[Jprogramming] Getting index in 2+d array

[Jon Hough]

Probably a very simple question. For a single dimension array I can do 
I.  1 0 1 1 1 0 0 0 1 0 1
0 2 3 4 8 10
to  get the nonzero indices.
But I am not sure how to do this for a larger dimension. i.e. get the (i,j) or 
(i,j,k) index of nonzero elements.
e.g. for this matrix:
 arr =. 3 3 $ 1 0 0 0 1 0 0 0 1
I. doesn't work because it only gives me the ith value of the position of the 
nonzero elements.
Any help appreciated.
Regards.                                          
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