numpy.take and duplicate indices

[andrewowens]

Hi,

First of all, thanks for making autograd! It's been extremely useful.

I noticed though that np.take produces the wrong result if the index array contains duplicate values. For example,
quick_grad_check(lambda x: np.sum(x[[0,0]]), np.array([1.]))
fails with:
AssertionError: Check failed! nd=2.0, ad=1.0

You can fix it by changing a line in primitive_sum_arrays (in numpy_extra.py) from
new_array[array.idx] += array.val
to:
numpy.add.at(new_array, array.idx, array.val). I'm not sure if this error occurs elsewhere, but it seems to at least fix the problem in all of the cases that I've tried.

[duvenaud]

Thanks for the bug report and the fix!