Python Excel float 大不同

Question

我使用以下代码在 Python 中计算平方根:

from math import *

#find average of two numbers
def average(a, b):
    print "the average is",(a + b) / 2.0,"\n"
    return (a + b) / 2.0


    #guess not good enouhgh
    def improve(guess, x):
        print "improved guess is ",average(guess, x/guess),"\n"
        return average(guess, x/guess)

    #As long as the guess is not good enough, keep on improving the guess
    def good_enough(guess, x):
        d = abs(guess*guess - x)
        print d," is the currentguess\n"
        return (d < 0.001)

    #As long as the guess is not good enough, keep on improving the guess
    def square_root(guess, x):
        while(not good_enough(guess, x)):
            guess = improve(guess, x)
            print "current guess is ",guess,"\n"
        return guess

    if __name__ == "__main__":
        x = square_root(5, 33)
        print "final answer is",x

33 的平方根的结果是:5.74456521739

我在 excel 2003 中使用了平方根函数:

=sqrt(33)

设置结果保留15位小数得到结果:5.744562646538030

然后我使用了:

math.sqrt(33) 

来自标准 Python 2.7.2 数学库

得到结果:5.74456264654

然后我提高了程序的准确性:return (d < 0.000001)

得到返回5.74456264654和我的程序一样

问题是为什么Python四舍五入和Excel 2003在不同的地方四舍五入。一个人怎么知道在危急情况下使用哪个更好？比如正在写数学方程式的 friend ，需要在物理上对PHD论文有很高的准确度？

Answer 1

Python 和Excel 都使用 double float ，精度取决于底层C 库，通常使用硬件浮点单元。普通 FPU 实现 IEEE-754 double。

话虽如此，我怀疑您正在使用执行格式设置的 print 语句。请参阅下面的差异。

>>> import math
>>> math.sqrt(33)
5.744562646538029
>>> print math.sqrt(33)
5.74456264654