numpy的范围和在哪里 [英] numpy arange and where

问题描述

我试图通过"where"在由"arange"创建的数组中查找值，但似乎无法正常工作.这是一个示例:

I am trying to find values in an array created by "arange" by means of "where", but it seems it does not work fine. Here is one example:

from numpy import arange, where

myarr = arange(6.6,10.25,0.05)
for item in [6.6,6.65,6.7,6.8,6.9,6.95,7.95,8.0,8.1,8.15,6.2,6.25,6.35]:
 print where(myarr == item)

(array([0]),)
(array([], dtype=int32),)
(array([], dtype=int32),)
(array([], dtype=int32),)
(array([], dtype=int32),)
(array([], dtype=int32),)
(array([], dtype=int32),)
(array([], dtype=int32),)
(array([], dtype=int32),)
(array([], dtype=int32),)
(array([], dtype=int32),)
(array([], dtype=int32),)
(array([], dtype=int32),)

使用Python 2.5.4，Numpy 1.3.0

Using Python 2.5.4, Numpy 1.3.0

提前谢谢！

推荐答案

注意:

In [32]: repr(myarr[1])
Out[32]: '6.6499999999999995'

In [33]: repr(6.65)
Out[33]: '6.6500000000000004'

因此，np.arange分配给myarr[1]的float64值与Python用来表示6.65的float并不完全相同.

So the float64 value that np.arange assigns to myarr[1] is not exactly the same float that Python uses to represent 6.65.

因此，除非您真的知道自己在做什么，否则从不为了相等而测试浮动.改用不平等:

So, unless you really know what you are doing, never test floats for equality. Use inequalities instead:

def near(a,b,rtol=1e-5,atol=1e-8):
    try:
        return np.abs(a-b)<(atol+rtol*np.abs(b))
    except TypeError:
        return False

myarr = np.arange(6.6,10.25,0.05)
for item in [6.6,6.65,6.7,6.8,6.9,6.95,7.95,8.0,8.1,8.15,6.2,6.25,6.35]:
    print (np.where(near(myarr,item)))

# (array([0]),)
# (array([1]),)
# (array([2]),)
# (array([4]),)
# (array([6]),)
# (array([7]),)
# (array([27]),)
# (array([28]),)
# (array([30]),)
# (array([31]),)
# (array([], dtype=int32),)
# (array([], dtype=int32),)
# (array([], dtype=int32),)

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