How can I convert a hex string to a u8 slice?

Question

I have a string that looks like this "090A0B0C" and I would like to convert it to a slice that looks something like this [9, 10, 11, 12]. How would I best go about doing that?

I don't want to convert a single hex char tuple to a single integer value. I want to convert a string consisting of multiple hex char tuples to a slice of multiple integer values.

Answer 1

You can also implement hex encoding and decoding yourself, in case you want to avoid the dependency on the hex crate:

use std::{fmt::Write, num::ParseIntError};

pub fn decode_hex(s: &str) -> Result<Vec<u8>, ParseIntError> {
    (0..s.len())
        .step_by(2)
        .map(|i| u8::from_str_radix(&s[i..i + 2], 16))
        .collect()
}

pub fn encode_hex(bytes: &[u8]) -> String {
    let mut s = String::with_capacity(bytes.len() * 2);
    for &b in bytes {
        write!(&mut s, "{:02x}", b);
    }
    s
}

Note that the decode_hex() function panics if the string length is odd. I've made a version with better error handling and an optimised encoder available on the playground.

Answer 2

You could use the hex crate for that. The decode function looks like it does what you want:

extern crate hex;

fn main() {
    let input = "090A0B0C";

    let decoded = hex::decode(input).expect("Decoding failed");

    println!("{:?}", decoded);
}

The above will print [9, 10, 11, 12]. Note that decode returns a heap allocated Vec<u8>, if you want to decode into an array you'd want to use the decode_to_slice function, which is not yet released on crates.io or the FromHex trait:

extern crate hex;

use hex::FromHex;

fn main() {
    let input = "090A0B0C";

    let decoded = <[u8; 4]>::from_hex(input).expect("Decoding failed");

    println!("{:?}", decoded);
}