl<'_, K, F, A> Iterator for DrainFilter<'_, K, F, A> where
    F: FnMut(&K) -> bool,
    A: Allocator + Clone, 

type Item = K

pub fn next(&mut self) -> Option<<DrainFilter<'_, K, F, A> as Iterator>::Item>

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

impl<K, A> Iterator for IntoIter<K, A> where
    A: Allocator + Clone, 

type Item = K

pub fn next(&mut self) -> Option<K>

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

impl<'_, K, V, F, A> Iterator for DrainFilter<'_, K, V, F, A> where
    F: FnMut(&K, &mut V) -> bool,
    A: Allocator + Clone, 

type Item = (K, V)

pub fn next(
    &mut self
) -> Option<<DrainFilter<'_, K, V, F, A> as Iterator>::Item>

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

impl<'a, K> Iterator for Iter<'a, K>

type Item = &'a K

pub fn next(&mut self) -> Option<&'a K>

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

impl<'a, K, V> Iterator for IterMut<'a, K, V>

type Item = (&'a K, &'a mut V)

pub fn next(&mut self) -> Option<(&'a K, &'a mut V)>

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

impl<K, V, A> Iterator for IntoIter<K, V, A> where
    A: Allocator + Clone, 

type Item = (K, V)

pub fn next(&mut self) -> Option<(K, V)>

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

impl<'_, K, A> Iterator for Drain<'_, K, A> where
    A: Allocator + Clone, 

type Item = K

pub fn next(&mut self) -> Option<K>

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

impl<'a, T, S, A> Iterator for Difference<'a, T, S, A> where
    T: Eq + Hash,
    S: BuildHasher,
    A: Allocator + Clone, 

type Item = &'a T

pub fn next(&mut self) -> Option<&'a T>

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

impl<'a, K, V, A> Iterator for Drain<'a, K, V, A> where
    A: Allocator + Clone, 

type Item = (K, V)

pub fn next(&mut self) -> Option<(K, V)>

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

impl<'a, T, S, A> Iterator for Intersection<'a, T, S, A> where
    T: Eq + Hash,
    S: BuildHasher,
    A: Allocator + Clone, 

type Item = &'a T

pub fn next(&mut self) -> Option<&'a T>

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

impl<'a, T, S, A> Iterator for SymmetricDifference<'a, T, S, A> where
    T: Eq + Hash,
    S: BuildHasher,
    A: Allocator + Clone, 

type Item = &'a T

pub fn next(&mut self) -> Option<&'a T>

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

impl<'a, K, V> Iterator for Keys<'a, K, V>

type Item = &'a K

pub fn next(&mut self) -> Option<&'a K>

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

impl<'a, K, V> Iterator for Iter<'a, K, V>

type Item = (&'a K, &'a V)

pub fn next(&mut self) -> Option<(&'a K, &'a V)>

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

impl<'a, K, V> Iterator for ValuesMut<'a, K, V>

type Item = &'a mut V

pub fn next(&mut self) -> Option<&'a mut V>

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

impl<'a, K, V> Iterator for Values<'a, K, V>

type Item = &'a V

pub fn next(&mut self) -> Option<&'a V>

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

Implementors

impl Iterator for std::ascii::EscapeDefault

impl Iterator for std::char::EscapeDebug

impl Iterator for std::char::EscapeDefault

impl Iterator for std::char::EscapeUnicode

impl Iterator for ToLowercase

impl Iterator for ToUppercase

impl Iterator for Args

impl Iterator for ArgsOs

impl Iterator for Vars

impl Iterator for VarsOs

impl Iterator for ReadDir

impl<'_> Iterator for std::str::Bytes<'_>

impl<'_> Iterator for std::string::Drain<'_>

impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized, 

impl<'_, I, A> Iterator for Splice<'_, I, A> where
    I: Iterator,
    A: Allocator, 

impl<'_, K, V, F> Iterator for std::collections::btree_map::DrainFilter<'_, K, V, F> where
    F: FnMut(&K, &mut V) -> bool, 

impl<'_, T> Iterator for std::collections::binary_heap::Drain<'_, T>

impl<'_, T> Iterator for DrainSorted<'_, T> where
    T: Ord, 

impl<'_, T, A> Iterator for std::collections::vec_deque::Drain<'_, T, A> where
    A: Allocator, 

impl<'_, T, A> Iterator for std::vec::Drain<'_, T, A> where
    A: Allocator, 

impl<'_, T, F> Iterator for std::collections::linked_list::DrainFilter<'_, T, F> where
    F: FnMut(&mut T) -> bool, 

impl<'_, T, F, A> Iterator for std::vec::DrainFilter<'_, T, F, A> where
    F: FnMut(&mut T) -> bool,
    A: Allocator, 

impl<'a> Iterator for SplitPaths<'a>

impl<'a> Iterator for std::error::Chain<'a>

impl<'a> Iterator for Incoming<'a>

impl<'a> Iterator for Ancestors<'a>

impl<'a> Iterator for Components<'a>

impl<'a> Iterator for std::path::Iter<'a>

impl<'a> Iterator for CommandArgs<'a>

impl<'a> Iterator for CommandEnvs<'a>

impl<'a> Iterator for CharIndices<'a>

impl<'a> Iterator for Chars<'a>

impl<'a> Iterator for EncodeUtf16<'a>

impl<'a> Iterator for std::str::EscapeDebug<'a>

impl<'a> Iterator for std::str::EscapeDefault<'a>

impl<'a> Iterator for std::str::EscapeUnicode<'a>

impl<'a> Iterator for std::str::Lines<'a>

impl<'a> Iterator for LinesAny<'a>

impl<'a> Iterator for SplitAsciiWhitespace<'a>

impl<'a> Iterator for SplitWhitespace<'a>

impl<'a, '_, T, F> Iterator for std::collections::btree_set::DrainFilter<'_, T, F> where
    F: 'a + FnMut(&T) -> bool, 

impl<'a, A> Iterator for std::option::Iter<'a, A>

impl<'a, A> Iterator for std::option::IterMut<'a, A>

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

impl<'a, I, T> Iterator for Copied<I> where
    T: 'a + Copy,
    I: Iterator<Item = &'a T>, 

impl<'a, K> Iterator for std::collections::hash_set::Drain<'a, K>

impl<'a, K> Iterator for std::collections::hash_set::Iter<'a, K>

impl<'a, K, V> Iterator for std::collections::btree_map::Iter<'a, K, V> where
    K: 'a,
    V: 'a, 

impl<'a, K, V> Iterator for std::collections::btree_map::IterMut<'a, K, V> where
    K: 'a,
    V: 'a, 

impl<'a, K, V> Iterator for std::collections::btree_map::Keys<'a, K, V>

impl<'a, K, V> Iterator for std::collections::btree_map::Range<'a, K, V>

impl<'a, K, V> Iterator for RangeMut<'a, K, V>

impl<'a, K, V> Iterator for std::collections::btree_map::Values<'a, K, V>

impl<'a, K, V> Iterator for std::collections::btree_map::ValuesMut<'a, K, V>

impl<'a, K, V> Iterator for std::collections::hash_map::Drain<'a, K, V>

impl<'a, K, V> Iterator for std::collections::hash_map::Iter<'a, K, V>

impl<'a, K, V> Iterator for std::collections::hash_map::IterMut<'a, K, V>

impl<'a, K, V> Iterator for std::collections::hash_map::Keys<'a, K, V>

impl<'a, K, V> Iterator for std::collections::hash_map::Values<'a, K, V>

impl<'a, K, V> Iterator for std::collections::hash_map::ValuesMut<'a, K, V>

impl<'a, P> Iterator for MatchIndices<'a, P> where
    P: Pattern<'a>, 

impl<'a, P> Iterator for Matches<'a, P> where
    P: Pattern<'a>, 

impl<'a, P> Iterator for RMatchIndices<'a, P> where
    P: Pattern<'a>,
    <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>, 

impl<'a, P> Iterator for RMatches<'a, P> where
    P: Pattern<'a>,
    <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>, 

impl<'a, P> Iterator for std::str::RSplit<'a, P> where
    P: Pattern<'a>,
    <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>, 

impl<'a, P> Iterator for std::str::RSplitN<'a, P> where
    P: Pattern<'a>,
    <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>, 

impl<'a, P> Iterator for RSplitTerminator<'a, P> where
    P: Pattern<'a>,
    <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>, 

impl<'a, P> Iterator for std::str::Split<'a, P> where
    P: Pattern<'a>, 

impl<'a, P> Iterator for std::str::SplitInclusive<'a, P> where
    P: Pattern<'a>, 

impl<'a, P> Iterator for std::str::SplitN<'a, P> where
    P: Pattern<'a>, 

impl<'a, P> Iterator for SplitTerminator<'a, P> where
    P: Pattern<'a>, 

impl<'a, T> Iterator for std::collections::binary_heap::Iter<'a, T>

impl<'a, T> Iterator for std::collections::btree_set::Difference<'a, T> where
    T: Ord, 

impl<'a, T> Iterator for std::collections::btree_set::Intersection<'a, T> where
    T: Ord, 

impl<'a, T> Iterator for std::collections::btree_set::Iter<'a, T>

impl<'a, T> Iterator for std::collections::btree_set::Range<'a, T>

impl<'a, T> Iterator for std::collections::btree_set::SymmetricDifference<'a, T> where
    T: Ord, 

impl<'a, T> Iterator for std::collections::btree_set::Union<'a, T> where
    T: Ord, 

impl<'a, T> Iterator for std::collections::linked_list::Iter<'a, T>

impl<'a, T> Iterator for std::collections::linked_list::IterMut<'a, T>

impl<'a, T> Iterator for std::collections::vec_deque::Iter<'a, T>

impl<'a, T> Iterator for std::collections::vec_deque::IterMut<'a, T>

impl<'a, T> Iterator for std::result::Iter<'a, T>

impl<'a, T> Iterator for std::result::IterMut<'a, T>

impl<'a, T> Iterator for Chunks<'a, T>

impl<'a, T> Iterator for ChunksExact<'a, T>

impl<'a, T> Iterator for ChunksExactMut<'a, T>

impl<'a, T> Iterator for ChunksMut<'a, T>

impl<'a, T> Iterator for std::slice::Iter<'a, T>

impl<'a, T> Iterator for std::slice::IterMut<'a, T>

impl<'a, T> Iterator for RChunks<'a, T>

impl<'a, T> Iterator for RChunksExact<'a, T>

impl<'a, T> Iterator for RChunksExactMut<'a, T>

impl<'a, T> Iterator for RChunksMut<'a, T>

impl<'a, T> Iterator for Windows<'a, T>

impl<'a, T> Iterator for std::sync::mpsc::Iter<'a, T>

impl<'a, T> Iterator for TryIter<'a, T>

impl<'a, T, P> Iterator for GroupBy<'a, T, P> where
    T: 'a,
    P: FnMut(&T, &T) -> bool, 

impl<'a, T, P> Iterator for GroupByMut<'a, T, P> where
    T: 'a,
    P: FnMut(&T, &T) -> bool, 

impl<'a, T, P> Iterator for std::slice::RSplit<'a, T, P> where
    P: FnMut(&T) -> bool, 

impl<'a, T, P> Iterator for RSplitMut<'a, T, P> where
    P: FnMut(&T) -> bool, 

impl<'a, T, P> Iterator for std::slice::RSplitN<'a, T, P> where
    P: FnMut(&T) -> bool, 

impl<'a, T, P> Iterator for RSplitNMut<'a, T, P> where
    P: FnMut(&T) -> bool, 

impl<'a, T, P> Iterator for std::slice::Split<'a, T, P> where
    P: FnMut(&T) -> bool, 

impl<'a, T, P> Iterator for SplitMut<'a, T, P> where
    P: FnMut(&T) -> bool, 

impl<'a, T, P> Iterator for std::slice::SplitN<'a, T, P> where
    P: FnMut(&T) -> bool, 

impl<'a, T, P> Iterator for SplitNMut<'a, T, P> where
    P: FnMut(&T) -> bool, 

impl<'a, T, S> Iterator for std::collections::hash_set::Difference<'a, T, S> where
    T: Eq + Hash,
    S: BuildHasher, 

impl<'a, T, S> Iterator for std::collections::hash_set::Intersection<'a, T, S> where
    T: Eq + Hash,
    S: BuildHasher, 

impl<'a, T, S> Iterator for std::collections::hash_set::SymmetricDifference<'a, T, S> where
    T: Eq + Hash,
    S: BuildHasher, 

impl<'a, T, S> Iterator for std::collections::hash_set::Union<'a, T, S> where
    T: Eq + Hash,
    S: BuildHasher, 

impl<'a, T, const N: usize> Iterator for ArrayChunks<'a, T, N>

impl<'a, T, const N: usize> Iterator for ArrayChunksMut<'a, T, N>

impl<'a, T, const N: usize> Iterator for ArrayWindows<'a, T, N>

impl<A> Iterator for std::ops::Range<A> where
    A: Step, 

impl<A> Iterator for RangeFrom<A> where
    A: Step, 

impl<A> Iterator for RangeInclusive<A> where
    A: Step, 

impl<A> Iterator for std::option::IntoIter<A>

impl<A> Iterator for Repeat<A> where
    A: Clone, 

impl<A, B> Iterator for std::iter::Chain<A, B> where
    B: Iterator<Item = <A as Iterator>::Item>,
    A: Iterator, 

impl<A, B> Iterator for Zip<A, B> where
    B: Iterator,
    A: Iterator, 

impl<A, F> Iterator for OnceWith<F> where
    F: FnOnce() -> A, 

impl<A, F> Iterator for RepeatWith<F> where
    F: FnMut() -> A, 

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

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

impl<B, I, P> Iterator for MapWhile<I, P> where
    I: Iterator,
    P: FnMut(<I as Iterator>::Item) -> Option<B>, 

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

impl<B: BufRead> Iterator for std::io::Lines<B>

impl<B: BufRead> Iterator for std::io::Split<B>

impl<I> Iterator for DecodeUtf16<I> where
    I: Iterator<Item = u16>, 

impl<I> Iterator for Cycle<I> where
    I: Clone + Iterator, 

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

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

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

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

impl<I> Iterator for Rev<I> where
    I: DoubleEndedIterator, 

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

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

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

impl<I, A> Iterator for Box<I, A> where
    I: Iterator + ?Sized,
    A: Allocator, 

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

impl<I, G> Iterator for IntersperseWith<I, G> where
    I: Iterator,
    G: FnMut() -> <I as Iterator>::Item, 

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

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

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

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

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

impl<K> Iterator for std::collections::hash_set::IntoIter<K>

impl<K, F> Iterator for std::collections::hash_set::DrainFilter<'_, K, F> where
    F: FnMut(&K) -> bool, 

impl<K, V> Iterator for std::collections::btree_map::IntoIter<K, V>

impl<K, V> Iterator for std::collections::btree_map::IntoKeys<K, V>

impl<K, V> Iterator for std::collections::btree_map::IntoValues<K, V>

impl<K, V> Iterator for std::collections::hash_map::IntoIter<K, V>

impl<K, V> Iterator for std::collections::hash_map::IntoKeys<K, V>

impl<K, V> Iterator for std::collections::hash_map::IntoValues<K, V>

impl<K, V, F> Iterator for std::collections::hash_map::DrainFilter<'_, K, V, F> where
    F: FnMut(&K, &mut V) -> bool, 

impl<R: Read> Iterator for std::io::Bytes<R>

impl<T> Iterator for std::collections::binary_heap::IntoIter<T>

impl<T> Iterator for IntoIterSorted<T> where
    T: Ord, 

impl<T> Iterator for std::collections::btree_set::IntoIter<T>

impl<T> Iterator for std::collections::linked_list::IntoIter<T>

impl<T> Iterator for std::result::IntoIter<T>

impl<T> Iterator for std::sync::mpsc::IntoIter<T>

impl<T> Iterator for Empty<T>

impl<T> Iterator for Once<T>

impl<T, A> Iterator for std::collections::vec_deque::IntoIter<T, A> where
    A: Allocator, 

impl<T, A> Iterator for std::vec::IntoIter<T, A> where
    A: Allocator, 

impl<T, F> Iterator for FromFn<F> where
    F: FnMut() -> Option<T>, 

impl<T, F> Iterator for Successors<T, F> where
    F: FnMut(&T) -> Option<T>, 

impl<T, const N: usize> Iterator for std::array::IntoIter<T, N>