Can I use '<' and '>' in match?

问题

I am trying to do a simple quadratic function that would return number of roots and their values via an enum:

enum QuadraticResult {
    None,
    OneRoot(f32),
    TwoRoots(f32, f32),
}

fn solveQuadratic(a: f32, b: f32, c: f32) -> QuadraticResult {
    let delta = b * b - 4.0 * a * c;
    match delta {
        < 0 => return QuadraticResult::None,
        > 0 => return QuadraticResult::TwoRoots(0.0, 1.0),
        _ => return QuadraticResult::OneRoot(0.0),
    }
}

This doesn't compile as it complains about '<' and '>'. Is there a way to achieve this with match or do I need to use if

回答1:

You can use a match guard, but that feels more verbose than a plain if statement:

return match delta {
    d if d < 0 => QuadraticResult::None,
    d if d > 0 => QuadraticResult::TwoRoots(0.0, 1.0),
    _   => QuadraticResult::OneRoot(0.0),
}

回答2:

If you want to handle the three cases where some value is greater than, equal to or less than another, you can match on an Ordering, which you can obtain by calling cmp (from the Ord trait) or partial_cmp (from the PartialOrd trait).

fn solve_quadratic(a: f32, b: f32, c: f32) -> QuadraticResult {
    let delta = b * b - 4.0 * a * c;
    match delta.partial_cmp(&0.0).expect("I don't like NaNs") {
        Ordering::Less => QuadraticResult::None,
        Ordering::Greater => QuadraticResult::TwoRoots(0.0, 1.0),
        Ordering::Equal => QuadraticResult::OneRoot(0.0),
    }
}

回答3:

You can, but you'll want to create a variable binding when you do it and turn it into an actual expression:

match delta {
    d if d < 0.0 => QuadraticResult::None,
    d if d > 0.0 => QuadraticResult::TwoRoots(0.0, 1.0),
    _ => QuadraticResult::OneRoot(0.0),
}

I'm not sure this is any better than just splitting this into an if statement though.

来源：https://stackoverflow.com/questions/47852269/can-i-use-and-in-match