List Comprehension in Rust

Question

In python if I have an array like [2, 4, 6] I can do the following

old_arr = [2, 4, 6]
new_arr = [x*2 for x in old_arr]

Is there a similar attribute in rust that will let me modify an array before returning it?

Answer 1

In Rust, iterators are used instead of comprehensions.

A very common pattern is .iter().map(...).collect().

• .iter() creates an iterator from a iterable data structure
• .map() creates a new iterator over modified elements
• .collect() stores all the elements back in a data structure.

For example, to mimic your case:

fn main() {
    let old_arr = vec![2, 4, 6];
    let new_arr: Vec<i32> = old_arr.iter().map(|x| 2 * x).collect();
    println!("old_arr: {:?}", old_arr);
    println!("new_arr: {:?}", new_arr);
}

old_arr: [2, 4, 6]
new_arr: [4, 8, 12]

|x| 2 * x is a closure.

Note that iterators in Rust are extremely fast, as they are mostly zero-cost abstractions.

---

In your special case, you could, however, also write:

fn main() {
    let old_arr = [2, 4, 6];
    let new_arr = old_arr.map(|x| 2 * x);
    println!("old_arr: {:?}", old_arr);
    println!("new_arr: {:?}", new_arr);
}

old_arr: [2, 4, 6]
new_arr: [4, 8, 12]

But that only works on arrays of constant size. For the general usecase, use vectors instead.