Robrecht W. Uyttenhove wrote:
> 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?
It is not really a 2D list; rather a list of lists. You cannot see the two
slices together, the slicing happens in two separate steps.
y[1:5:2] is a list containing two items (that happen to be lists)
>>> x = y[1:5:2]
>>> x
[[7, 8, 9, 10, 11, 12, 13], [21, 22, 23, 24, 25, 26, 27]]
x[::3] then operates on the outer list
>>> x[::3]
[[7, 8, 9, 10, 11, 12, 13]]
By the way, numpy offers the behaviour you expected:
>>> import numpy
>>> a = numpy.array(y)
>>> a
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, 25, 26, 27],
[28, 29, 30, 31, 32, 33, 34]])
>>> a[1:5:2,::3]
array([[ 7, 10, 13],
[21, 24, 27]])
Note that both slices are passed in the same a[...] operation.
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