# Trait core::iter::Iterator 1.0.0[−][src]

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

        fn size_hint(&self) -> (usize, Option<usize>) { ... }
        fn count(self) -> usize
        where
            Self: Sized,
        { ... }
        fn last(self) -> Option<Self::Item>
        where
            Self: Sized,
        { ... }
        fn nth(&mut self, n: usize) -> Option<Self::Item> { ... }
        fn step_by(self, step: usize) -> StepBy<Self>
        where
            Self: Sized,
        { ... }
        fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
        where
            Self: Sized,
            U: IntoIterator<Item = Self::Item>,
        { ... }
        fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
        where
            Self: Sized,
            U: IntoIterator,
        { ... }
        fn map<B, F>(self, f: F) -> Map<Self, F>
        where
            Self: Sized,
            F: FnMut(Self::Item) -> B,
        { ... }
        fn for_each<F>(self, f: F)
        where
            Self: Sized,
            F: FnMut(Self::Item),
        { ... }
        fn filter<P>(self, predicate: P) -> Filter<Self, P>
        where
            Self: Sized,
            P: FnMut(&Self::Item) -> bool,
        { ... }
        fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
        where
            Self: Sized,
            F: FnMut(Self::Item) -> Option<B>,
        { ... }
        fn enumerate(self) -> Enumerate<Self>
        where
            Self: Sized,
        { ... }
        fn peekable(self) -> Peekable<Self>
        where
            Self: Sized,
        { ... }
        fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
        where
            Self: Sized,
            P: FnMut(&Self::Item) -> bool,
        { ... }
        fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
        where
            Self: Sized,
            P: FnMut(&Self::Item) -> bool,
        { ... }
        fn skip(self, n: usize) -> Skip<Self>
        where
            Self: Sized,
        { ... }
        fn take(self, n: usize) -> Take<Self>
        where
            Self: Sized,
        { ... }
        fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
        where
            Self: Sized,
            F: FnMut(&mut St, Self::Item) -> Option<B>,
        { ... }
        fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
        where
            Self: Sized,
            U: IntoIterator,
            F: FnMut(Self::Item) -> U,
        { ... }
        fn flatten(self) -> Flatten<Self>
        where
            Self: Sized,
            Self::Item: IntoIterator,
        { ... }
        fn fuse(self) -> Fuse<Self>
        where
            Self: Sized,
        { ... }
        fn inspect<F>(self, f: F) -> Inspect<Self, F>
        where
            Self: Sized,
            F: FnMut(&Self::Item),
        { ... }
        fn by_ref(&mut self) -> &mut Self
        where
            Self: Sized,
        { ... }
        #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
    fn collect<B: FromIterator<Self::Item>>(self) -> B
        where
            Self: Sized,
        { ... }
        fn partition<B, F>(self, f: F) -> (B, B)
        where
            Self: Sized,
            B: Default + Extend<Self::Item>,
            F: FnMut(&Self::Item) -> bool,
        { ... }
        fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
        where
            Self: Sized,
            F: FnMut(B, Self::Item) -> R,
            R: Try<Ok = B>,
        { ... }
        fn try_for_each<F, R>(&mut self, f: F) -> R
        where
            Self: Sized,
            F: FnMut(Self::Item) -> R,
            R: Try<Ok = ()>,
        { ... }
        fn fold<B, F>(self, init: B, f: F) -> B
        where
            Self: Sized,
            F: FnMut(B, Self::Item) -> B,
        { ... }
        fn all<F>(&mut self, f: F) -> bool
        where
            Self: Sized,
            F: FnMut(Self::Item) -> bool,
        { ... }
        fn any<F>(&mut self, f: F) -> bool
        where
            Self: Sized,
            F: FnMut(Self::Item) -> bool,
        { ... }
        fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
        where
            Self: Sized,
            P: FnMut(&Self::Item) -> bool,
        { ... }
        fn find_map<B, F>(&mut self, f: F) -> Option<B>
        where
            Self: Sized,
            F: FnMut(Self::Item) -> Option<B>,
        { ... }
        fn position<P>(&mut self, predicate: P) -> Option<usize>
        where
            Self: Sized,
            P: FnMut(Self::Item) -> bool,
        { ... }
        fn rposition<P>(&mut self, predicate: P) -> Option<usize>
        where
            P: FnMut(Self::Item) -> bool,
            Self: Sized + ExactSizeIterator + DoubleEndedIterator,
        { ... }
        fn max(self) -> Option<Self::Item>
        where
            Self: Sized,
            Self::Item: Ord,
        { ... }
        fn min(self) -> Option<Self::Item>
        where
            Self: Sized,
            Self::Item: Ord,
        { ... }
        fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
        where
            Self: Sized,
            F: FnMut(&Self::Item) -> B,
        { ... }
        fn max_by<F>(self, compare: F) -> Option<Self::Item>
        where
            Self: Sized,
            F: FnMut(&Self::Item, &Self::Item) -> Ordering,
        { ... }
        fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
        where
            Self: Sized,
            F: FnMut(&Self::Item) -> B,
        { ... }
        fn min_by<F>(self, compare: F) -> Option<Self::Item>
        where
            Self: Sized,
            F: FnMut(&Self::Item, &Self::Item) -> Ordering,
        { ... }
        fn rev(self) -> Rev<Self>
        where
            Self: Sized + DoubleEndedIterator,
        { ... }
        fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
        where
            FromA: Default + Extend<A>,
            FromB: Default + Extend<B>,
            Self: Sized + Iterator<Item = (A, B)>,
        { ... }
        fn cloned<'a, T: 'a>(self) -> Cloned<Self>
        where
            Self: Sized + Iterator<Item = &'a T>,
            T: Clone,
        { ... }
        fn cycle(self) -> Cycle<Self>
        where
            Self: Sized + Clone,
        { ... }
        fn sum<S>(self) -> S
        where
            Self: Sized,
            S: Sum<Self::Item>,
        { ... }
        fn product<P>(self) -> P
        where
            Self: Sized,
            P: Product<Self::Item>,
        { ... }
        fn cmp<I>(self, other: I) -> Ordering
        where
            I: IntoIterator<Item = Self::Item>,
            Self::Item: Ord,
            Self: Sized,
        { ... }
        fn partial_cmp<I>(self, other: I) -> Option<Ordering>
        where
            I: IntoIterator,
            Self::Item: PartialOrd<I::Item>,
            Self: Sized,
        { ... }
        fn eq<I>(self, other: I) -> bool
        where
            I: IntoIterator,
            Self::Item: PartialEq<I::Item>,
            Self: Sized,
        { ... }
        fn ne<I>(self, other: I) -> bool
        where
            I: IntoIterator,
            Self::Item: PartialEq<I::Item>,
            Self: Sized,
        { ... }
        fn lt<I>(self, other: I) -> bool
        where
            I: IntoIterator,
            Self::Item: PartialOrd<I::Item>,
            Self: Sized,
        { ... }
        fn le<I>(self, other: I) -> bool
        where
            I: IntoIterator,
            Self::Item: PartialOrd<I::Item>,
            Self: Sized,
        { ... }
        fn gt<I>(self, other: I) -> bool
        where
            I: IntoIterator,
            Self::Item: PartialOrd<I::Item>,
            Self: Sized,
        { ... }
        fn ge<I>(self, other: I) -> bool
        where
            I: IntoIterator,
            Self::Item: PartialOrd<I::Item>,
            Self: Sized,
        { ... }
    }

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());Run

## 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());Run

A more complex example:

    // The even numbers from zero to ten.
    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());Run

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_value(), None), iter.size_hint());Run

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

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

This method will evaluate the iterator until its `next` returns `None`. Once `None` is encountered, `count()` returns the number of times it called `next`.

# 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);Run

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

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));Run

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

Returns the `n`th 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));Run

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);Run

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

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

ⓘImportant traits for StepBy<I>

### Important traits for StepBy<I>

`impl<I> Iterator for StepBy<I> where
I: Iterator, type Item = I::Item;`

### `fn step_by(self, step: usize) -> StepBy<Self> where
Self: Sized,`
1.28.0

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 `next(), nth(step-1), nth(step-1), …`, but is also free to behave like the sequence `advance_n_and_return_first(step), advance_n_and_return_first(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, total_step: usize) -> Option<I::Item>
    where
        I: Iterator,
    {
        let next = iter.next();
        if total_step > 1 {
            iter.nth(total_step-2);
        }
        next
    }Run

# 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.into_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);Run

ⓘImportant traits for Chain<A, B>

### Important traits for Chain<A, B>

`impl<A, B> Iterator for Chain<A, B> where
A: Iterator,
B: Iterator<Item = A::Item>, type Item = A::Item;`

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

Takes two iterators and creates a new iterator over both in sequence.

`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. 🔗

# 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);Run

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);Run

ⓘImportant traits for Zip<A, B>

### Important traits for Zip<A, B>

`impl<A, B> Iterator for Zip<A, B> where
A: Iterator,
B: Iterator, type Item = (A::Item, B::Item);`

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

'Zips up' two iterators into a single iterator of pairs.

`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);Run

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);Run

`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]);Run

ⓘImportant traits for Map<I, F>

### Important traits for Map<I, F>

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

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

Takes a closure and creates an iterator which calls that closure on each element.

`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.into_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);Run

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);
    }Run

### `fn for_each<F>(self, f: F) where
Self: Sized,
F: FnMut(Self::Item),`
1.21.0

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 adaptors 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]);Run

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));Run

ⓘImportant traits for Filter<I, P>

### Important traits for Filter<I, P>

`impl<I: Iterator, P> Iterator for Filter<I, P> where
P: FnMut(&I::Item) -> bool, type Item = I::Item;`

### `fn filter<P>(self, predicate: P) -> Filter<Self, P> where
Self: Sized,
P: FnMut(&Self::Item) -> bool,`

Creates an iterator which uses a closure to determine if an element should be yielded.

The closure must return `true` or `false`. `filter()` creates an iterator which calls this closure on each element. If the closure returns `true`, then the element is returned. If the closure returns `false`, it will try again, and call the closure on the next element, seeing if it passes the test.

# Examples

Basic usage:

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

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

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

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.into_iter().filter(|x| **x > 1); // need two *s!

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

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

    let a = [0, 1, 2];

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

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

or both:

    let a = [0, 1, 2];

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

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

of these layers.

ⓘImportant traits for FilterMap<I, F>

### Important traits for FilterMap<I, F>

`impl<B, I: Iterator, F> Iterator for FilterMap<I, F> where
F: FnMut(I::Item) -> Option<B>, type Item = B;`

### `fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F> where
Self: Sized,
F: FnMut(Self::Item) -> Option<B>,`

Creates an iterator that both filters and maps.

The closure must return an `Option<T>`. `filter_map` creates an iterator which calls this closure on each element. If the closure returns `Some(element)`, then that element is returned. If the closure returns `None`, it will try again, and call the closure on the next element, seeing if it will return `Some`.

Why `filter_map` and not just `filter` and `map`? The key is in this part:

> If the closure returns `Some(element)`, then that element is returned.

In other words, it removes the `Option<T>` layer automatically. If your mapping is already returning an `Option<T>` and you want to skip over `None`s, then `filter_map` is much, much nicer to use.

# Examples

Basic usage:

    let a = ["1", "lol", "3", "NaN", "5"];

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

    assert_eq!(iter.next(), Some(1));
    assert_eq!(iter.next(), Some(3));
    assert_eq!(iter.next(), Some(5));
    assert_eq!(iter.next(), None);Run

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

    let a = ["1", "lol", "3", "NaN", "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(3));
    assert_eq!(iter.next(), Some(5));
    assert_eq!(iter.next(), None);Run

ⓘImportant traits for Enumerate<I>

### Important traits for Enumerate<I>

`impl<I> Iterator for Enumerate<I> where
I: Iterator, type Item = (usize, <I as Iterator>::Item);`

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

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

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);Run

ⓘImportant traits for Peekable<I>

### Important traits for Peekable<I>

`impl<I: Iterator> Iterator for Peekable<I> type Item = I::Item;`

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

Creates an iterator which can use `peek` to look at the next element of the iterator without consuming it.

Adds a `peek` method to an iterator. See its documentation for more information.

Note that the underlying iterator is still advanced when `peek` is 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);Run

ⓘImportant traits for SkipWhile<I, P>

### Important traits for SkipWhile<I, P>

`impl<I: Iterator, P> Iterator for SkipWhile<I, P> where
P: FnMut(&I::Item) -> bool, type Item = I::Item;`

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

Creates an iterator that `skip`s elements based on a predicate.

`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.into_iter().skip_while(|x| x.is_negative());

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

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 is a double reference:

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

    let mut iter = a.into_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);Run

Stopping after an initial `false`:

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

    let mut iter = a.into_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);Run

ⓘImportant traits for TakeWhile<I, P>

### Important traits for TakeWhile<I, P>

`impl<I: Iterator, P> Iterator for TakeWhile<I, P> where
P: FnMut(&I::Item) -> bool, type Item = I::Item;`

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

Creates an iterator that yields elements based on a predicate.

`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.into_iter().take_while(|x| x.is_negative());

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

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.into_iter().take_while(|x| **x < 0); // need two *s!

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

Stopping after an initial `false`:

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

    let mut iter = a.into_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);Run

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.into_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]);Run

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 or some similar thing.

ⓘImportant traits for Skip<I>

### Important traits for Skip<I>

`impl<I> Iterator for Skip<I> where
I: Iterator, type Item = <I as Iterator>::Item;`

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

Creates an iterator that skips the first `n` elements.

After they have been consumed, the rest of the elements are yielded.

# 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);Run

ⓘImportant traits for Take<I>

### Important traits for Take<I>

`impl<I> Iterator for Take<I> where
I: Iterator, type Item = <I as Iterator>::Item;`

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

Creates an iterator that yields its first `n` elements.

# 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);Run

`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);Run

ⓘImportant traits for Scan<I, St, F>

### Important traits for Scan<I, St, F>

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

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

An iterator adaptor similar to `fold` that holds internal state and produces a new iterator.

`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);Run

ⓘImportant traits for FlatMap<I, U, F>

### Important traits for FlatMap<I, U, F>

`impl<I: Iterator, U: IntoIterator, F> Iterator for FlatMap<I, U, F> where
F: FnMut(I::Item) -> U, type Item = U::Item;`

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

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

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 `map`ping, and then `flatten`ing 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");Run

ⓘImportant traits for Flatten<I>

### Important traits for Flatten<I>

`impl<I, U> Iterator for Flatten<I> where
I: Iterator,
U: Iterator,
I::Item: IntoIterator<IntoIter = U, Item = U::Item>, type Item = U::Item;`

### `fn flatten(self) -> Flatten<Self> where
Self: Sized,
Self::Item: IntoIterator,`
1.29

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]);Run

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");Run

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");Run

Flattening once only removes one level of nesting:

    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]);Run

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.

ⓘImportant traits for Fuse<I>

### Important traits for Fuse<I>

`impl<I> Iterator for Fuse<I> where
I: Iterator, type Item = <I as Iterator>::Item;impl<I> Iterator for Fuse<I> where
I: FusedIterator,`

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

Creates an iterator which ends after the first `None`.

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.

# 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);Run

ⓘImportant traits for Inspect<I, F>

### Important traits for Inspect<I, F>

`impl<I: Iterator, F> Iterator for Inspect<I, F> where
F: FnMut(&I::Item), type Item = I::Item;`

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

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

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);Run

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);Run

This will print:

    Parsing error: invalid digit found in string
    Sum: 3

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

Borrows an iterator, rather than consuming it.

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

# Examples

Basic usage:

    let a = [1, 2, 3];

    let iter = a.into_iter();

    let sum: i32 = iter.take(5).fold(0, |acc, i| acc + i );

    assert_eq!(sum, 6);

    // if we try to use iter again, it won't work. The following line
    // gives "error: use of moved value: `iter`
    // assert_eq!(iter.next(), None);

    // let's try that again
    let a = [1, 2, 3];

    let mut iter = a.into_iter();

    // instead, we add in a .by_ref()
    let sum: i32 = iter.by_ref().take(2).fold(0, |acc, i| acc + i );

    assert_eq!(sum, 3);

    // now this is just fine:
    assert_eq!(iter.next(), Some(&3));
    assert_eq!(iter.next(), None);Run

### `
#[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
fn collect<B: FromIterator<Self::Item>>(self) -> B where
Self: Sized,`

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.

One of the keys to `collect()`'s power is that many things you might not think of as 'collections' actually are. For example, a `String` is a collection of `char`s. And a collection of `Result<T, E>` can be thought of as single `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);Run

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]);Run

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);Run

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);Run

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);Run

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);Run

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

Consumes an iterator, creating two collections from it.

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`.

# Examples

Basic usage:

    let a = [1, 2, 3];

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

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

### `fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R where
Self: Sized,
F: FnMut(B, Self::Item) -> R,
R: Try<Ok = B>,`
1.27.0

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

Most 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));Run

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));Run

### `fn try_for_each<F, R>(&mut self, f: F) -> R where
Self: Sized,
F: FnMut(Self::Item) -> R,
R: Try<Ok = ()>,`
1.27.0

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"));Run

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

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

`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, may not terminate for infinite iterators, even on traits for which a result is determinable in finite time.

# 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);Run

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`.

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);Run

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

Tests if every element of the iterator matches a predicate.

`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));Run

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));Run

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

Tests if any element of the iterator matches a predicate.

`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));Run

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));Run

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

Searches for an element of an iterator that satisfies a predicate.

`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);Run

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));Run

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

🔬 This is a nightly-only experimental API. (`iterator_find_map` #49602)

unstable new API

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

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

# Examples

    #![feature(iterator_find_map)]
    let a = ["lol", "NaN", "2", "5"];

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

    assert_eq!(first_number, Some(2));Run

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

Searches for an element in an iterator, returning its index.

`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);Run

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));
    Run

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

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

`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);Run

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));Run

### `fn max(self) -> Option<Self::Item> where
Self: Sized,
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.

# 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);Run

### `fn min(self) -> Option<Self::Item> where
Self: Sized,
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.

# 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);Run

### `fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item> where
Self: Sized,
F: FnMut(&Self::Item) -> B,`
1.6.0

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);Run

### `fn max_by<F>(self, compare: F) -> Option<Self::Item> where
Self: Sized,
F: FnMut(&Self::Item, &Self::Item) -> Ordering,`
1.15.0

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);Run

### `fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item> where
Self: Sized,
F: FnMut(&Self::Item) -> B,`
1.6.0

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);Run

### `fn min_by<F>(self, compare: F) -> Option<Self::Item> where
Self: Sized,
F: FnMut(&Self::Item, &Self::Item) -> Ordering,`
1.15.0

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);Run

ⓘImportant traits for Rev<I>

### Important traits for Rev<I>

`impl<I> Iterator for Rev<I> where
I: DoubleEndedIterator, type Item = <I as Iterator>::Item;`

### `fn rev(self) -> Rev<Self> where
Self: Sized + DoubleEndedIterator,`

Reverses an iterator's direction.

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 `DoubleEndedIterator`s.

# 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);Run

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

Converts an iterator of pairs into a pair of containers.

`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]);Run

ⓘImportant traits for Cloned<I>

### Important traits for Cloned<I>

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

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

Creates an iterator which `clone`s all of its elements.

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]);Run

ⓘImportant traits for Cycle<I>

### Important traits for Cycle<I>

`impl<I> Iterator for Cycle<I> where
I: Clone + Iterator, type Item = <I as Iterator>::Item;`

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

Repeats an iterator endlessly.

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));Run

### `fn sum<S>(self) -> S where
Self: Sized,
S: Sum<Self::Item>,`
1.11.0

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);Run

### `fn product<P>(self) -> P where
Self: Sized,
P: Product<Self::Item>,`
1.11.0

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..).take_while(|&i| i <= n).product()
    }
    assert_eq!(factorial(0), 1);
    assert_eq!(factorial(1), 1);
    assert_eq!(factorial(5), 120);Run

### `fn cmp<I>(self, other: I) -> Ordering where
I: IntoIterator<Item = Self::Item>,
Self::Item: Ord,
Self: Sized,`
1.5.0

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

### `fn partial_cmp<I>(self, other: I) -> Option<Ordering> where
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,`
1.5.0

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

### `fn eq<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialEq<I::Item>,
Self: Sized,`
1.5.0

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

### `fn ne<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialEq<I::Item>,
Self: Sized,`
1.5.0

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

### `fn lt<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,`
1.5.0

Determines if the elements of this `Iterator` are lexicographically less than those of another.

### `fn le<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,`
1.5.0

Determines if the elements of this `Iterator` are lexicographically less or equal to those of another.

### `fn gt<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,`
1.5.0

Determines if the elements of this `Iterator` are lexicographically greater than those of another.

### `fn ge<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,`
1.5.0

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

## Implementors

- `impl Iterator for core::ascii::EscapeDefault type Item = u8;` | [src]

- `impl<I: Iterator<Item = u8>> Iterator for DecodeUtf8<I> type Item = Result<char, InvalidSequence>;` | [src]

- `impl<I: Iterator<Item = u16>> Iterator for DecodeUtf16<I> type Item = Result<char, DecodeUtf16Error>;` | [src]

- `impl Iterator for EscapeUnicode type Item = char;` | [src]

- `impl Iterator for core::char::EscapeDefault type Item = char;` | [src]

- `impl Iterator for EscapeDebug type Item = char;` | [src]

- `impl Iterator for ToLowercase type Item = char;` | [src]

- `impl Iterator for ToUppercase type Item = char;` | [src]

- `impl<'a, I: Iterator + ?Sized> Iterator for &'a mut I type Item = I::Item;` | [src]

- `impl<A: Step> Iterator for Range<A> type Item = A;` | [src]

- `impl<A: Step> Iterator for RangeFrom<A> type Item = A;` | [src]

- `impl<A: Step> Iterator for RangeInclusive<A> type Item = A;` | [src]

- `impl<A: Clone> Iterator for Repeat<A> type Item = A;` | [src]

- `impl<A, F: FnMut() -> A> Iterator for RepeatWith<F> type Item = A;` | [src]

- `impl<T> Iterator for Empty<T> type Item = T;` | [src]

- `impl<T> Iterator for Once<T> type Item = T;` | [src]

- `impl<I> Iterator for Rev<I> where I: DoubleEndedIterator, type Item = <I as Iterator>::Item;` | [src]

- `impl<'a, I, T: 'a> Iterator for Cloned<I> where I: Iterator<Item = &'a T>, T: Clone, type Item = T;` | [src]

- `impl<I> Iterator for Cycle<I> where I: Clone + Iterator, type Item = <I as Iterator>::Item;` | [src]

- `impl<I> Iterator for StepBy<I> where I: Iterator, type Item = I::Item;` | [src]

- `impl<A, B> Iterator for Chain<A, B> where A: Iterator, B: Iterator<Item = A::Item>, type Item = A::Item;` | [src]

- `impl<A, B> Iterator for Zip<A, B> where A: Iterator, B: Iterator, type Item = (A::Item, B::Item);` | [src]

- `impl<B, I: Iterator, F> Iterator for Map<I, F> where F: FnMut(I::Item) -> B, type Item = B;` | [src]

- `impl<I: Iterator, P> Iterator for Filter<I, P> where P: FnMut(&I::Item) -> bool, type Item = I::Item;` | [src]

- `impl<B, I: Iterator, F> Iterator for FilterMap<I, F> where F: FnMut(I::Item) -> Option<B>, type Item = B;` | [src]

- `impl<I> Iterator for Enumerate<I> where I: Iterator, type Item = (usize, <I as Iterator>::Item);` | [src]

- `impl<I: Iterator> Iterator for Peekable<I> type Item = I::Item;` | [src]

- `impl<I: Iterator, P> Iterator for SkipWhile<I, P> where P: FnMut(&I::Item) -> bool, type Item = I::Item;` | [src]

- `impl<I: Iterator, P> Iterator for TakeWhile<I, P> where P: FnMut(&I::Item) -> bool, type Item = I::Item;` | [src]

- `impl<I> Iterator for Skip<I> where I: Iterator, type Item = <I as Iterator>::Item;` | [src]

- `impl<I> Iterator for Take<I> where I: Iterator, type Item = <I as Iterator>::Item;` | [src]

- `impl<B, I, St, F> Iterator for Scan<I, St, F> where I: Iterator, F: FnMut(&mut St, I::Item) -> Option<B>, type Item = B;` | [src]

- `impl<I: Iterator, U: IntoIterator, F> Iterator for FlatMap<I, U, F> where F: FnMut(I::Item) -> U, type Item = U::Item;` | [src]

- `impl<I, U> Iterator for Flatten<I> where I: Iterator, U: Iterator, I::Item: IntoIterator<IntoIter = U, Item = U::Item>, type Item = U::Item;` | [src]

- `impl<I> Iterator for Fuse<I> where I: Iterator, type Item = <I as Iterator>::Item;` | [src]

- `impl<I> Iterator for Fuse<I> where I: FusedIterator,` | [src]

- `impl<I: Iterator, F> Iterator for Inspect<I, F> where F: FnMut(&I::Item), type Item = I::Item;` | [src]

- `impl<'a, A> Iterator for core::option::Iter<'a, A> type Item = &'a A;` | [src]

- `impl<'a, A> Iterator for core::option::IterMut<'a, A> type Item = &'a mut A;` | [src]

- `impl<A> Iterator for core::option::IntoIter<A> type Item = A;` | [src]

- `impl<'a, T> Iterator for core::result::Iter<'a, T> type Item = &'a T;` | [src]

- `impl<'a, T> Iterator for core::result::IterMut<'a, T> type Item = &'a mut T;` | [src]

- `impl<T> Iterator for core::result::IntoIter<T> type Item = T;` | [src]

- `impl<'a, T> Iterator for core::slice::Iter<'a, T> type Item = &'a T;` | [src]

- `impl<'a, T> Iterator for core::slice::IterMut<'a, T> type Item = &'a mut T;` | [src]

- `impl<'a, T, P> Iterator for core::slice::Split<'a, T, P> where P: FnMut(&T) -> bool, type Item = &'a [T];` | [src]

- `impl<'a, T, P> Iterator for SplitMut<'a, T, P> where P: FnMut(&T) -> bool, type Item = &'a mut [T];` | [src]

- `impl<'a, T, P> Iterator for core::slice::RSplit<'a, T, P> where P: FnMut(&T) -> bool, type Item = &'a [T];` | [src]

- `impl<'a, T, P> Iterator for RSplitMut<'a, T, P> where P: FnMut(&T) -> bool, type Item = &'a mut [T];` | [src]

- `impl<'a, T, P> Iterator for core::slice::SplitN<'a, T, P> where P: FnMut(&T) -> bool, type Item = &'a [T];` | [src]

- `impl<'a, T, P> Iterator for core::slice::RSplitN<'a, T, P> where P: FnMut(&T) -> bool, type Item = &'a [T];` | [src]

- `impl<'a, T, P> Iterator for SplitNMut<'a, T, P> where P: FnMut(&T) -> bool, type Item = &'a mut [T];` | [src]

- `impl<'a, T, P> Iterator for RSplitNMut<'a, T, P> where P: FnMut(&T) -> bool, type Item = &'a mut [T];` | [src]

- `impl<'a, T> Iterator for Windows<'a, T> type Item = &'a [T];` | [src]

- `impl<'a, T> Iterator for Chunks<'a, T> type Item = &'a [T];` | [src]

- `impl<'a, T> Iterator for ChunksMut<'a, T> type Item = &'a mut [T];` | [src]

- `impl<'a, T> Iterator for ExactChunks<'a, T> type Item = &'a [T];` | [src]

- `impl<'a, T> Iterator for ExactChunksMut<'a, T> type Item = &'a mut [T];` | [src]

- `impl<'a> Iterator for Utf8LossyChunksIter<'a> type Item = Utf8LossyChunk<'a>;` | [src]

- `impl<'a> Iterator for Chars<'a> type Item = char;` | [src]

- `impl<'a> Iterator for CharIndices<'a> type Item = (usize, char);` | [src]

- `impl<'a> Iterator for Bytes<'a> type Item = u8;` | [src]

- `impl<'a, P: Pattern<'a>> Iterator for core::str::Split<'a, P> type Item = &'a str;` | [src]

- `impl<'a, P: Pattern<'a>> Iterator for core::str::RSplit<'a, P> where P::Searcher: ReverseSearcher<'a>, type Item = &'a str;` | [src]

- `impl<'a, P: Pattern<'a>> Iterator for SplitTerminator<'a, P> type Item = &'a str;` | [src]

- `impl<'a, P: Pattern<'a>> Iterator for RSplitTerminator<'a, P> where P::Searcher: ReverseSearcher<'a>, type Item = &'a str;` | [src]

- `impl<'a, P: Pattern<'a>> Iterator for core::str::SplitN<'a, P> type Item = &'a str;` | [src]

- `impl<'a, P: Pattern<'a>> Iterator for core::str::RSplitN<'a, P> where P::Searcher: ReverseSearcher<'a>, type Item = &'a str;` | [src]

- `impl<'a, P: Pattern<'a>> Iterator for MatchIndices<'a, P> type Item = (usize, &'a str);` | [src]

- `impl<'a, P: Pattern<'a>> Iterator for RMatchIndices<'a, P> where P::Searcher: ReverseSearcher<'a>, type Item = (usize, &'a str);` | [src]

- `impl<'a, P: Pattern<'a>> Iterator for Matches<'a, P> type Item = &'a str;` | [src]

- `impl<'a, P: Pattern<'a>> Iterator for RMatches<'a, P> where P::Searcher: ReverseSearcher<'a>, type Item = &'a str;` | [src]

- `impl<'a> Iterator for Lines<'a> type Item = &'a str;` | [src]

- `impl<'a> Iterator for LinesAny<'a> type Item = &'a str;` | [src]

- `impl<'a> Iterator for SplitWhitespace<'a> type Item = &'a str;` | [src]

- `impl<'a> Iterator for SplitAsciiWhitespace<'a> type Item = &'a str;` | [src]

- `impl<'a> Iterator for EncodeUtf16<'a> type Item = u16;` | [src]

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