[Numpy-discussion] Logical Selector

Xavier Gnata gnata@obs.univ-lyon1...
Wed Jul 18 16:43:51 CDT 2007


Eric Firing wrote:
> Robert Kern wrote:
>   
>> Geoffrey Zhu wrote:
>>     
>>> Hi Everyone,
>>>
>>> I am finding that numpy cannot operate on boolean arrays. For example,
>>> the following does not work:
>>>
>>> x=3Darray([(1,2),(2,1),(3,1),(4,1)])
>>>
>>> x[x[:,0]>x[:,1] and x[1:]>1,:]
>>>
>>> It gives me an syntax error:
>>>
>>> -------------------
>>> Traceback (most recent call last):
>>>   File "<pyshell#74>", line 1, in <module>
>>>     x[x[:,0]>x[:,1] and x[1:]>1,:]
>>> ValueError: The truth value of an array with more than one element is
>>> ambiguous. Use a.any() or a.all()
>>> -------------------
>>>
>>> However, this is not what I want. I want a "piece-wise" "AND" operation
>>> on the two boolean vectors. In other words, the row is selected if both
>>> are true. How do I accomplish this?
>>>       
>> The "and" keyword tries to coerce each of its operands into a Boolean True or
>> False value. This behavior cannot be overridden in the current Python language
>> to yield arrays of Boolean values. However, for Boolean arrays, the bitwise
>> logical operations &, |, and ~ work just fine for this purpose instead of "and",
>> "or", and "not".
>>
>>     
>
> except that their precedence is higher than that of the logical 
> operators, so one must remember to use parentheses:
> (a<b) & (c>d)
> which is good for clarity anyway.
>
> Eric
>   
Hi,

Well maybe it is a bug on my box (thunderbird) but the topic of the 
thread is "-lmkl_lapack64 on i368 ??".
Nothing to do with  "Logical Selector" ;)
Should I post another mail about this topic?

Xavier
ps : I'm just sorry for the noise if it is a bug on my side.

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Xavier Gnata
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E-mail: gnata@obs.univ-lyon1.fr
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