python - getting the opposite diagonal of a numpy array -


so in numpy arrays there built in function getting diagonal indices, can't seem figure out how diagonal starting top right rather top left.

this normal code starting top left:

>>> import numpy 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 use if want return:

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

there 

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 two, np.diag(np.fliplr(array)) faster:

in [50]: %timeit np.diag(np.fliplr(array)) 100000 loops, best of 3: 4.29 per loop  in [51]: %timeit np.diag(np.rot90(array)) 100000 loops, best of 3: 6.09 per loop

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