CODEWITHSUNDEEP
rustmutable
Mike Land
Mike Land

Reputation: 506

Re-assignment of immutable variable that should be a mutable borrow in a loop

I am trying to modify a mutable sums: Vec<i64> while iterating over it. The loop code is as follows:

for (j, &mut sum) in sums.iter_mut().enumerate() {
    if !(j == i) {
        sum += n;
    }
}

And here is the error I get:

error[E0384]: re-assignment of immutable variable `sum`
  --> mini_max_sum.rs:27:17
   |
25 |         for (j, &mut sum) in sums.iter_mut().enumerate() {
   |                      --- first assignment to `sum`
26 |             if !(j == i) {
27 |                 sum += n;
   |                 ^^^^^^^^ re-assignment of immutable variable

This seems totally arcane to me. Rust lets me mutably borrow sum from sums, but the compiler prevents me from actually modifying it. Omitting .enumerate() does not even alter the resulting error code.

I would like to know how to fix the loop.

Upvotes: 1

Views: 1926

Answers (1)

Shepmaster
Shepmaster

Reputation: 430671

I don't know why you decided to add &mut to the pattern for the loop variable, but that's the problem. You need to take the mutable reference directly and then dereference it when you increment it:

fn main() {
    let mut sums = vec![1, 2, 3];
    let i = 0;
    let n = 0;

    for (j, sum) in sums.iter_mut().enumerate() {
        if j != i {
            *sum += n;
        }
    }
}

With &mut in the pattern, you are actually destructuring the variable and removing the mutable reference. If you print the type of your sum variable, you'll see that it's an i64.

Upvotes: 6

Related Questions