A Tour of Go: Exercise: Loops and Functions (NEXT)

exercise_loops_and_functions_next.go

package main

import (
	"fmt"
	"math"
)

func Sqrt(x float64) float64 {
	z := 1.0
	z_before := 0.0
	count := 0
	for i := 0; math.Abs(z-z_before) > 0.0000001; i++ {
		z_before = z
		z = z - (math.Pow(z, 2)-x)/(2*z)
		count += 1
	}
	fmt.Printf("(loop count:%d) ", count)
	return z
}

func main() {
	for i := 1.0; i < 10; i++ {
		fmt.Printf("Sqrt(%f) = ", i)
		fmt.Println(
			math.Sqrt(i),
			Sqrt(i),
		)
	}
}

停止条件の差分を0.0000001にするとmath.Sqrtと同じ値になった。一桁減らすと一部の数字で結果が異なった。

Sqrt(1.000000) = (loop count:1) 1 1
Sqrt(2.000000) = (loop count:5) 1.4142135623730951 1.4142135623730951
Sqrt(3.000000) = (loop count:5) 1.7320508075688772 1.7320508075688772
Sqrt(4.000000) = (loop count:5) 2 2.000000000000002
Sqrt(5.000000) = (loop count:6) 2.23606797749979 2.23606797749979
Sqrt(6.000000) = (loop count:6) 2.449489742783178 2.449489742783178
Sqrt(7.000000) = (loop count:6) 2.6457513110645907 2.6457513110645907
Sqrt(8.000000) = (loop count:6) 2.8284271247461903 2.8284271247461903
Sqrt(9.000000) = (loop count:6) 3 3

4, 9は5,6回ループしてるけどこれでいいのかなあ