getting the opposite diagonal of a numpy array

Question

So in numpy arrays there is the built in function for getting the diagonal indices, but I can't seem to figure out how to get the diagonal starting from the top right rather than top left.

This is the normal code to get starting from the top left:

>>> import numpy as np
>>> array = np.arange(25).reshape(5,5)
>>> diagonal = np.diag_indices(5)
>>> array
array([[ 0,  1,  2,  3,  4],
   [ 5,  6,  7,  8,  9],
   [10, 11, 12, 13, 14],
   [15, 16, 17, 18, 19],
   [20, 21, 22, 23, 24]])
>>> array[diagonal]
array([ 0,  6, 12, 18, 24])

so what do I use if I want it to return:

array([ 4,  8, 12, 16, 20])

Answer 1

There is

In [47]: np.diag(np.fliplr(array))
Out[47]: array([ 4,  8, 12, 16, 20])

or

In [48]: np.diag(np.rot90(array))
Out[48]: array([ 4,  8, 12, 16, 20])

Of the two, np.diag(np.fliplr(array)) is faster:

In [50]: %timeit np.diag(np.fliplr(array))
100000 loops, best of 3: 4.29 us per loop

In [51]: %timeit np.diag(np.rot90(array))
100000 loops, best of 3: 6.09 us per loop

Answer 2

Here are two ideas:

step = len(array) - 1

# This will make a copy
array.flat[step:-step:step]

# This will make a veiw
array.ravel()[step:-step:step]