How can I do a mutable borrow in a for loop?

Question

I tried:

fn main() {
    let mut vec = [1, 2, 3];
    
    for mut x in &vec { *x = 3; }
    for mut &x in &vec { x = 3; }
    for mut *x in &vec { x = 3; }
    for mut x in mut &vec { *x = 3; }
    for mut x in &(mut vec) { *x = 3; }
}

None of these work; how should I do it?

I get errors like:

• mut must be attached to each individual binding
• expected identifier, found *
• expected expression, found keyword mut
• cannot assign to *x which is behind a & reference

Answer 1

You may want to re-read The Rust Programming Language, specifically the sections on:

• mutability
• mutable references
• vectors

We can also iterate over mutable references to each element in a mutable vector in order to make changes to all the elements. The for loop in Listing 8-9 will add 50 to each element.

let mut v = vec![100, 32, 57];
for i in &mut v {
    *i += 50;
}

Listing 8-9: Iterating over mutable references to elements in a vector

To change the value that the mutable reference refers to, we have to use the dereference operator (*) to get to the value in i before we can use the += operator.

In addition, you can call the iter_mut method:

let mut v = vec![100, 32, 57];
for i in v.iter_mut() {
    *i += 50;
}

See also:

• What's the difference between placing "mut" before a variable name and after the ":"?
• What is the difference between iter and into_iter?

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Note that your variable is not a vector. It is an array.