[Numpy-discussion] ndarray with double comparison

[Chao YUE]

Thanks. I starts to use python do some real data processing and has bunch of
questions.

Chao

2011/10/13 Benjamin Root <ben.root at ou.edu>

> On Thu, Oct 13, 2011 at 11:13 AM, Chao YUE <chaoyuejoy at gmail.com> wrote:
>
>> Dear all,
>>
>> sorry for this stupid question but I cannot find it in numpy tutorial or
>> google.
>> suppose I have a=np.arange(11).
>>
>> In [32]: a < 8
>> Out[32]:
>> array([ True,  True,  True,  True,  True,  True,  True,  True, False,
>>        False, False], dtype=bool)
>>
>> In [34]: a > 4
>> Out[34]:
>> array([False, False, False, False, False,  True,  True,  True,  True,
>>         True,  True], dtype=bool)
>>
>> how can I have boolean index like 4 < a < 8
>> np.where(a>4 and a<8);or plainly input "a>4 and a<8" doesn't work.
>>
>> thanks,
>>
>> Chao
>>
>>
> Unfortunately, you can't use "and", "or", "not" keywords with boolean
> arrays because numpy can't overload them.  Instead, use the bitwise
> operators: '&', '|', and '~'.  Be careful, though, because of operator
> precedence is different for bitwise operators than the boolean keywords.  I
> am in the habit of always wrapping my boolean expressions in parentheses,
> just in case.
>
> (a > 4) & (a < 8)
>
> is what you want.  Note that "a > 4 & a < 8" would be evaluated in a
> different order -- "4 & a" would be first.
>
> I hope that helps!
>
> Ben Root
>
>
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>


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Chao YUE
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