How to modify the last item of an array?

Question

Since arr is borrowed as mutable, the length of arr can't be gotten by calling len(). I'm stuck here, what's the right way to do it?

fn double_last(arr: &mut[i32]) -> &i32 {
    let last = &mut arr[arr.len() - 1];  // borrow checker error.
    //let last = &mut arr[3];            // fine
    *last *= 2;
    last
}

fn main() {
    let mut a = [1,2,3,4];
    println!("{}", double_last(&mut a));
    println!("{:?}", a);
}

Answer 1

If you only need the last, you can use std::slice::last_mut

fn double_last(arr: &mut[i32]) -> &i32 {
    let last = arr.last_mut().unwrap();
    *last *= 2;
    last
}