resistancefm resistancefm - 2 months ago 8
Python Question

How can I create a vector from a matrix using specified colums from each row without looping in Python?

Say I have a matrix of shape (N,d) and a vector of size N which says which column in the matrix is of relevance to a given row. How can I return the vector of size N which is given by the values in the matrix and the relevant column?

For example:

M = [[ 2, 4, 1, 8],
[3, 5, 7, 1],
[2, 5, 3, 9],
[1, 2, 3, 4]]
V = [2, 1, 0, 1]


I tried something like:

M[:,V]


but this returns a matrix which is NXN

Is there a simple way to format this which does not involve writing a for-loop so that I could get the following vector:

V' = [1,5,2,2]

Answer

Use np.arange(len(V)) for indexing the row numbers and V for columns:

In [110]: M = [[ 2, 4, 1, 8],
   .....:     [3, 5, 7, 1],
   .....:     [2, 5, 3, 9],
   .....:     [1, 2, 3, 4]]

In [111]: V = [2, 1, 0, 1]

In [112]: 

In [112]: M = np.array(M)

In [113]: M[np.arange(len(V)),V]
Out[113]: array([1, 5, 2, 2])
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