Numpy array cannot index within a single []

Question

>>> allData.shape
Out[72]: (8L, 161L)
>>> mask = allData[2,:]
>>> allData[[0,1,3],:][:,mask == 1]  # works fine
>>> allData[[0,1,3],mask == 1]  # error: ValueError: shape mismatch: objects cannot be broadcast to a single shape

Why is it that numpy arrays cannot be indexed within a single bracket []?

CT Zhu · Accepted Answer · 2013-10-02 20:37:16Z

5

Changing allData[[0,1,3],mask == 1] to allData[[0,1,3],argwhere(mask == 1)] should fix it.

Brief explanation, if you slice an array by [[list1], [list2]], both lists are supposed to be lists of indices. If one of them is substituted by : to take all the element among that axis, then the other list can be a Boolean array of the same size. Only ONE of them need to be substituted by :. Consider the following 3D array example:

b=random.random((5, 10,2))
b[[0,1,3],:, some_mask==1] #works
b[:,:, some_mask==1] #works
b[[0,1,3],[2,4], some_mask==1] #ValueError

edited Oct 2, 2013 at 20:37

answered Oct 2, 2013 at 20:04

CT Zhu

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