[Numpy-discussion] yet another indexing question

Neal Becker ndbecker2@gmail....
Fri Oct 14 09:33:27 CDT 2011


Fabrice Silva wrote:

> Le vendredi 14 octobre 2011 à 08:04 -0400, Neal Becker a écrit :
>> suppose I have:
>> 
>> In [10]: u
>> Out[10]:
>> array([[0, 1, 2, 3, 4],
>>        [5, 6, 7, 8, 9]])
>> 
>> And I have a vector v:
>>  v = np.array ((0,1,0,1,0))
>> 
>> I want to form an output vector which selects items from u where v is the
>> index of the row of u to be selected.
>> 
>> In the above example, I want:
>> 
>> w = [0,6,2,8,4]
>> 
>> I can't seem to find a syntax that does this.
>> 
>> Now, more importantly, I need the result to be a reference to the original
>> array
>> (not a copy), because I'm going to use it on the LHS of an assignment.  Is
>> this possible?
> 
> What about np.where?
> http://docs.scipy.org/doc/numpy/reference/generated/numpy.where.html
> 
> w = np.where(v, u[1], u[0])
> 
> if you may want to have more than two options (more than two lines for
> u), then np.choose may be more appropriate
> http://docs.scipy.org/doc/numpy/reference/generated/numpy.choose.html

Will np.choose result in a lval?



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