I'm not going to debug you program for you - you'll learn more doing it yourself, but I glanced at it and saw immediately that you're missing an easy optimization. Raising a number to an integer power can be done much faster by repeated squaring according to the bit pattern of the exponent. I'll leave that cryptic comment alone to let you puzzle it out yourself. (Don't look it up; it's much more fun to figure out.)
-rob On Tue, Sep 22, 2020 at 3:11 AM Hau Phan <haupc....@gmail.com> wrote: > i can't find get n-th root in document of go big package so i decided to > do it myself using newton's method. i found a solution at > https://www.geeksforgeeks.org/n-th-root-number/ and start to implement in > go. but my code only work fine for base 2. other base result gone too far > from correct. > > Anyone could show me where am i wrong. > > Here's my code > > ```go > package main > > import ( > "fmt" > "math/big" > "math/rand" > ) > > // PowFloat return x^n > func PowFloat(x *big.Float, n int64) *big.Float { > result := new(big.Float).SetInt64(1) > for i := 0; i < int(n); i++ { > result.Mul(result, x) > } > return result > } > > // GetNthRoothFloat get nth root of a using newton's method > func GetNthRoothFloat(a *big.Float, n int64) *big.Float { > N := new(big.Float).SetInt64(n) > xPre := new(big.Float).SetInt64(int64(rand.Intn(10) + 1)) > eps := new(big.Float).SetFloat64(0.00000000001) > delX := new(big.Float).SetInt64(2147483647) > xK := new(big.Float).SetInt64(0) > > for delX.Cmp(eps) > 0 { > t1 := new(big.Float).Sub(N, new(big.Float).SetFloat64(1.0)) // t1 = n-1 > t1 = t1.Mul(t1, xPre) // t1 = (N-1) > * xPre > t2 := new(big.Float).Quo(a, PowFloat(xPre, n-1)) // t2 = a/( > xPre^(n-1) ) > t3 := new(big.Float).Add(t1, t2) // t3 = t1 + t2 > xK.Quo(t3, N) > delX = new(big.Float).Sub(xK, xPre) > delX.Abs(delX) > xPre = xK.Copy(xK) > } > return xK > } > > func main() { > t := new(big.Float).SetInt64(64) > fmt.Println(GetNthRoothFloat(t, 3)) > } > ``` > > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to golang-nuts+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/golang-nuts/5a38a7fe-426b-4f94-905e-79b42dcaa611n%40googlegroups.com > <https://groups.google.com/d/msgid/golang-nuts/5a38a7fe-426b-4f94-905e-79b42dcaa611n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAOXNBZQrjBjT-dDEK1Atu7R32aT1mgTsSAYhXAF7hXmj35BjuQ%40mail.gmail.com.
