dongtuoao7987 2017-01-24 22:23
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Golang中的奇怪Pow实现

I just came across the Pow implementation in golang:

func Pow(x, y float64) float64 {
    // ...
    case x == 0:
        switch {
        case y < 0:
            if isOddInt(y) {
                return Copysign(Inf(1), x)
            }
            return Inf(1)
        case y > 0:
            if isOddInt(y) {
                return x
            }
            return 0
        }
    //...
}

Isn't the case y > 0 part over complicated? I would just return 0. Or did I miss something?

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  • doulang9953 2017-01-24 23:02
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    there are two types of zero's, +0 and -0. return value of Pow(-0,1) should be -0 not +0

    to create -0 in golang, use math.Copysign.

    x := math.Copysign(0, -1)
    if x == 0 {
        fmt.Println("x is zero")
    }
    fmt.Println("x ** 3 is", math.Pow(x, 3))
    

    the output of above code is

    x is zero
    x ** 3 is -0
    

    you can check it in Go Playground

    why we have to distinguish +0 and -0, see: https://softwareengineering.stackexchange.com/questions/280648/why-is-negative-zero-important

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