[Numpy-discussion] Index array confusion...

Todd Miller jmiller at stsci.edu
Sun Dec 15 13:34:02 CST 2002


Magnus Lie Hetland wrote:

>I can see why this happens, to some degree, but it is a bit confusing
>still:
>
>  
>
>>>>a = zeros([5,5])
>>>>a[[1,2,3]]
>>>>        
>>>>
>array([[0, 0, 0, 0, 0],
>       [0, 0, 0, 0, 0],
>       [0, 0, 0, 0, 0]])
>  
>
>>>>a[[1,2]]
>>>>        
>>>>
>0
>  
>
>>>>a[[1]]
>>>>        
>>>>
>array([[0, 0, 0, 0, 0]])
>
>I was planning to use index arrays to select a subset of the rows in a
>two-dimensional array, but because of the special-casing for
>one-dimensional arrays of the same length as the dimensionality of the
>array being indexed, it seems I can't do that (without using
>put/take, which may be quite OK, I guess)... Am I right? (I see why
>this behaviour is needed for tuples and slice objects; and I guess
>type checking would be sort of "ugly"...)
>
>Of course a[[1,2],] works just as expected.
>
>This may not be important, but perhaps worth a sentence or example in
>the manual? I.e. if you are using an index array idx and you don't
>want to risk having it cast as a single index lookup, you should do
>
>  a[idx,]
>
>Wouldn't that be sound advice in general?
>
>  
>
Assuming you're talking about numarray here,  I regard this as a 0.4 bug 
which you just found.  Thanks!

Todd






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