Rust by Example

Iterators

The Iterator trait is used to implement iterators over collections such as arrays.

The trait requires only a method to be defined for the next element, which may be manually defined in an impl block or automatically defined (as in arrays and ranges).

As a point of convenience for common situations, the for construct turns some collections into iterators using the .into_iter() method.

struct Fibonacci {

curr: u32,

next: u32,

}

// Implement `Iterator` for `Fibonacci`.

// The `Iterator` trait only requires a method to be defined for the `next` element.

impl Iterator for Fibonacci {

type Item = u32;

// Here, we define the sequence using `.curr` and `.next`.

// The return type is `Option`:

//     * When the `Iterator` is finished, `None` is returned.

//     * Otherwise, the next value is wrapped in `Some` and returned.

fn next(&mut self) -> Option {

let new_next = self.curr + self.next;

self.curr = self.next;

self.next = new_next;

// Since there's no endpoint to a Fibonacci sequence, the `Iterator`

// will never return `None`, and `Some` is always returned.

Some(self.curr)

}

}

// Returns a Fibonacci sequence generator

fn fibonacci() -> Fibonacci {

Fibonacci { curr: 0, next: 1 }

}

fn main() {

// `0..3` is an `Iterator` that generates: 0, 1, and 2.

let mut sequence = 0..3;

println!("Four consecutive `next` calls on 0..3");

println!("> {:?}", sequence.next());

println!("> {:?}", sequence.next());

println!("> {:?}", sequence.next());

println!("> {:?}", sequence.next());

// `for` works through an `Iterator` until it returns `None`.

// Each `Some` value is unwrapped and bound to a variable (here, `i`).

println!("Iterate through 0..3 using `for`");

for i in 0..3 {

println!("> {}", i);

}

// The `take(n)` method reduces an `Iterator` to its first `n` terms.

println!("The first four terms of the Fibonacci sequence are: ");

for i in fibonacci().take(4) {

println!("> {}", i);

}

// The `skip(n)` method shortens an `Iterator` by dropping its first `n` terms.

println!("The next four terms of the Fibonacci sequence are: ");

for i in fibonacci().skip(4).take(4) {

println!("> {}", i);

}

let array = [1u32, 3, 3, 7];

// The `iter` method produces an `Iterator` over an array/slice.

println!("Iterate the following array {:?}", &array);

for i in array.iter() {

println!("> {}", i);

}

}

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