Python NaN

NaN, Not a Number, 非数. 它即不是无穷大, 也不是无穷小, 而是python/numpy/... 觉得无法计算时返回的一个符号(自己的推测, 未考证(TODO)).

import numpy as np

无穷大减无穷大会导致NaN

a = np.infty

print a - a

nan

print a * a, a * a - a

inf nan

无穷大乘以0或无穷小或除以无穷大会导致NaN

print a * 0

nan

print a * 1/ a

nan

print a / a

nan

print a / 1

inf

print a / 1

inf

总结起来就是, 涉及到无穷大的四则运算, 若无法确定运算结果仍为无穷大, 那么运算结果就是一个NaN.

另外很明显的是:

有NaN参与的运算, 其结果也一定是NaN

b = np.nan

print b + 1

nan

print b - b

nan

NaN != NaN

print b == b

False

利用这个性质可以判断数组中是否含有NaN:

def has_nan(x):
    test = x != x
    return np.sum(test) > 0

也可以使用系统api：

import math
val = float('nan')

if math.isnan(val):
    print('Detected NaN')
    import pdb; pdb.set_trace() # Break into debugger to look around

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markdown由jupyter notebook生成. [notebook](https://github.com/dengdan/blog_posts/blob/master/python/NaN.ipynb)