Cython的计算不正确 [英] Cython's calculations are incorrect

问题描述

我实现了Madhava–Leibniz系列，先用Python计算pi，然后在Cython中提高速度。 Python版本：

I implemented the Madhava–Leibniz series to calculate pi in Python, and then in Cython to improve the speed. The Python version:

from __future__ import division
pi = 0
l = 1
x = True
while True:
    if x:
        pi += 4/l
    else:
        pi -= 4/l
    x = not x
    l += 2
    print str(pi)

Cython版本：

cdef float pi = 0.0
cdef float l = 1.0
cdef unsigned short x = True
while True:
    if x:
        pi += 4.0/l
    else:
        pi -= 4.0/l
    x = not x
    l += 2
    print str(pi)

当我停止Python版本时，它已正确计算出pi为3.141592 。 Cython版本最终以3.141597结尾，出现了我不记得的更多数字（我的终端崩溃了），但是不正确。为什么Cython版本的计算不正确？

When I stopped the Python version it had correctly calculated pi to 3.141592. The Cython version eventually ended up at 3.141597 with some more digits that I don't remember (my terminal crashed) but were incorrect. Why are the Cython version's calculations incorrect?

推荐答案

您正在使用 float 在Cython版本中-这是单精度！使用 double 代替，它对应于Python的 float （很有趣）。 C类型 float 仅具有约8个有效十进制数字，而 double 或Python的 float 大约有16位数字。

You are using float in the Cython version -- that's single precision! Use double instead, which corresponds to Python's float (funnily enough). The C type float only has about 8 significant decimal digits, whereas double or Python's float have about 16 digits.

这篇关于Cython的计算不正确的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！