[Numpy-discussion] Re: ***[Possible UCE]*** [SciPy-user] Regarding what "where" returns

Travis Oliphant oliphant at ee.byu.edu
Wed Apr 12 15:56:03 CDT 2006


Perry Greenfield wrote:

>We've noticed that in numpy that the where() function behaves  
>differently than for numarray. In numarray, where() (when used with a  
>mask or condition array only) always returns a tuple of index arrays,  
>even for the 1D case whereas numpy returns an index array for the 1D  
>case and a tuple for higher dimension cases. While the tuple is a  
>annoyance for users when they want to manipulate the 1D case, the  
>benefit is that one always knows that where is returning a tuple, and  
>thus can write code accordingly. The  problem with the current numpy  
>behavior is that it requires special case testing to see which kind  
>return one has before manipulating if you aren't certain of what the  
>dimensionality of the argument is going to be.
>  
>
I went ahead and made this change to the code.    The nonzero function 
still behaves as before (and in fact only works for 1-d arrays as it did 
in Numeric).

The where(condition)  function works the same as condition.nonzero() and 
both always return a tuple.

I had to change exactly one piece of code that used the new where syntax.

This does represent a code breakage with the where syntax (but only if 
you used the newer, numarray-introduced usage).  I think this is a 
small-enough segment that we can make this change.  

-Travis





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