Rust float formatted as integer by generics

Question

I have the following Rust code:

#[derive(Debug)]
struct Point<T, U> {
    x: T,
    y: U
}

impl<T: std::fmt::Display, U: std::fmt::Display> Point<T, U> {
    fn print(&self) {
        println!("{} {}", &self.x, &self.y );
    }
}

impl<X1, Y1> Point<X1, Y1> {
    fn mixup<X2, Y2>(self, other: Point<X2, Y2>) -> Point<X1, Y2> {
        Point {
            x: self.x,
            y: other.y,
        }
    }
}

fn main() {
    let integer = Point {x: 1, y:2};
    let float = Point{x: 2.0, y:3.0};

    let floaty = integer.mixup(float);

    println!("{:?}", floaty); // output: Point { x: 1, y: 3.0 } - OK
    floaty.print(); // output: 1 3 - NOT OK
}

Why does floaty.print() coerce 3.0 to 3? Is there anything wrong with how the print method is implemented?

Note that println!("{}", 3.0f64) also prints 3 so this is just how the Display implementation of floats works. — apilat
– apilat, Commented Aug 31, 2022 at 12:43
If you use {:?} in your print() function, you should see the decimal point. — prog-fh
– prog-fh, Commented Aug 31, 2022 at 12:46
"{:?}" marker formats arguments in Debug mode, "{}" marker formats arguments in Display mode: and their implementations: doc.rust-lang.org/src/core/fmt/float.rs.html#193, for other kinds of formats you can check this link: doc.rust-lang.org/std/fmt/index.html#traits — Ömer Erden
– Ömer Erden, Commented Aug 31, 2022 at 12:49

Cerberus · Accepted Answer · 2022-08-31 12:46:46Z

5

Minimized example:

fn main() {
    println!("{} {:?}", 3.0, 3.0);
}

(Playground)

This prints 3 3.0, i.e. floats which don't have a fractional part are formatted via Display as integers. That's not something wrong with print function - just the fact that Display implies human-readable output, and for human-readable output it is chosen to print integers where possible.

answered Aug 31, 2022 at 12:46

Cerberus

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1 Comment

Finomnis Over a year ago

Additional remark: if you want a specific precision shown, println expects you to specify that explicitly: stackoverflow.com/questions/26576889/…

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