[SciPy-user] Slicing 3D array using two 1D arrays not working

[Travis E. Oliphant]

Dharhas Pothina wrote:
> Hi,
>
> I have an array a 3D array 'a' (a.shape = a(6, 21216, 2) ) and I want to slice it using a[levels,nodes,:] where
> levels=array([2,4]),nodes=array([12,1234,4566,1233]) etc.
>
> I can do    
>
> b=a[:,nodes,:]
> c=b[levels,:,:]
>
> but if I try 
> c=a[levels,nodes,:]
>
> ValueError: shape mismatch: objects cannot be broadcast to a single shape
>
> is there something I am doing wrong or is this just not possible?
>   
What is a[levels, nodes, :] supposed to return?  If you are expecting 
the cross-product, then I suspect what you want is

il, in = numpy.ix_(levels, nodes)
c = a[il, in,:]


Indexing with a list in NumPy returns an "element-byelement" result so 
the two indexing lists have to have commensurate shapes.  You get the 
cross-product by fiddling with the dimensions and turn levels into a 
(2,1)-array and nodes into a (1,4)-array.   Then broadcasting handles 
createing the (2,4)-shape set of indices that you may have been expecting.

That's all numpy.ix_ does is fiddle with the shapes to give you index 
arrays for getting the cross-product.

-Travis