golang 浮点计算后转 big.Int 精度会丢失?

Question

大数运算丢失精度

func calculate(a string, b string) *big.Int {
    v, _ := new(big.Float).SetString(a)
    decimals, _ := new(big.Float).SetString(b)
    value := new(big.Float).Mul(v, decimals)
    transVal, _ := value.Int(new(big.Int))
    return transVal
}

func main() {
    rst := calculate("0.001", "1000000000000000000")
    fmt.Println(rst) // output: 999999999999999, 精度丢失
}

期望精度不丢失

Answer 1

建议使用第三方库 decimal

package main

import (
    "fmt"
    "math/big"

    "github.com/shopspring/decimal"
)

func calculate(a string, b string) *big.Int {
    v, _ := decimal.NewFromString(a)
    decimals, _ := decimal.NewFromString(b)
    value := v.Mul(decimals)
    transVal := value.BigInt()
    return transVal
}

func main() {
    rst := calculate("0.001", "1000000000000000000")
    fmt.Println(rst) // output: 1000000000000000
}

Answer 2

可以使用 big.Rat 来处理

package main

import (
    "fmt"
    "math/big"
)

func calculate(a string, b string) (*big.Int, error) {
    v, ok := new(big.Rat).SetString(a)
    if (!ok) {
        return nil, fmt.Errorf("invalid value for a: %s", a)
    }
    decimals, ok := new(big.Rat).SetString(b)
    if (!ok) {
        return nil, fmt.Errorf("invalid value for b: %s", b)
    }
    value := new(big.Rat).Mul(v, decimals)
    floatValue := new(big.Float).SetRat(value)
    transVal, _ := floatValue.Int(nil)
    return transVal, nil
}

func main() {
    rst, err := calculate("0.001", "1000000000000000000")
    if err != nil {
        fmt.Println("Error:", err)
        return
    }
    fmt.Println(rst) // output: 1000000000000000, 精度保持
}