Python 언어로 평방근, 입방근, n제곱근 구하는 함수를 구현하고 테스트하기

음이 아닌 실수 A 의 평방근 sqrt(A) 를 구하는 Heron 의 방법:

        반복함수  g(x) = (x + A/x) / 2   를 이용

 

실수 A 의 n제곱근 root(n, A) 를 구하는 Newton-Raphson 의 방법

        반복함수  g(x) = ((n-1)*x + A/(x**(n - 1))) / n    를 이용

n = 2 인 경우에는 Newton-Raphson 의 방법이 Heron 의 방법과 동일하다.

(참조. http://en.wikipedia.org/wiki/Newton's_method )

 

Python 언어에는 지수 연산자 ** 를 (밑수)**(지수) 의 형식으로 언어 자체에서 지원하고 있다. 하지만 차후 필요한 데가 있을 것 같아서 이와 유사한 n 제곱 함수와 n 제곱근 함수를 구현해 보았다.

지수가 정수인 거듭제곱을 계산하는  함수도 nPow(), gPow, mPow() 세 개 구현해 놓았는데, 이들 세 함수는 절차적 언어의 성능상 재귀호출이 아니고 단순 반복 기법을 사용하는 함수이다. 이 세 함수 중 mPow() 의 성능이 가장 우수하다. 큰 지수의 경우 for 반복문의 반복회수를 따져 보면 성능 비교를 할 수 있을 것이다. (성능 비교를 위해 세 가지를 모두 소스에 남겨 두었다.) mPow() 함수는 n 제곱근을 구하는 재귀함수 newtonNthRoot(int, float) 의 구현에 사용되기도 한다. if ... else ... 구문이 많아 소스가 복잡하게 보일지 모르겠으나 이는 밑수나 지수가 음수이거나 0인 경우의 처리를 위함이다. 구현된 모든 함수의 구현에는 예외상황(예를 들어, 음수의 짝수 제곱근 같은 예외상황) 처리 과정이 있다. (참고로 Pythn 언어에는 double 타입이 없고, float 타입이 C, C++, Java, C# 언어의 double 타입에 해당한다.)

아래의 소스는 2.x 버전대의 파이썬에서 실행되도록 작성된 소스이다.

#!/usr/bin/env python
# -*- encoding: utf-8 -*-

# Filename: testNthRoot.py
#
#            Approximate square roots, cubic roots and n-th roots of a given number.
#
# Execute: python testNthRoot.py
#
#   Or
#
# Execute: jython testNthRoot.py
#
#   Or
#
# Execute: ipy testNthRoot.py
#
#   Or
#
# Compile: ipy %IRONPYTHON_HOME%\tools\scripts\pyc.py /main:testNthRoot.py /target:exe
# Execute: testNthRoot
#
# Date: 2013. 1. 6.
# Copyright (c) 2013 PH Kim  (pkim __AT__ scripts.pe.kr)

import math

MAX_ITER = 20000
M_EPSILON = 1.0e-15

#
# Compute the n-th power of x to a given scale, x > 0.
#
def nPow(a, n):
    if n > 0:
        if n == 1:
            return a
        else:
            if a == 0.0 or a == 1.0:
                return a
            elif a == -1.0:
                if n % 2 == 1:
                    return -1.0
                else:
                    return 1.0
            elif a < 0.0:
                if n % 2 == 1:
                    return -nPow(-a, n)
                else:
                    return nPow(-a, n)
            else:
                y = 1.0
                for i in range(n):
                    y *= a
                return y
    elif n == 0:
        return 1.0
    else:      #  when n < 0
        if a == 0.0:
            raise Error,'Negative powering exception of zero.'
        else:
            if n == -1:
                return 1.0/a
            else:
                return 1.0/nPow(a, -n)

 

#
# Compute the n-th power of x to a given scale, x > 0.
#
def gPow(a, n):
    if n > 0:
        if n == 1:
            return a
        else:
            if a == 0.0 or a == 1.0:
                return a
            elif a == -1.0:
                if n % 2 == 1:
                    return -1.0
                else:
                    return 1.0
            elif a < 0.0:
                if n % 2 == 1:
                    return -gPow(-a, n)
                else:
                    return gPow(-a, n)
            else:
                y = 1.0
                r = a
                m = 8*4 - 1            #  8*sizeof(int) - 1;
                one = 1
                for i in range(m + 1):
                    if (n & one) == 0:
                        y *= 1.0
                    else:
                        y *= r
                    r = r*r
                    one <<= 1
                    if one > n:
                        break
                return y
    elif n == 0:
        return 1.0
    else:      #  when n < 0
        if a == 0.0:
            raise Error,'Negative powering exception of zero.'
        else:
            if n == -1:
                return 1.0/a
            else:
                return 1.0/gPow(a, -n)

 

#
# Compute the n-th power of x to a given scale, x > 0.
#
def mPow(a, n):
    if n > 0:
        if n == 1:
            return a
        else:
            if a == 0.0 or a == 1.0:
                return a
            elif a == -1.0:
                if n % 2 == 1:
                    return -1.0
                else:
                    return 1.0
            elif a < 0.0:
                if n % 2 == 1:
                    return -mPow(-a, n)
                else:
                    return mPow(-a, n)
            else:
                y = 1.0
                r = a
                m = n
                while m > 0:
                    if (m & 0x1) == 1:
                        y *= r
                    r = r*r
                    m >>= 1
                return y
    elif n == 0:
        return 1.0
    else:      #  when n < 0
        if a == 0.0:
            raise Error,'Negative powering exception of zero.'
        else:
            if n == -1:
                return 1.0/a
            else:
                return 1.0/mPow(a, -n)

 

#
# Compute the square root of x to a given scale, x > 0.
#
def heronSqrt(a):
    if a < 0.0:
        raise ValueError,'Cannot find the sqrt of a negative number.'
    elif a == 0.0 or a == 1.0:
        return a
    else:
        x1 = a
        x2 = (x1 + a/x1)/2.0
        er = x1 - x2
        counter = 0
        while x1 + er != x1:
            x1 = x2
            x2 = (x1 + a/x1)/2.0
            er = x1 - x2
            if abs(er) < abs(M_EPSILON*x1):
                break
            counter += 1
            if counter > MAX_ITER:
                break
        if counter >= MAX_ITER:
            raise ValueError,'Inaccurate sqrt exception by too many iterations.'
        return x2

#
# Compute the cubic root of x to a given scale, x > 0.
#
def newtonCbrt(a):
    if a == 0.0 or a == 1.0 or a == -1.0:
        return a
    elif a < 0.0:
        return -newtonCbrt(-a)
    else:
        x1 = a
        x2 = (2.0*x1 + a/(x1*x1))/3.0
        er = x1 - x2
        counter = 0
        while x1 + er != x1:
            x1 = x2
            x2 = (2.0*x1 + a/(x1*x1))/3.0
            er = x1 - x2
            if abs(er) < abs(M_EPSILON*x1):
                break
            counter += 1
            if counter > MAX_ITER:
                break
        if counter >= MAX_ITER:
            raise Error,'Inaccurate cbrt exception by too many iterations.'
        return x2

#
# Compute the n-th root of x to a given scale, x > 0.
#
def newtonNthRoot(n, a):
    if n == 0:
        return 1.0
    elif n == 1:
        return a
    elif n > 0:
        if a == 0.0 or a == 1.0:
            return a
        elif a == -1.0:
            if n % 2 == 1:
                return a
            else:
                raise ValueError,'Cannot find the even n-th root of a negative number.'
        elif a < 0.0:
            if n % 2 == 1:
                return -newtonNthRoot(n, -a)
            else:
                raise ValueError,'Cannot find the even n-th root of a negative number.'
        elif a < 1.0:
            return 1.0/newtonNthRoot(n, 1.0/a)
        else:
            x1 = a
            xn = mPow(x1, n - 1)
            x2 = ((n - 1)*x1 + a/xn)/n
            er = x1 - x2
            counter = 0
            while x1 + er != x1:
                x1 = x2
                xn = mPow(x1, n - 1)
                x2 = ((n - 1)*x1 + a/xn)/n
                er = x1 - x2
                if abs(er) < abs(M_EPSILON*x1):
                    break
                counter += 1
                if counter > MAX_ITER:
                    break
            if counter >= MAX_ITER:
                raise ValueError, 'Inaccurate n-th root exception by too many iterations.'
            return x2
    else:
        if a == 0.0:
            raise Error, 'Cannot find the negative n-th root of zero.'
        else:
            return 1.0/newtonNthRoot(-n, a)

if __name__ == "__main__":

    x = 16.0
    u = math.sqrt(x)

    print "[ Testing heronSqrt(double) ]--------------------"
    print "x = %g" % x
    print "u = sqrt(%g) = %g" % (x, u)
    y = heronSqrt(x)
    print "y = heronSqrt(%g) = %g" % (x, y)
    print "y*y = %g" % (y*y)
    print

    print "[ Testing newtonCbrt(double) ]--------------------"
    x = -216.0
    print "x = %g" % x
    print "-exp(log(-x)/3.0) = %g" % -math.exp(math.log(-x)/3.0)
    w = newtonCbrt(x)
    print "w = newtonCbrt(%g) = %g" % (x, w)
    print "w*w*w = %g" % (w*w*w)
    print

    x = 729000000000.0
    print "x = %g" % x
    print "exp(log(x)/3.0) = %g" % math.exp(math.log(x)/3.0)
    w = newtonCbrt(x)
    print "w = newtonCbrt(%g) = %g" % (x, w)
    print "w*w*w = %g" % (w*w*w)
    print

    print "[ Testing newtonNthRoot(int, double) ]--------------------"
    z = newtonNthRoot(3, x)
    print "x = %g" % x
    print "z = newtonNthRoot(3, %g) = %g" % (x, z)
    print "z*z*z = %g" % (z*z*z)
    print

    x = 12960000000000000000.0
    z = newtonNthRoot(4, x)
    print "x = %g" % x
    print "z = newtonNthRoot(4, x) = newtonNthRoot(4, %g) =  %g" % (x, z)
    print "z*z*z*z = %g" % (z*z*z*z)
    print

    x = 1.0/12960000000000000000.0
    z = newtonNthRoot(4, x)
    print "x = %g" % x
    print "exp(log(x)/4.0) = %g" % math.exp(math.log(x)/4.0)
    print "z = newtonNthRoot(4, x) = newtonNthRoot(4, %g) =  %g" % (x, z)
    print "z*z*z*z = %g" % (z*z*z*z)
    print

    try:
        x = -4.0
        print "[ Test Exception heronSqrt(double) ]--------------------"
        print "x = %g" % x
        print "Calculating heronSqrt(%g)" % x
        y = heronSqrt(x)
        print "y = heronSqrt(%g) = %g" % (x, y)
        print "y*y = %g" % (y*y)
        print
    except ValueError, ex:
        print "%s\nCaught some exception in calculating heronSqrt(%g)" % (ex, x)
        print

    try:
        x = -4.0
        print "[ Test Exception in newtonCbrt(double) ]--------------------"
        print "x = %g" % x
        print "Calculating newtonCbrt(%g)" % x
        y = newtonCbrt(x)
        print "y = newtonCbrt(%g) = %g" % (x, y)
        print "y*y*y = %g" % (y*y*y)
        print
    except ValueError, ex:
        print "%s\nCaught some exception in calculating newtonCbrt(%g)" % (ex, x)
        print

    print "[ Test calculations by powering ]-----------------------------"
    x = 200.0
    z = newtonNthRoot(10, x)
    print "x = %g" % x
    print "exp(log(x)/10.0) = %g" % math.exp(math.log(x)/10.0)
    print "z = newtonNthRoot(10, x) = newtonNthRoot(10, %g) = %g" % (x, z)
    print "pow(z, 10) = %g" % math.pow(z, 10)
    print

    x = 3001.0
    z = newtonNthRoot(99, x)
    print "x = %g" % x
    print "exp(log(x)/99.0) = %g" % math.exp(math.log(x)/99.0)
    print "z = newtonNthRoot(99, x) = newtonNthRoot(99, %g) = %g" % (x, z)
    print "pow(z, 99) = %g" % math.pow(z, 99)
    print

    x = 3001.0
    z = newtonNthRoot(-99, x)
    print "x = %g" % x
    print "exp(log(x)/-99.0) = %g" % math.exp(math.log(x)/-99.0)
    print "z = newtonNthRoot(-99, x) = newtonNthRoot(-99, %g) = %g" % (x,z)
    print "1.0/pow(z, 99) = %g" % (1.0/math.pow(z, 99))
    print

    print "2.1**2.1 = pow(2.1, 2.1) = %g" % math.pow(2.1, 2.1)
    print "2.1**(-2.1) = pow(2.1, -2.1) = %g" % math.pow(2.1, -2.1)
    print "2.1**2.1 * 2.1**(-2.1) = pow(2.1, 2.1) * pow(2.1, -2.1) = %g" % (math.pow(2.1, 2.1)*math.pow(2.1, -2.1))
    print "2.1**2.1 = exp(2.1*log(2.1)) = %g" % math.exp(2.1*math.log(2.1))
    print "2.1**(-2.1) = exp(-2.1*log(2.1)) = %g" % math.exp(-2.1*math.log(2.1))
    print "2.1**2.1 * 2.1**(-2.1) = exp(2.1*log(2.1)) * exp(-2.1*log(2.1)) = %g" % (math.exp(2.1*math.log(2.1)) * math.exp(-2.1*math.log(2.1)))
    print

    k = 301
    x = -1.029
    t1 = nPow(x, k)
    t2 = gPow(x, k)
    t3 = mPow(x, k)
    print "math.pow(%g, %d) = %g" % (x, k,  math.pow(x, k))
    print "t1 = nPow(%g, %d) = %g" % (x, k,  t1)
    print "t2 = gPow(%g, %g) = %g" % (x, k, t2)
    print "t3 = mPow(%g, %g) = %g" % (x, k, t3)
    print "t1 / t2 = %g" % (t1 / t2)
    print "t1 - t2 = %g" % (t1 - t2)
    print "t1 == t2 ?", 
    if t1 == t2: print "yes"
    else: print "no"
    print "t1 / t3 = %g" % (t1 / t3)
    print "t1 - t3 = %g" % (t1 - t3)
    print "t1 == t3 ?", 
    if t1 == t3: print "yes"
    else: print "no"
    print "t2 / t3 = %g" % (t2 / t3)
    print "t2 - t3 = %g" % (t2 - t3)
    print "t2 == t3 ?", 
    if t2 == t3: print "yes"
    else: print "no"
    print

    print "Done."

"""
Output:
[ Testing heronSqrt(double) ]--------------------
x = 16
u = sqrt(16) = 4
y = heronSqrt(16) = 4
y*y = 16

[ Testing newtonCbrt(double) ]--------------------
x = -216
-exp(log(-x)/3.0) = -6
w = newtonCbrt(-216) = -6
w*w*w = -216

x = 7.29e+11
exp(log(x)/3.0) = 9000
w = newtonCbrt(7.29e+11) = 9000
w*w*w = 7.29e+11

[ Testing newtonNthRoot(int, double) ]--------------------
x = 7.29e+11
z = newtonNthRoot(3, 7.29e+11) = 9000
z*z*z = 7.29e+11

x = 1.296e+19
z = newtonNthRoot(4, x) = newtonNthRoot(4, 1.296e+19) =  60000
z*z*z*z = 1.296e+19

x = 7.71605e-20
exp(log(x)/4.0) = 1.66667e-05
z = newtonNthRoot(4, x) = newtonNthRoot(4, 7.71605e-20) =  1.66667e-05
z*z*z*z = 7.71605e-20

[ Test Exception heronSqrt(double) ]--------------------
x = -4
Calculating heronSqrt(-4)
Cannot find the sqrt of a negative number.
Caught some exception in calculating heronSqrt(-4)

[ Test Exception in newtonCbrt(double) ]--------------------
x = -4
Calculating newtonCbrt(-4)
y = newtonCbrt(-4) = -1.5874
y*y*y = -4

[ Test calculations by powering ]-----------------------------
x = 200
exp(log(x)/10.0) = 1.69865
z = newtonNthRoot(10, x) = newtonNthRoot(10, 200) = 1.69865
pow(z, 10) = 200

x = 3001
exp(log(x)/99.0) = 1.08424
z = newtonNthRoot(99, x) = newtonNthRoot(99, 3001) = 1.08424
pow(z, 99) = 3001

x = 3001
exp(log(x)/-99.0) = 0.922308
z = newtonNthRoot(-99, x) = newtonNthRoot(-99, 3001) = 0.922308
1.0/pow(z, 99) = 3001

2.1**2.1 = pow(2.1, 2.1) = 4.74964
2.1**(-2.1) = pow(2.1, -2.1) = 0.210542
2.1**2.1 * 2.1**(-2.1) = pow(2.1, 2.1) * pow(2.1, -2.1) = 1
2.1**2.1 = exp(2.1*log(2.1)) = 4.74964
2.1**(-2.1) = exp(-2.1*log(2.1)) = 0.210542
2.1**2.1 * 2.1**(-2.1) = exp(2.1*log(2.1)) * exp(-2.1*log(2.1)) = 1

math.pow(-1.029, 301) = -5457.93
t1 = nPow(-1.029, 301) = -5457.93
t2 = gPow(-1.029, 301) = -5457.93
t3 = mPow(-1.029, 301) = -5457.93
t1 / t2 = 1
t1 - t2 = 6.18456e-11
t1 == t2 ? no
t1 / t3 = 1
t1 - t3 = 6.18456e-11
t1 == t3 ? no
t2 / t3 = 1
t2 - t3 = 0
t2 == t3 ? yes

Done.
"""