# 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 {
// We can refer to this type using Self::Item
type Item = u32;

// Here, we define the sequence using `.curr` and `.next`.
// The return type is `Option<T>`:
//     * When the `Iterator` is finished, `None` is returned.
//     * Otherwise, the next value is wrapped in `Some` and returned.
// We use Self::Item in the return type, so we can change
// the type without having to update the function signatures.
fn next(&mut self) -> Option<Self::Item> {
let current = self.curr;

self.curr = self.next;
self.next = current + self.next;

// Since there's no endpoint to a Fibonacci sequence, the `Iterator`
// will never return `None`, and `Some` is always returned.
Some(current)
}
}

// 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);
}
}
``````