Arrays and Functions

Value Semantics

Arrays are passed by copy. The function receives its own copy — the caller’s array is not affected:

fn modify_copy(mut arr: [i32; 3]) -> i32 {
    arr[0] = 99;
    return arr[0];
}

pub fn original_unchanged() -> i32 {
    let data: [i32; 3] = [1, 2, 3];
    let ignored: i32 = modify_copy(data);
    return data[0];  // returns 1, not 99
}

Here, mut attributes the function parameter so that arr can be modified within the function. However, this does not affect the caller’s data array, which remains unchanged.

Returning Arrays

Functions can return fixed-size arrays:

pub fn make_sequence() -> [i32; 3] {
    return [10, 20, 30];
}

pub fn use_sequence() -> i32 {
    let arr: [i32; 3] = make_sequence();
    return arr[1];
}

Array return values must be assigned to a variable first — make_sequence()[0] is not supported.