Rounding error with f64::from_str_radix

[TatriX]

Consider the following program:

use num::Num;

fn main() {
    let y = "2.89".parse::<f64>();
    println!("{:?}", y);
    let x = f64::from_str_radix("2.89", 10);
    println!("{:?}", x);
}

It prints:

Ok(2.89)
Ok(2.8899999999999997)

Am I missing anything here or results should be equal?

[cuviper]

Well, 2.89 is not a number that can be precisely represented in binary floating point. If you force it to print more digits like `"{:.30?}", you'll get:

Ok(2.890000000000000124344978758018)
Ok(2.889999999999999680255768907955)

We can also print to_bits():

0100000000000111000111101011100001010001111010111000010100011111
0100000000000111000111101011100001010001111010111000010100011110

So parse did indeed choose a closer approximation, and yes this look like a rounding error.

We could special case radix=10 to use parse, but it would be nicer to improve this for all bases.