How can I simultaneously select all odd rows and all even columns of an array

Question

I'm new to programming and I need a program, that can select all odd rows and all even columns of a Numpy array at the same time in one code. here is what I tried:

>In [78]: a  >Out[78]: >array([[ 1,  2,  3,  4,  5], >       [ 6,  7,  8,  9, 10], >       [11, 12, 13, 14, 15], >       [16, 17, 18, 19, 20]]) > >In [79]: for value in range(a.shape[0]): >     if value %2 == 0: >        print a[value,:]  >[1 2 3 4 5] >[11 12 13 14 15] > >In [82]: for value in range(a.shape[1]): >    if value %2 == 1: >        print a[:,value]  >[ 2  7 12 17] >[ 4  9 14 19] 

I've read something with (: even) but don't know in which way I could use it. Thanks for your Help.

Han

Answer 1

Let's say you have this array, x:

>>> import numpy >>> x = numpy.array([[ 1,  2,  3,  4,  5], ... [ 6,  7,  8,  9, 10], ... [11, 12, 13, 14, 15], ... [16, 17, 18, 19, 20]]) 

To get every other odd row, like you mentioned above:

>>> x[::2] array([[ 1,  2,  3,  4,  5],        [11, 12, 13, 14, 15]]) 

To get every other even column, like you mentioned above:

>>> x[:, 1::2] array([[ 2,  4],        [ 7,  9],        [12, 14],        [17, 19]]) 

Then, combining them together yields:

>>> x[::2, 1::2] array([[ 2,  4],        [12, 14]]) 

For more details, see the Indexing docs page.

Answer 2

To get every other odd column:

 x[:,0::2]