IEEE 754二进制浮点数不精确 [英] IEEE 754 binary floating-point numbers imprecise for money

问题描述

当我将 math.Floor 与浮点变量一起使用时(将精度部分向下舍入/截断)，我遇到了问题.我该怎么做呢?

 程序包主要进口 ("fmt"数学")func main(){var st float64 = 1980var salePrice1 = st * 0.1/1.1fmt.Printf(％T:％v \ n"，salePrice1，salePrice1)//179.9999var salePrice2 = math.Floor(st * 0.1/1.1)fmt.Printf(％T:％v \ n"，salePrice2，salePrice2)//179} 

游乐场: https://play.golang.org/p/49TjJwwEdEJ

输出:

  float64:179.99999999999997浮动64:179 

我希望 1980 * 0.1/1.1 的输出为 180 ，但实际输出为 179 .

解决方案

原始问题:

---

golang的楼层号不正确

在将Math.Floor与float变量一起使用时，我遇到问题(舍入降低/截断精度部分).我该怎么做呢?

 程序包主要进口 ("fmt"数学")func main(){var st float64 = 1980var salePrice1 = st * 0.1/1.1fmt.Printf(％T:％v \ n"，salePrice1，salePrice1)var salePrice2 = math.Floor(st * 0.1/1.1)fmt.Printf(％T:％v \ n"，salePrice2，salePrice2)} 

我希望1980 * 0.1/1.1的输出为180，但实际输出是179."

游乐场:

输出:

  float64:179.99999999999997浮动64:179 

---

XY问题是在询问您尝试的解决方案，而不是实际问题: XY问题.

>

---

很明显，这是 salePrice1 的货币计算.货币计算使用精确的十进制计算，而不是不精确的二进制浮点计算.

对于货币计算，请使用整数.例如，

 程序包主要导入"fmt"func main(){var st int64 = 198000//$ 1980.00作为美分fmt.Printf(％[1] T:％[1] v \ n"，st)fmt.Printf("$％d.％02d \ n"，st/100，st％100)var n，d int64 = 1，11fmt.Printf(％d，％d \ n"，n，d)var salePrice1 int64 =(st * n)/d//向下取整fmt.Printf(％[1] T:％[1] v \ n"，salePrice1)fmt.Printf("$％d.％02d \ n"，salePrice1/100，salePrice1％100)var salePrice2 int64 =((st * n)* 10/d + 5)/10//四舍五入fmt.Printf(％[1] T:％[1] v \ n"，salePrice2)fmt.Printf("$％d.％02d \ n"，salePrice2/100，salePrice2％100)var salePrice3 int64 =(st * n +(d-1))/d//向上舍入fmt.Printf(％[1] T:％[1] v \ n"，salePrice1)fmt.Printf("$％d.％02d \ n"，salePrice3/100，salePrice3％100)} 

游乐场: https://play.golang.org/p/HbqVJUXXR-N

输出:

  int64:198000$ 1980.001、11int64:18000$ 180.00int64:18000$ 180.00int64:18000$ 180.00 

---

参考文献:

每个计算机科学家应了解的浮点运算法则

我们应如何计算货币(十进制，大浮点数)

常规十进制算术

I have a problem when I use math.Floor with a floating-point variable (round down/truncate the precision part). How can I do it correctly?

package main

import (
    "fmt"
    "math"
)

func main() {
    var st float64 = 1980
    var salePrice1 = st * 0.1 / 1.1
    fmt.Printf("%T:%v\n", salePrice1, salePrice1) // 179.9999
    var salePrice2 = math.Floor(st * 0.1 / 1.1)
    fmt.Printf("%T:%v\n", salePrice2, salePrice2) // 179
}

Playground: https://play.golang.org/p/49TjJwwEdEJ

Output:

float64:179.99999999999997
float64:179

I expect the output of 1980 * 0.1 / 1.1 to be 180, but the actual output is 179.

解决方案

The original question:

---

Incorrect floor number in golang

I have problem when use Math.Floor with float variable (round down/truncate the precision part). How can i do it correctly?

package main

import (
    "fmt"
    "math"
)

func main() {
    var st float64 = 1980
    var salePrice1 = st * 0.1 / 1.1
    fmt.Printf("%T:%v\n", salePrice1, salePrice1)
    var salePrice2 = math.Floor(st * 0.1 / 1.1)
    fmt.Printf("%T:%v\n", salePrice2, salePrice2)
}

I expect the output of 1980 * 0.1 / 1.1 to be 180, but the actual output is 179."

Playground:

Output:

float64:179.99999999999997
float64:179

---

The XY problem is asking about your attempted solution rather than your actual problem: The XY Problem.

---

Clearly, this is a money calculation for salePrice1. Money calculations use precise decimal calculations, not imprecise binary floating-point calculations.

For money calculations use integers. For example,

package main

import "fmt"

func main() {
    var st int64 = 198000 // $1980.00 as cents

    fmt.Printf("%[1]T:%[1]v\n", st)
    fmt.Printf("$%d.%02d\n", st/100, st%100)

    var n, d int64 = 1, 11
    fmt.Printf("%d, %d\n", n, d)

    var salePrice1 int64 = (st * n) / d // round down

    fmt.Printf("%[1]T:%[1]v\n", salePrice1)
    fmt.Printf("$%d.%02d\n", salePrice1/100, salePrice1%100)

    var salePrice2 int64 = ((st*n)*10/d + 5) / 10 // round half up

    fmt.Printf("%[1]T:%[1]v\n", salePrice2)
    fmt.Printf("$%d.%02d\n", salePrice2/100, salePrice2%100)

    var salePrice3 int64 = (st*n + (d - 1)) / d // round up

    fmt.Printf("%[1]T:%[1]v\n", salePrice1)
    fmt.Printf("$%d.%02d\n", salePrice3/100, salePrice3%100)
}

Playground: https://play.golang.org/p/HbqVJUXXR-N

Output:

int64:198000
$1980.00
1, 11
int64:18000
$180.00
int64:18000
$180.00
int64:18000
$180.00

---

References:

What Every Computer Scientist Should Know About Floating-Point Arithmetic

How should we calc money (decimal, big.Float)

General Decimal Arithmetic

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