Explain this code!

[Amigo]

fn main() {
    let num_vec = vec![10, 9, 8];

    num_vec
        .iter()
        .enumerate()
        .map(|(index, number)| println!("Index number {} has number {}", index, number));

}

1.how this code works?
2.what happen when calling .iter method
3.what happen when calling .enumarate method
4.what happen when calling .map method

[Cerber-Ursi]

1. Please check the pinned topic and format your post accordingly.
2. This code doesn't "work", in that it doesn't do anything when run, as you can see in the playground. Compiler even hints you on the reason why. And by "doesn't do anything" I mean, literally - in release mode it compiles to nothing.
3. For other questions, see: Vec::iter, Iterator::enumerate, Iterator::map.

[quinedot]

@Amigo

num_vec
    .iter()
//  ^^^^^^^ Returns a lazy iterator over references to the values in `num_vec`
    .enumerate()
//  ^^^^^^^^^^^^ Wraps iterator in a new one that returns (0, _), (1, _),
//               etc. for each value out of the original iterator
    .map(|(index, number)| println!("Index number {} has number {}", index, number));
//  ^^^^ Wraps _that_ iterator in a new one that maps each value out of
//       the iterator to the results of the closure.  The result of the
//       closure is going to be `()` every time because that's what
//       `println!()` returns.

But at the top, you didn't save the iterator anywhere, or make it iterate, so it does nothing. You create a lazy, nested iterator and then throw it away. What you probably wanted:

    for (index, number) in num_vec.iter().enumerate() {
        println!("Index number {} has number {}", index, number);
    }

Or

    num_vec.iter().enumerate().for_each(|(index, number)| {
        println!("Index number {} has number {}", index, number);
    });

Both of these consume the iterator and make it iterate over all the values.

[H2CO3]

@Amigo

what happen when calling .iter method
3.what happen when calling .enumarate method
4.what happen when calling .map method

Please read the documentation.