Difference between python numpy's where and boolean array

Question

I have two numpy arrays: myarray and mask, which are both bitewise arrays(1s and 0s only).

What is the difference between

myarray[mask] = 0

and

myarray = np.where( mask, 0, myarray )

? Because I get different results and can't figure out why.

Not easy. Its a thresholded skeletonized image of 600x800 pixels. — fmonegaglia
– fmonegaglia, Commented Oct 23, 2014 at 8:45
Doesn't matter, make the data up and reduce it to 4X4 or something? Though I suspect John has answered your question anyway. — Ffisegydd
– Ffisegydd, Commented Oct 23, 2014 at 8:46

John Zwinck · Accepted Answer · 2014-10-23 08:44:19Z

3

Since you say mask contains 1's and 0's, the problem is that NumPy treats these as indexes, not as a mask. You probably want to make mask be of boolean type (True/False), in which case it can be the same length as myarray and will select those elements where mask is True.

np.where() always treats the first argument as a boolean array, so it probably does what you want already.

answered Oct 23, 2014 at 8:44

John Zwinck

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