Robrecht W. Uyttenhove <ruyttenhove <at> gmail.com> writes:
>
> Hello,
> I tried out the following
code:y=[range(0,7),range(7,14),range(14,21),range(21,28),range(28,35)]
> >>> y[[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, 25, 26, 27], [28, >
29, 30, 31, 32, 33, 34]]
> >>> y[1:5:2][::3]
> [[7, 8, 9, 10, 11, 12, 13]]
> I expected the 2D list:[[ 7, 10, 13],
> [21, 24, 27]]
>
> Any ideas?
>
> Thanks,
> Rob
> PS: I used Python 2.7.3
>
The explanation is rather simple, just break up your complex slicing into
its parts:
y[1:5:2] => [[7, 8, 9, 10, 11, 12, 13],[21, 22, 23, 24, 25, 26, 27]]
and [::3] is asking for the first,4th,7th,... element from this list.
Obviously, only the first one's existing, so [7, 8, 9, 10, 11, 12, 13]
What you expected is kind of vertical slicing through the rows. I don't
think you can achieve this with slicing alone in standard Python, but it's
possible with numpy arrays.
In Python you will have to combine slicing with a comprehension, like this:
[x[::3] for x in y[1:5:2]]
Best,
Wolfgang
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