Trait std::iter::Iterator1.0.0 [−] [src]

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

    fn size_hint(&self) -> (usize, Option<usize>) { ... }
    fn count(self) -> usize { ... }
    fn last(self) -> Option<Self::Item> { ... }
    fn nth(&mut self, n: usize) -> Option<Self::Item> { ... }
    fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter> where U: IntoIterator<Item=Self::Item> { ... }
    fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter> where U: IntoIterator { ... }
    fn map<B, F>(self, f: F) -> Map<Self, F> where F: FnMut(Self::Item) -> B { ... }
    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 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 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 B: Default + Extend<Self::Item>, F: FnMut(&Self::Item) -> bool { ... }
    fn fold<B, F>(self, init: B, f: F) -> B where F: FnMut(B, Self::Item) -> B { ... }
    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 position<P>(&mut self, predicate: P) -> Option<usize> where P: FnMut(Self::Item) -> bool { ... }
    fn rposition<P>(&mut self, predicate: P) -> Option<usize> where P: FnMut(Self::Item) -> bool,
              Self: ExactSizeIterator + DoubleEndedIterator { ... }
    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 B: Ord, F: FnMut(&Self::Item) -> B { ... }
    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 B: Ord, F: FnMut(&Self::Item) -> B { ... }
    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 FromA: Default + Extend<A>,
              FromB: Default + Extend<B>,
              Self: Iterator<Item=(A, B)> { ... }
    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 partial_cmp<I>(self, other: I) -> Option<Ordering> where I: IntoIterator, Self::Item: PartialOrd<I::Item> { ... }
    fn eq<I>(self, other: I) -> bool where I: IntoIterator, Self::Item: PartialEq<I::Item> { ... }
    fn ne<I>(self, other: I) -> bool where I: IntoIterator, Self::Item: PartialEq<I::Item> { ... }
    fn lt<I>(self, other: I) -> bool where I: IntoIterator, Self::Item: PartialOrd<I::Item> { ... }
    fn le<I>(self, other: I) -> bool where I: IntoIterator, Self::Item: PartialOrd<I::Item> { ... }
    fn gt<I>(self, other: I) -> bool where I: IntoIterator, Self::Item: PartialOrd<I::Item> { ... }
    fn ge<I>(self, other: I) -> bool where I: IntoIterator, Self::Item: PartialOrd<I::Item> { ... }
}

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

The type of the elements being iterated over.

Required Methods

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

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 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 one 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
let iter = 0..;

assert_eq!((0, None), iter.size_hint());

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);

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));

Returns the nth element of the iterator.

Note that all preceding elements will be consumed (i.e. discarded).

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

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);

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);

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);

'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.

When either iterator returns None, all further calls to next() will return None.

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]);

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);

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);
}

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);

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);

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);

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);

of these layers.

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().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 Nones, then filter_map() is much, much nicer to use.

Examples

Basic usage:

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

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

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

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

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

let mut iter = a.iter()
                .map(|s| s.parse().ok())
                .filter(|s| s.is_some());

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

There's an extra layer of Some in there.

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);

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 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);

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);

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);

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);

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);

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);

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);

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]);

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.

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);

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);

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);

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;

    // the value passed on to the next iteration
    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);

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.

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");

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);

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 much more common for inspect() to be used as a debugging tool than to exist in your final code, but never say never.

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:

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

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);

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 chars. 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);

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() 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);

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]);

An iterator adaptor 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.

Examples

Basic usage:

let a = [1, 2, 3];

// the sum of all of the elements of a
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.

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);

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));

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));

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));

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];

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));

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));

Returns the maximum element of an iterator.

If several elements are equally maximum, the last element is returned.

Examples

Basic usage:

let a = [1, 2, 3];

assert_eq!(a.iter().max(), Some(&3));

Returns the minimum element of an iterator.

If several elements are equally minimum, the first element is returned.

Examples

Basic usage:

let a = [1, 2, 3];

assert_eq!(a.iter().min(), Some(&1));

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

If several elements are equally maximum, the last element is returned.

Examples

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

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.

Examples

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

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

If several elements are equally minimum, the first element is returned.

Examples

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

Examples

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

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);

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]);

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]);

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));

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);

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);

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

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

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

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

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

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

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

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

Implementors