Is there a built-in function that converts a number to a string in any base?

Question

I want to replace the inner match statement and work for all values up to when the alphabet runs out. I know I can write it myself, but I want to use built-in functions.

fn convert(inp: u32, out: u32, numb: &String) -> Result<String, String> {
    match isize::from_str_radix(numb, inp) {
        Ok(a) => match out {
            2 => Ok(format!("{:b}", a)),
            8 => Ok(format!("{:o}", a)),
            16 => Ok(format!("{:x}", a)),
            10 => Ok(format!("{}", a)),
            0 | 1 => Err(format!("No base lower than 2!")),
            _ => Err(format!("printing in this base is not supported")),
        },
        Err(e) => Err(format!(
            "Could not convert {} to a number in base {}.\n{:?}\n",
            numb, inp, e
        )),
    }
}

Answer 1

For now, you cannot do it using the standard library, but you can:

• use my crate radix_fmt

• or roll your own implementation:

  fn format_radix(mut x: u32, radix: u32) -> String {
      let mut result = vec![];
  
      loop {
          let m = x % radix;
          x = x / radix;
  
          // will panic if you use a bad radix (< 2 or > 36).
          result.push(std::char::from_digit(m, radix).unwrap());
          if x == 0 {
              break;
          }
      }
      result.into_iter().rev().collect()
  }
  
  fn main() {
      assert_eq!(format_radix(1234, 10), "1234");
      assert_eq!(format_radix(1000, 10), "1000");
      assert_eq!(format_radix(0, 10), "0");
  }
  

Answer 2

If you wanted to eke out a little more performance, you can create a struct and implement Display or Debug for it. This avoids allocating a String. For maximum over-engineering, you can also have a stack-allocated array instead of the Vec.

Here is Boiethios' answer with these changes applied:

struct Radix {
    x: i32,
    radix: u32,
}

impl Radix {
    fn new(x: i32, radix: u32) -> Result<Self, &'static str> {
        if radix < 2 || radix > 36 {
            Err("Unnsupported radix")
        } else {
            Ok(Self { x, radix })
        }
    }
}

use std::fmt;

impl fmt::Display for Radix {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let mut x = self.x;
        // Good for binary formatting of `u128`s
        let mut result = ['\0'; 128];
        let mut used = 0;
        let negative = x < 0;
        if negative {
            x*=-1;
        }
        let mut x = x as u32;
        loop {
            let m = x % self.radix;
            x /= self.radix;

            result[used] = std::char::from_digit(m, self.radix).unwrap();
            used += 1;

            if x == 0 {
                break;
            }
        }

        if negative {
            write!(f, "-")?;
        }

        for c in result[..used].iter().rev() {
            write!(f, "{}", c)?;
        }

        Ok(())
    }
}

fn main() {
    assert_eq!(Radix::new(1234, 10).to_string(), "1234");
    assert_eq!(Radix::new(1000, 10).to_string(), "1000");
    assert_eq!(Radix::new(0, 10).to_string(), "0");
}

This could still be optimized by:

• creating an ASCII array instead of a char array
• not zero-initializing the array

Since these avenues require unsafe or an external crate like arraybuf, I have not included them. You can see sample code in internal implementation details of the standard library.