Golang ParseFloat在示例中不准确

Question

I've been working on a project where I have to convert a string to a uint, to make sure some money values are matching:

total, err := strconv.ParseFloat(paymentResp.Transactions[0].Amount.Total, 64)
if err != nil {
    return ctx.JSON(http.StatusBadRequest, err.Error())
}

if o.TotalPrice != uint(total*100) {
    return ctx.JSON(http.StatusBadRequest, "Unable to verify amount paid")
}

But I've seemingly found a problem when trying to do the strconv.ParseFloat() on a couple of numbers, then attempting to multiply them by 100 (to get the cents value).

I've created an example here: Go Playground

f, _ := strconv.ParseFloat("79.35", 64)
fmt.Println(uint(f*100))  //7934

f2, _ := strconv.ParseFloat("149.20", 64)
fmt.Println(uint(f2*100)) //14919

Is ParseFloat() what I should be using in this scenario? If not, I'd love to hear a brief explanation on this, as I'm still a programmer in learning.

Answer 1

Go uses IEEE-754 binary floating-point numbers. Floating-point numbers are imprecise. Don't use them for financial transactions. Use integers.

For example,

package main

import (
    "fmt"
    "strconv"
    "strings"
)

func parseCents(s string) (int64, error) {
    n := strings.SplitN(s, ".", 3)
    if len(n) != 2 || len(n[1]) != 2 {
        err := fmt.Errorf("format error: %s", s)
        return 0, err
    }
    d, err := strconv.ParseInt(n[0], 10, 56)
    if err != nil {
        return 0, err
    }
    c, err := strconv.ParseUint(n[1], 10, 8)
    if err != nil {
        return 0, err
    }
    if d < 0 {
        c = -c
    }
    return d*100 + int64(c), nil
}

func main() {
    s := "79.35"
    fmt.Println(parseCents(s))
    s = "149.20"
    fmt.Println(parseCents(s))
    s = "-149.20"
    fmt.Println(parseCents(s))
    s = "149.2"
    fmt.Println(parseCents(s))
}

Playground: https://play.golang.org/p/mGuO51QWyIv

Output:

7935 <nil>
14920 <nil>
-14920 <nil>
0 format error: 149.2