当指数为负数时，为什么numpy.power不对数组进行元素处理?

问题描述

使用负数组时，numpy.power或**和/处理数组时有什么区别?以及为什么numpy.power不能按文档.

What is the difference between numpy.power or ** for negative exponents and / when working with arrays? and why does numpy.power not act element-wise as described in the documentation.

例如，使用python 2.7.3:

For example, using python 2.7.3:

>>> from __future__ import division
>>> import numpy as np
>>> A = arange(9).reshape(3,3)
>>> A
array([[0, 1, 2],
       [3, 4, 5],
       [6, 7, 8]])

当指数为负时，**和numpy.power似乎不是按元素进行操作

It appears that ** and numpy.power are not acting element-wise when the exponent is negative

>>> A**-1
array([[-9223372036854775808,                    1,                    0],
       [                   0,                    0,                    0],
       [                   0,                    0,                    0]])
>>> np.power(A, -1)
array([[-9223372036854775808,                    1,                    0],
       [                   0,                    0,                    0],
       [                   0,                    0,                    0]])

而/在元素方面起作用

>>> 1/A
array([[        inf,  1.        ,  0.5       ],
       [ 0.33333333,  0.25      ,  0.2       ],
       [ 0.16666667,  0.14285714,  0.125     ]])

当指数为正时，我没有此类问题.为什么负指数的行为会有所不同?

I have no such problems when the exponent is positive. Why does it behave differently for negative exponents?

推荐答案

这主要是转换问题.运营商自然会以为您不希望将号码上载到其他类型. 2**-1与整数的最接近值是0，可以验证int(2**-1) >>>0.

This is primarily an issue of casting. Operators naturally assume that you do not wish to upcast a number to a different type. The closest value of 2**-1 with integers is 0, this can be verified int(2**-1) >>>0.

首先创建类型为int的数组A:

First create array A of type int:

A = np.arange(9).reshape(3,3)
>>> A.dtype
dtype('int64')

将数组A复制到A_float作为类型float:

Copy array A to A_float as type float:

>>> A_float = A.astype(float)
>>> A_float.dtype
dtype('float64')

同时运行**-1操作

>>> A_float**-1
array([[        inf,  1.        ,  0.5       ],
       [ 0.33333333,  0.25      ,  0.2       ],
       [ 0.16666667,  0.14285714,  0.125     ]])

>>> A**-1
array([[-9223372036854775808,                    1,                    0],
       [                   0,                    0,                    0],
       [                   0,                    0,                    0]])

观察numpy不会自动假定您要以float形式完成此操作，而是尝试使用整数来完成此操作.如果您在两个操作数中都表示浮点数，则由于安全"转换规则，您将获得浮点数输出:

Observe numpy does not automatically assume you want to complete this operation as float and attempts to accomplish this with integers. If you signify a float in either operand you will obtain a float output due to the "safe" casting rules:

>>> A**-1.0
array([[        inf,  1.        ,  0.5       ],
       [ 0.33333333,  0.25      ,  0.2       ],
       [ 0.16666667,  0.14285714,  0.125     ]])

另一种选择是强制np.power将操作强制转换为浮点型:

Another option is to force np.power to cast the operation as a float:

>>> np.power(A,-1,dtype=float)
array([[        inf,  1.        ,  0.5       ],
       [ 0.33333333,  0.25      ,  0.2       ],
       [ 0.16666667,  0.14285714,  0.125     ]])

我不确定为什么要使用1/A获得浮点结果. 1.0/A正常工作.

I am not sure why you are obtaining a float result with 1/A. 1.0/A works just fine however.

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