Question

I have a matrix that should have ones on the diagonal but the columns are mixed up.

But I don’t know how, without the obvious for loop, to efficiently interchange rows to get unity on the diagonals. I’m not even sure what key I would pass to sort on.

Any suggestions?

Answer 1

You can use numpy’s argmax to determine the goal column ordering and reorder your matrix using the argmax results as column indices:

>>> z = numpy.array([[ 0.1 ,  0.1 ,  1.  ],
...                  [ 1.  ,  0.1 ,  0.09],
...                  [ 0.1 ,  1.  ,  0.2 ]])

numpy.argmax(z, axis=1)

>>> array([2, 0, 1]) #Goal column indices

z[:,numpy.argmax(z, axis=1)]

>>> array([[ 1.  ,  0.1 ,  0.1 ],
...        [ 0.09,  1.  ,  0.1 ],
...        [ 0.2 ,  0.1 ,  1.  ]])