How to convert a `f64` to a `f32`?

Question

How can I convert a f64 into the closest f32?

There is neither a From nor a TryFrom implementation, presumably because such implementations are only provided for lossless conversions. I also searched for references to f32 in the f64 docs and vice versa, and found nothing.

    let double: f64 = 0.;

    // doesn't work
    // let single: f32 = double.into();
    // let single: f32 = double.try_into().unwrap();

Answer 1

In this case, we can use the as keyword for casting. It is usually not desired for integer conversions because "casting from a larger integer to a smaller integer (e.g. u32 -> u8) will truncate", and this is usually not wanted. For f64 to f32, however, it's very sensible:

Casting from an f64 to an f32 will produce the closest possible f32

• if necessary, rounding is according to roundTiesToEven mode
• on overflow, infinity (of the same sign as the input) is produced

Rust Reference

    let double: f64 = 42.;
    dbg!(double as f32);

[src/main.rs:13] double as f32 = 42.0

If an overflow to infinity is not desired and you want to panic instead, the simplest solution might be to check for is_finite:

fn f64_to_f32(x: f64) -> f32 {
    let y = x as f32;
    assert_eq!(
        x.is_finite(),
        y.is_finite(),
        "f32 overflow during conversion"
    );
    y
}

fn main() {
    dbg!(f64_to_f32(42_f64));
    // dbg!(f64_to_f32(f64::MAX / 10.)); // panics
    dbg!(f64_to_f32(f64::NAN));
    dbg!(f64_to_f32(f64::INFINITY));
    dbg!(f64_to_f32(f64::NEG_INFINITY));
}

[src/main.rs:2] f64_to_f32(42_f64) = 42.0
[src/main.rs:4] f64_to_f32(f64::NAN) = NaN
[src/main.rs:5] f64_to_f32(f64::INFINITY) = inf
[src/main.rs:6] f64_to_f32(f64::NEG_INFINITY) = -inf