How to convert hex string to a float in Rust?

Question

What's the most straightforward way to convert a hex string into a float? (without using 3rd party crates).

Does Rust provide some equivalent to Python's struct.unpack('!f', bytes.fromhex('41973333'))

See this question for Python & Java, mentioning for reference.

Since this can be done in a few lines in other languages, I'd like to know how to do primitive operations without introducing middle-ware. — ideasman42, Apr 30 '17 at 09:51

score 5 · Accepted Answer · answered Apr 30 '17 at 11:06

This is quite easy without external crates:

fn main() {
    // Hex string to 4-bytes, aka. u32
    let bytes = u32::from_str_radix("41973333", 16).unwrap();

    // Reinterpret 4-bytes as f32:
    let float = unsafe { std::mem::transmute::<u32, f32>(bytes) };

    // Print 18.9
    println!("{}", float);
}

Playground link.

score 4 · Answer 2 · edited Oct 12 '17 at 18:56

4

There's f32::from_bits which performs the transmute in safe code. Note that transmuting is not the same as struct.unpack, since struct.unpack lets you specify endianness and has a well-defined IEEE representation.

edited Oct 12 '17 at 18:56

Shepmaster

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answered Apr 30 '17 at 16:12

Veedrac

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The endianness issue can easily be dealt with with [`{from,to}_{be,le}`](https://doc.rust-lang.org/std/primitive.u32.html#method.from_be). `f32::from_bits` also has the advantage to deal with NaN values correctly. – mcarton Apr 30 '17 at 17:50
@mcarton It's not entirely certain that `from_bits` handles NaN more correctly; it's just more conservative. Whether signalling NaNs can actually cause unsafe behaviour is still under debate. – Veedrac Apr 30 '17 at 19:10
@mcarton `from_bits` is now exactly equivalent to a `transmute`. https://github.com/rust-lang/rust/pull/46012 – Veedrac Dec 01 '17 at 02:51

How to convert hex string to a float in Rust?

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