Asked By – Zee
I’m struggling to select the specific columns per row of a NumPy matrix.
Suppose I have the following matrix which I would call
[1, 2, 3] [4, 5, 6] [7, 8, 9]
I also have a
list of column indexes per every row which I would call
[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
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?
Now we will see solution for issue: NumPy selecting specific column index per row by using a list of indexes
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])
This question is answered By – Slater Victoroff
This answer is collected from stackoverflow and reviewed by FixPython community admins, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0