How do I pass an array to a function in Rust and change its content?

Question

I want to pass an array to a function and change the content inside it:

fn change_value(mut arr: &[i32]) {     arr[1] = 10; }  fn main() {     let mut arr: [i32; 4] = [1, 2, 3, 4];     change_value(&arr);     println!("this is {}", arr[1]); } 

I'm getting this error:

warning: variable does not need to be mutable  --> src/main.rs:2:17   | 2 | fn change_value(mut arr: &[i32]) {   |                 ----^^^   |                 |   |                 help: remove this `mut`   |   = note: `#[warn(unused_mut)]` on by default  error[E0594]: cannot assign to `arr[_]` which is behind a `&` reference  --> src/main.rs:3:5   | 2 | fn change_value(mut arr: &[i32]) {   |                          ------ help: consider changing this to be a mutable reference: `&mut [i32]` 3 |     arr[1] = 10;   |     ^^^^^^^^^^^ `arr` is a `&` reference, so the data it refers to cannot be written  warning: variable does not need to be mutable  --> src/main.rs:7:9   | 7 |     let mut arr: [i32; 4] = [1, 2, 3, 4];   |         ----^^^   |         |   |         help: remove this `mut` 

I've been searching around, but I can't find anything.

Answer 1

Rust references (denoted by the & sign) are of two kinds: immutable (&T) and mutable (&mut T). In order to change the value behind the reference, this reference has to be mutable, so you need to:

1. accept &mut [i32] as the function argument, not &[i32]
2. pass &mut arr to the function, not &arr:

fn change_value(arr: &mut [i32]) {     arr[1] = 10; }  fn main() {     let mut arr: [i32; 4] = [1, 2, 3, 4];     change_value(&mut arr);     println!("this is {}", arr[1]); } 

You don't need mut arr in change_value's argument because mut there denotes mutability of that variable, not of the data it points to. With mut arr: &[i32] you can reassign arr itself (for it to point to a different slice), but you can't change the data it references.

If you wanted to accept an array instead of a slice, you could also do that:

fn change_value(arr: &mut [i32; 4]) {     arr[1] = 10; } 

See also:

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