Composable external iteration. If you’ve found yourself with a collection of some kind, and needed to perform an operation on the elements of said collection, you’ll quickly run into ‘iterators’. Iterators are heavily used in idiomatic code, so it’s worth becoming familiar with them. Before explaining more, let’s talk about how this module is structured:

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Organization

This module is largely organized by type:
  • Traits are the core portion: these traits define what kind of iterators exist and what you can do with them. The methods of these traits are worth putting some extra study time into.
  • Functions provide some helpful ways to create some basic iterators.
  • Structs are often the return types of the various methods on this module’s traits. You’ll usually want to look at the method that creates the struct, rather than the struct itself. For more detail about why, see ‘Implementing Iterator’.
That’s it! Let’s dig into iterators.

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Iterator

The heart and soul of this module is the Iterator trait. The core of Iterator looks like this: 
trait Iterator {
    type Item;
    fn next(ref self) -> Option;
}
An iterator has a method, next, which when called, returns an Option. Calling next will return Some(Item) as long as there are elements, and once they’ve all been exhausted, will return None to indicate that iteration is finished. Iterator’s full definition includes a number of other methods as well, but they are default methods, built on top of next, and so you get them for free. Iterators are also composable, and it’s common to chain them together to do more complex forms of processing. See the Adapters section below for more details.

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Forms of iteration

There is currently only one common method which can create iterators from a collection:
  • into_iter(), which iterates over T.

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Implementing Iterator

Creating an iterator of your own involves two steps: creating a struct to hold the iterator’s state, and then implementing Iterator for that struct. This is why there are so many structs in this module: there is one for each iterator and iterator adapter. Let’s make an iterator named Counter which counts from 1 to 5: 
// First, the struct:

/// An iterator which counts from one to five
#[derive(Drop)]
struct Counter {
    count: usize,
}

// we want our count to start at one, so let's add a new() method to help.
// This isn't strictly necessary, but is convenient. Note that we start
// `count` at zero, we'll see why in `next()`'s implementation below.
#[generate_trait]
impl CounterImpl of CounterTrait {
    fn new() -> Counter {
        Counter { count: 0 }
    }
}

// Then, we implement `Iterator` for our `Counter`:

impl CounterIter of core::iter::Iterator {
    // we will be counting with usize
    type Item = usize;

    // next() is the only required method
    fn next(ref self: Counter) -> Option {
        // Increment our count. This is why we started at zero.
        self.count += 1;

        // Check to see if we've finished counting or not.
        if self.count  next = val,
                None => {
                    break;
                },
            };
            let x = next;
            let () = { println!("{x}"); };
        };
    result
}
First, we call into_iter() on the value. Then, we match on the iterator that returns, calling next over and over until we see a None. At that point, we break out of the loop, and we’re done iterating. There’s one more subtle bit here: the core library contains an interesting implementation of IntoIterator:
(only-for-syntax-highlight)
impl IteratorIntoIterator> of IntoIterator
In other words, all Iterators implement IntoIterator, by just returning themselves. This means two things:
  1. If you’re writing an Iterator, you can use it with a for loop.
  2. If you’re creating a collection, implementing IntoIterator for it will allow your collection to be used with the for loop.

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Adapters

Functions which take an Iterator and return another Iterator are often called ‘iterator adapters’, as they’re a form of the ‘adapter pattern’. Common iterators adapters include map, enumerate and zip.

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Laziness

Iterators (and iterator adapters) are lazy. This means that just creating an iterator doesn’t do a whole lot. Nothing really happens until you call next. This is sometimes a source of confusion when creating an iterator solely for its side effects. For example, the map method calls a closure on each element it iterates over: 
let v = array![1, 2, 3, 4, 5];
let _ = v.into_iter().map(|x| println!("{x}"));
This will not print any values, as we only created an iterator, rather than using it. The compiler will warn us about this kind of behavior: 
Unhandled `#[must_use]` type

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Modules

adapters—
traits—

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Re-exports:

  • ​
    Free functions

zipConverts the arguments to iterators and zips them. See the documentation of Iterator::zip for more…
PeekableTrait—
ExtendExtend a collection with the contents of an iterator. Iterators produce a series of values, and collections can also be thought of as a series of values. The Extend…
FromIteratorConversion from an Iterator . By implementing FromIterator for a type, you define how it will be…
IntoIteratorConversion into an Iterator . By implementing IntoIterator for a type, you define how it will be…
IteratorA trait for dealing with iterators. This is the main iterator trait. For more about the concept of iterators generally, please see the module-level documentation…