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% Iterators

Let's talk about loops.

Remember Rust's for loop? Here's an example:

for x in 0..10 {
    println!("{}", x);
}

Now that you know more Rust, we can talk in detail about how this works. Ranges (the 0..10) are 'iterators'. An iterator is something that we can call the .next() method on repeatedly, and it gives us a sequence of things.

(By the way, a range with two dots like 0..10 is inclusive on the left (so it starts at 0) and exclusive on the right (so it ends at 9). A mathematician would write "[0, 10)". To get a range that goes all the way up to 10 you can write 0...10.)

Like this:

let mut range = 0..10;

loop {
    match range.next() {
        Some(x) => {
            println!("{}", x);
        },
        None => { break }
    }
}

We make a mutable binding to the range, which is our iterator. We then loop, with an inner match. This match is used on the result of range.next(), which gives us a reference to the next value of the iterator. next returns an Option<i32>, in this case, which will be Some(i32) when we have a value and None once we run out. If we get Some(i32), we print it out, and if we get None, we break out of the loop.

This code sample is basically the same as our for loop version. The for loop is a handy way to write this loop/match/break construct.

for loops aren't the only thing that uses iterators, however. Writing your own iterator involves implementing the Iterator trait. While doing that is outside of the scope of this guide, Rust provides a number of useful iterators to accomplish various tasks. But first, a few notes about limitations of ranges.

Ranges are very primitive, and we often can use better alternatives. Consider the following Rust anti-pattern: using ranges to emulate a C-style for loop. Let’s suppose you needed to iterate over the contents of a vector. You may be tempted to write this:

let nums = vec![1, 2, 3];

for i in 0..nums.len() {
    println!("{}", nums[i]);
}

This is strictly worse than using an actual iterator. You can iterate over vectors directly, so write this:

let nums = vec![1, 2, 3];

for num in &nums {
    println!("{}", num);
}

There are two reasons for this. First, this more directly expresses what we mean. We iterate through the entire vector, rather than iterating through indexes, and then indexing the vector. Second, this version is more efficient: the first version will have extra bounds checking because it used indexing, nums[i]. But since we yield a reference to each element of the vector in turn with the iterator, there's no bounds checking in the second example. This is very common with iterators: we can ignore unnecessary bounds checks, but still know that we're safe.

There's another detail here that's not 100% clear because of how println! works. num is actually of type &i32. That is, it's a reference to an i32, not an i32 itself. println! handles the dereferencing for us, so we don't see it. This code works fine too:

let nums = vec![1, 2, 3];

for num in &nums {
    println!("{}", *num);
}

Now we're explicitly dereferencing num. Why does &nums give us references? Firstly, because we explicitly asked it to with &. Secondly, if it gave us the data itself, we would have to be its owner, which would involve making a copy of the data and giving us the copy. With references, we're only borrowing a reference to the data, and so it's only passing a reference, without needing to do the move.

So, now that we've established that ranges are often not what you want, let's talk about what you do want instead.

There are three broad classes of things that are relevant here: iterators, iterator adaptors, and consumers. Here's some definitions:

  • iterators give you a sequence of values.
  • iterator adaptors operate on an iterator, producing a new iterator with a different output sequence.
  • consumers operate on an iterator, producing some final set of values.

Let's talk about consumers first, since you've already seen an iterator, ranges.

Consumers

A consumer operates on an iterator, returning some kind of value or values. The most common consumer is collect(). This code doesn't quite compile, but it shows the intention:

let one_to_one_hundred = (1..101).collect();

As you can see, we call collect() on our iterator. collect() takes as many values as the iterator will give it, and returns a collection of the results. So why won't this compile? Rust can't determine what type of things you want to collect, and so you need to let it know. Here's the version that does compile:

let one_to_one_hundred = (1..101).collect::<Vec<i32>>();

If you remember, the ::<> syntax allows us to give a type hint, and so we tell it that we want a vector of integers. You don't always need to use the whole type, though. Using a _ will let you provide a partial hint:

let one_to_one_hundred = (1..101).collect::<Vec<_>>();

This says "Collect into a Vec<T>, please, but infer what the T is for me." _ is sometimes called a "type placeholder" for this reason.

collect() is the most common consumer, but there are others too. find() is one:

let greater_than_forty_two = (0..100)
                             .find(|x| *x > 42);

match greater_than_forty_two {
    Some(_) => println!("Found a match!"),
    None => println!("No match found :("),
}

find takes a closure, and works on a reference to each element of an iterator. This closure returns true if the element is the element we're looking for, and false otherwise. find returns the first element satisfying the specified predicate. Because we might not find a matching element, find returns an Option rather than the element itself.

Another important consumer is fold. Here's what it looks like:

let sum = (1..4).fold(0, |sum, x| sum + x);

fold() is a consumer that looks like this: fold(base, |accumulator, element| ...). It takes two arguments: the first is an element called the base. The second is a closure that itself takes two arguments: the first is called the accumulator, and the second is an element. Upon each iteration, the closure is called, and the result is the value of the accumulator on the next iteration. On the first iteration, the base is the value of the accumulator.

Okay, that's a bit confusing. Let's examine the values of all of these things in this iterator:

base accumulator element closure result
0 0 1 1
0 1 2 3
0 3 3 6

We called fold() with these arguments:

# (1..4)
.fold(0, |sum, x| sum + x);

So, 0 is our base, sum is our accumulator, and x is our element. On the first iteration, we set sum to 0, and x is the first element of nums, 1. We then add sum and x, which gives us 0 + 1 = 1. On the second iteration, that value becomes our accumulator, sum, and the element is the second element of the array, 2. 1 + 2 = 3, and so that becomes the value of the accumulator for the last iteration. On that iteration, x is the last element, 3, and 3 + 3 = 6, which is our final result for our sum. 1 + 2 + 3 = 6, and that's the result we got.

Whew. fold can be a bit strange the first few times you see it, but once it clicks, you can use it all over the place. Any time you have a list of things, and you want a single result, fold is appropriate.

Consumers are important due to one additional property of iterators we haven't talked about yet: laziness. Let's talk some more about iterators, and you'll see why consumers matter.

Iterators

As we've said before, an iterator is something that we can call the .next() method on repeatedly, and it gives us a sequence of things. Because you need to call the method, this means that iterators can be lazy and not generate all of the values upfront. This code, for example, does not actually generate the numbers 1-99, instead creating a value that merely represents the sequence:

let nums = 1..100;

Since we didn't do anything with the range, it didn't generate the sequence. Let's add the consumer:

let nums = (1..100).collect::<Vec<i32>>();

Now, collect() will require that the range gives it some numbers, and so it will do the work of generating the sequence.

Ranges are one of two basic iterators that you'll see. The other is iter(). iter() can turn a vector into a simple iterator that gives you each element in turn:

let nums = vec![1, 2, 3];

for num in nums.iter() {
   println!("{}", num);
}

These two basic iterators should serve you well. There are some more advanced iterators, including ones that are infinite.

That's enough about iterators. Iterator adaptors are the last concept we need to talk about with regards to iterators. Let's get to it!

Iterator adaptors

Iterator adaptors take an iterator and modify it somehow, producing a new iterator. The simplest one is called map:

(1..100).map(|x| x + 1);

map is called upon another iterator, and produces a new iterator where each element reference has the closure it's been given as an argument called on it. So this would give us the numbers from 2-100. Well, almost! If you compile the example, you'll get a warning:

warning: unused result which must be used: iterator adaptors are lazy and
         do nothing unless consumed, #[warn(unused_must_use)] on by default
(1..100).map(|x| x + 1);
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Laziness strikes again! That closure will never execute. This example doesn't print any numbers:

(1..100).map(|x| println!("{}", x));

If you are trying to execute a closure on an iterator for its side effects, use for instead.

There are tons of interesting iterator adaptors. take(n) will return an iterator over the next n elements of the original iterator. Let's try it out with an infinite iterator:

for i in (1..).take(5) {
    println!("{}", i);
}

This will print

1
2
3
4
5

filter() is an adapter that takes a closure as an argument. This closure returns true or false. The new iterator filter() produces only the elements that the closure returns true for:

for i in (1..100).filter(|&x| x % 2 == 0) {
    println!("{}", i);
}

This will print all of the even numbers between one and a hundred. (Note that, unlike map, the closure passed to filter is passed a reference to the element instead of the element itself. The filter predicate here uses the &x pattern to extract the integer. The filter closure is passed a reference because it returns true or false instead of the element, so the filter implementation must retain ownership to put the elements into the newly constructed iterator.)

You can chain all three things together: start with an iterator, adapt it a few times, and then consume the result. Check it out:

(1..)
    .filter(|&x| x % 2 == 0)
    .filter(|&x| x % 3 == 0)
    .take(5)
    .collect::<Vec<i32>>();

This will give you a vector containing 6, 12, 18, 24, and 30.

This is just a small taste of what iterators, iterator adaptors, and consumers can help you with. There are a number of really useful iterators, and you can write your own as well. Iterators provide a safe, efficient way to manipulate all kinds of lists. They're a little unusual at first, but if you play with them, you'll get hooked. For a full list of the different iterators and consumers, check out the iterator module documentation.