Dealing with floating point number precision in Go arithmetic?

Question

I'm interested in a way to accurately subtract 2 float's in Go.

I've tried to use the math/big library but I can't get an accurate result.

I've used the big.js library in Javascript which solves this problem. Is there a similar library/method for Go arithmetic?

    package main

    import (
        "fmt"
        "math/big"
    )

    func main() {
        const prec = 200
        a := new(big.Float).SetPrec(prec).SetFloat64(5000.0)
        b := new(big.Float).SetPrec(prec).SetFloat64(4000.30)
        result := new(big.Float).Sub(a, b)
        fmt.Println(result)
    }
    Result: 999.6999999999998181010596454143524169921875

https://play.golang.org/p/vomAr87Xln

Answer 1

Package big

import "math/big"

func (*Float) String

func (x *Float) String() string

String formats x like x.Text('g', 10). (String must be called explicitly, Float.Format does not support %s verb.)

Use string input and round the output, for example,

package main

import (
    "fmt"
    "math/big"
)

func main() {
    const prec = 200
    a, _ := new(big.Float).SetPrec(prec).SetString("5000")
    b, _ := new(big.Float).SetPrec(prec).SetString("4000.30")
    result := new(big.Float).Sub(a, b)
    fmt.Println(result)
    fmt.Println(result.String())
}

Output:

999.6999999999999999999999999999999999999999999999999999999995
999.7

For decimal, by definition, binary floating-point numbers are an approximation. For example, the decimal number 0.1 cannot be represented exactly, it is approximately 1.10011001100110011001101 * (2**(-4)).

You are already used to this sort of thing since you know about repeating decimals, an approximation for rational numbers: 1 / 3 = .333... and 3227 / 555 = 5.8144144144....

See What Every Computer Scientist Should Know About Floating-Point Arithmetic.