Zee Zee -4 years ago 172
Python Question

Numpy selecting specific column index per row by using a list of indexes

I'm struggling to select the specific columns per row of a numpy matrix. I'm after a good way of solving the following issue

Suppose I have the following matrix which I would call X

[1, 2, 3]
[4, 5, 6]
[7, 8, 9]

I also have a list of column indexes per every row which I would call Y

[1, 0, 2]

I need to get the values


Instead of a list with indexes Y I can also produce a matrix with the same shape as X where every column is a bool/(int in the range 0-1 value indicating whether this is the required column.

[0, 1, 0]
[1, 0, 0]
[0, 0, 1]

I know this can be done with iterating over the array and selecting the column values I need, however I was wondering if there is a better solution. This will be executed frequently on big arrays of data and that's why i has to run as fast as it can.

Thank you

Answer Source

If you've got a boolean array you can do direct selection based on that like so:

>>> a = np.array([True, True, True, False, False])
>>> b = np.array([1,2,3,4,5])
>>> b[a]
array([1, 2, 3])

To go along with your initial example you could do the following:

>>> a = np.array([[1,2,3], [4,5,6], [7,8,9]])
>>> b = np.array([[False,True,False],[True,False,False],[False,False,True]])
>>> a[b]
array([2, 4, 9])

You can also add in an arange and do direct selection on that, though depending on how you're generating your boolean array and what your code looks like YMMV.

>>> a = np.array([[1,2,3], [4,5,6], [7,8,9]])
>>> a[np.arange(len(a)), [1,0,2]]
array([2, 4, 9])

Hope that helps, let me know if you've got any more questions.

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