Negative Zero in Rust Pattern Matching

Question

I'm reading the Rust tutorial on rust-lang.org, and I came across the following code:

use std::f64;
fn angle(vector: (f64, f64)) -> f64 {
    let pi = f64::consts::PI;
    match vector {
      (0.0, y) if y < 0.0 => 1.5 * pi,
      (0.0, _) => 0.5 * pi,
      (x, y) => (y / x).atan()
    }
}

Normally for atan2 I would expect to see special cases for -0 and 0, for example to implement cases like these:

atan2(+0, +0) = +0
atan2(+0, −0) = +π
atan2(−0, +0) = −0
atan2(−0, −0) = −π

I'm not saying the tutorial should include those examples. After all it's just a demonstration of the match structure.

I'm just wondering if 0.0 would match -0.0 as well? Or if the two would be recognized as disjoint values?

Vladimir Matveev · Accepted Answer · 2014-07-09 19:43:44Z

5

Well, you can just try and test it! Here is a link to playtest: http://is.gd/NiS0gF

Apparently, 0.0 matches -0.0. This code:

fn main() {
    let x = -0.0f64;
    match x {
        0.0f64 => println!("Zero!"),
        _ => println!("Something else!")
    }
}

when run on playtest prints "Zero!", so 0.0 and -0.0 are the same thing from match perspective. And this is quite natural, provided that

fn main() {
    println!("{}", -0.0f64 == 0.0f64);
}

also prints "true".

answered Jul 9, 2014 at 19:43

Vladimir Matveev

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1 Comment

Thomas Ahle Over a year ago

That makes sense, these kinds of comparisons are better done using specialized floating point functions.

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