The trait bound is not satisfied in Rust

Question

I'm trying to write a Tic Tac Toe game in Rust, but this function for changing a field doesn't work and I don't know what's wrong with it:

fn change_field(mut table: [char; 9], field: i32, player: char) -> bool {
    if field > 0 && field < 10 {
        if table[field - 1] == ' ' {
            table[field - 1] = player;
            return true;
        } else {
            println!("That field isn't empty!");
        }
    } else {
        println!("That field doesn't exist!");
    }

    return false;
}

I'm getting these errors:

src/main.rs:16:12: 16:26 error: the trait bound `[char]: std::ops::Index<i32>` is not satisfied [E0277]
src/main.rs:16         if table[field-1] == ' ' {
                          ^~~~~~~~~~~~~~
src/main.rs:16:12: 16:26 help: run `rustc --explain E0277` to see a detailed explanation
src/main.rs:16:12: 16:26 note: slice indices are of type `usize`
src/main.rs:17:13: 17:27 error: the trait bound `[char]: std::ops::Index<i32>` is not satisfied [E0277]
src/main.rs:17             table[field-1] = player;
                           ^~~~~~~~~~~~~~
src/main.rs:17:13: 17:27 help: run `rustc --explain E0277` to see a detailed explanation
src/main.rs:17:13: 17:27 note: slice indices are of type `usize`
src/main.rs:17:13: 17:27 error: the trait bound `[char]: std::ops::IndexMut<i32>` is not satisfied [E0277]
src/main.rs:17             table[field-1] = player;
                           ^~~~~~~~~~~~~~

In later versions of Rust, I get these errors:

error[E0277]: the trait bound `i32: std::slice::SliceIndex<[char]>` is not satisfied
 --> src/main.rs:3:12
  |
3 |         if table[field - 1] == ' ' {
  |            ^^^^^^^^^^^^^^^^ slice indices are of type `usize` or ranges of `usize`
  |
  = help: the trait `std::slice::SliceIndex<[char]>` is not implemented for `i32`
  = note: required because of the requirements on the impl of `std::ops::Index<i32>` for `[char]`

error[E0277]: the trait bound `i32: std::slice::SliceIndex<[char]>` is not satisfied
 --> src/main.rs:4:13
  |
4 |             table[field - 1] = player;
  |             ^^^^^^^^^^^^^^^^ slice indices are of type `usize` or ranges of `usize`
  |
  = help: the trait `std::slice::SliceIndex<[char]>` is not implemented for `i32`
  = note: required because of the requirements on the impl of `std::ops::Index<i32>` for `[char]`

This is my first project in Rust so I don't have much experience with it. I tried to change the field to u32 too.

Answer 1

The reason is given to you in the notes:

note: slice indices are of type `usize`

slice indices are of type `usize` or ranges of `usize`

You need to cast the i32 value to usize, for example:

table[(field - 1) as usize]

Alternatively, consider using usize as the type of the field variable, if it makes sense in your application.