可以将函数作为for循环条件的一部分来调用吗？

Question

Below are two attempts to solve the exercise at https://tour.golang.org/flowcontrol/8. One version makes a function call as part of the for condition, but this doesn't work - it doesn't even execute the loop body. If I move the condition inside the loop itself, it works as I expected. Why?

package main

import (
    "fmt"
    "math"
)

func Sqrt_working(x float64) float64 {
    var z float64 = 1.0

    for {
        if math.Abs((z*z) - x) < 0.0001 {
            break
        }
        z -= ((z*z - x) / (2*z))
    }

    return z
}

func Sqrt_not_working(x float64) float64 {
    var z float64 = 1.0

    for math.Abs((z*z) - x) < 0.0001 {
        z -= ((z*z - x) / (2*z))
    }

    return z
}

func main() {
    fmt.Println(Sqrt_working(2))
    fmt.Println(Sqrt_not_working(2))
}

Output

1.4142156862745099
1

Answer 1

Your if condition is signalling when the loop should stop, but the for condition signals when the loop should continue.

To see the desired result, invert your for condition:

for math.Abs((z*z) - x) >= 0.0001 {
    z -= ((z*z - x) / (2*z))
}