An iterator gives you items one at a time, like drawing cards from a deck. Call `.iter()` on a vector when you want an iterator of references: you can read its items without taking the values out of the vector. section 8.2
Choose the method that matches what you need to do with each item:
- `.iter()` gives references for reading items while keeping the original vector. section 8.2
- `.iter_mut()` gives mutable references for changing items; `.into_iter()` gives values instead of references. section 8.2
Make a new vector without using up the old one
`let numbers = vec![1, 2, 3];` `let increased = numbers.iter().map(|x| x + 1).collect::<Vec<i32>>();` `increased` is `[2, 3, 4]`, and `numbers` is still available. Here `.map()` prepares a change for each item; `.collect()` makes the new vector. section 8.2 section 8.3.2
fn main() {
let my_vec = vec!['a', 'b', '거', '柳']; #1
let mut my_vec_iter = my_vec.iter(); #2
assert_eq!(my_vec_iter.next(), Some(&'a')); #3
assert_eq!(my_vec_iter.next(), Some(&'b'));
assert_eq!(my_vec_iter.next(), Some(&'거'));
assert_eq!(my_vec_iter.next(), Some(&'柳'));
assert_eq!(my_vec_iter.next(), None); #4
assert_eq!(my_vec_iter.next(), None); #5
}
Follow successive `.next()` calls: each item arrives as a reference inside `Some`, and the exhausted iterator returns `None`. section 8.2
Calling `.map()` after `.iter()` does not run the changes immediately. The iterator waits until something uses it, such as `.collect()`. If a mapped iterator is left unused, the compiler warns you. section 8.3.2
