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I need to extract one element from each column of a matrix according to an index vector. Say:

index = [0,1,1]
matrix = [[1,4,7],[2,5,8],[3,6,9]]

Index vector tells me I need the first element from column 1, second element from column 2, and third element from column 3.

The output should be [1,5,8]. How can I write it out without explicit loop?

Thanks

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  • Your example seems pretty non-general. What about an index vector like [0, 2]? What about [0, 0, 1, 2]? Should these also be covered? Commented Aug 28, 2018 at 16:37
  • No need for considering this. Dim of index vector is equal to number of columns of matrix Commented Aug 28, 2018 at 16:54
  • I change the index vector, which is first one from column 1, the second from column 2 and 3 Commented Aug 28, 2018 at 16:56

3 Answers 3

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4

You can use advanced indexing:

index = np.array([0,1,2])
matrix = np.array([[1,4,7],[2,5,8],[3,6,9]])

res = matrix[np.arange(matrix.shape[0]), index]
# array([1, 5, 9])

For your second example, reverse your indices:

index = np.array([0,1,1])
matrix = np.array([[1,4,7],[2,5,8],[3,6,9]])

res = matrix[index, np.arange(matrix.shape[1])]
# array([1, 5, 8])
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4 Comments

Thanks a lot. It works. But I need to select w.r.t columns. This one works for rows.
@MichaelSun, Can you be clearer? The output is [1, 5, 9] as per desired output in your question.
V0.15 has added a np.take_along_axis function that does this kind of indexing. It's interesting reading.
@hpaulj, Thanks, I definitely need to upgrade!
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Since you're working with 2-dimensional matrices, I'd suggest using numpy. Then, in your case, you can just use np.diag:

>>> import numpy as np
>>> matrix = np.array([[1,4,7],[2,5,8],[3,6,9]])
>>> matrix
array([[1, 4, 7],
       [2, 5, 8],
       [3, 6, 9]])
>>> np.diag(matrix)
array([1, 5, 9])

However, @jpp's solution is more generalizable. My solution is useful in your case because you really just want the diagonal of your matrix.

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2 Comments

I should make question more clear. That one is just one example. I need more general thing like get i,j,k element from three columns respectively
Then you should go with @jpp's solution.
1 
val = [matrix[i][index[i]] for i in range(0, len(index))]
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1 Comment

I think OP said How can I write it out without explicit loop

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