# 30 Iterators

The Iterator trait is used to implement iterators over collections (like arrays) and lazy value generators.

struct Fibonacci { curr: u32, next: u32, } // Implement 'Iterator' for 'Fibonacci' impl Iterator for Fibonacci { type Item = u32; // The 'Iterator' trait only requires the 'next' method to be defined. The // return type is 'Option<T>', 'None' is returned when the 'Iterator' is // over, otherwise the next value is returned wrapped in 'Some' fn next(&mut self) -> Option<u32> { let new_next = self.curr + self.next; self.curr = self.next; self.next = new_next; // 'Some' is always returned, this is an infinite value generator Some(self.curr) } } // Returns a fibonacci sequence generator fn fibonacci() -> Fibonacci { Fibonacci { curr: 1, next: 1 } } fn main() { // 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()); // The for construct will iterate an 'Iterator' until it returns 'None'. // Every 'Some' value is unwrapped and bound to a variable. println!("Iterate over 0..3 using for"); for i in 0..3 { println!("> {}", i); } // The 'take(n)' method will reduce an iterator to its first 'n' terms, // which is pretty useful for infinite value generators println!("The first four terms of the Fibonacci sequence are: "); for i in fibonacci().take(4) { println!("> {}", i); } // The 'skip(n)' method will shorten 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.as_slice()); for i in array.iter() { println!("> {}", i); } }