Why unused_assignment before if statment?

May 2, 2023

I have the following method and I get a unused_assignment warning for let mut res = 1.0:

fn my_func(pld: &f64, lambda: &[f64;NUMF]) -> [f64;NUMF] {
    let mut result = [0.0; NUMF];
    for i in 0..NUMF{
        let pdrel = pld / lambda[i];
        let mut res = 1.0;
        if pdrel >= 0.0 {
            res = 1.0;
        } else {
            res = 1.0 - (7.5 * pdrel.abs()).sqrt();
            if res >= 0.0 {
            } else {
                res = 0.0;
            }
        }
        result[i] = res;
    }
    result
}

I know I can #[allow(unused_assignment)]to avoid from showing. But how could I write more "correct" code to avoid the warning?

>Solution :

You never use the first assignment of res. That is why rustc is complaining. You assign it in all of the branches of the if-else statement below, therefore never reading the default 1.0 you assign it. You can just remove the assignment and rustc will stop giving you the warning:

fn my_func(pld: &f64, lambda: &[f64;NUMF]) -> [f64;NUMF] {
    let mut result = [0.0; NUMF];
    for i in 0..NUMF{
        let pdrel = pld / lambda[i];
        let mut res = 1.0;
        if pdrel >= 0.0 {
            res = 1.0;
        } else {
            res = 1.0 - (7.5 * pdrel.abs()).sqrt();
            if res >= 0.0 {
            } else {
                res = 0.0;
            }
        }
        result[i] = res;
    }
    result
}

Alternatively you can rely on the fact that each block has a return value in Rust, so you can return values from your if-else blocks directly. Then you don’t have to make res mutable, which you may prefer:

fn my_func(pld: &f64, lambda: &[f64;NUMF]) -> [f64;NUMF] {
    let mut result = [0.0; NUMF];
    for i in 0..NUMF{
        let pdrel = pld / lambda[i];
        
        let res = if pdrel >= 0.0 {
            1.0
        } else {
            let tmp = 1.0 - (7.5 * pdrel.abs()).sqrt();
            if tmp >= 0.0 {
                tmp
            } else {
                0.0
            }
        };
        
        result[i] = res;
    }
    result
}