# 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_iterator()` 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<T>`:
        //     * When the `Iterator` is finished, `None` is returned.
        //     * Otherwise, the next value is wrapped in `Some` and returned.
        fn next(&mut self) -> Option<u32> {
            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: 1, 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);
        }
    }