How to swap values from a multidimensional array in Rust?

Question

Using a matrix (multidimensional fixed sized array) defined as [[f64; 4]; 4], is it possible to swap two values?

std::mem::swap(&mut matrix[i][k], &mut matrix[k][l]);

Gives the error:

error[E0499]: cannot borrow `matrix[..][..]` as mutable more than once at a time
   --> math_matrix.rs:100
    |
100 | std::mem::swap(&mut matrix[i][j], &mut matrix[k][l]);
    |                     ------------       ^^^^^^^^^^^^^^^- first borrow ends here
    |                     |                  |
    |                     |                  second mutable borrow occurs here
    |                     first mutable borrow occurs here

---

The only way I could figure out how to accomplish this was to use a temp value, e.g.:

macro_rules! swap_value {
    ($a_ref:expr, $b_ref:expr) => {
        {
            let t = *$a_ref;
            *$a_ref = *$b_ref;
            *$b_ref = t;
        }
    }
}

Then use:

swap_value!(&mut matrix[i][k], &mut matrix[maxj][k]);

Is there a better alternative?

Answer 1

You will need to split the outside layers using split_at_mut. This creates two disjoint mutable references which can then be individually swapped:

use std::mem;

fn main() {
    let mut matrix = [[42.0f64; 4]; 4];

    // instead of 
    // mem::swap(&mut matrix[0][1], &mut b[2][3]);

    let (x, y) = matrix.split_at_mut(2);
    mem::swap(&mut x[0][1], &mut y[0][3]);
    //                             ^-- Note that this value is now `0`!
}

In the most general case, you'll likely need to add some code to figure out where to split and in which order.