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.

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:

    [2]
    [4]
    [9]

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, this will be executed frequently on big arrays of data and that's why it has to run as fast as it can.

I was thus wondering if there is a better solution?

Thank you.

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 q.

From: stackoverflow.com/q/23435782