Discontinuous slice in python list

Question

I'm looking for an efficient way of achieving this, which I think is a slicing-like operation:

>>> mylist = range(100)
>>>magicslicer(mylist, 10, 20)
[0,1,2,3,4,5,6,7,8,9,30,31,32,33,34,35,36,37,38,39,60,61,62,63......,97,98,99]

the idea is: the slicing gets 10 elements, then skips 20 elements, then gets next 10, then skips next 20, and so on.

I think I should not use loops if possible, for the very reason to use slice is (I guess) to do the "extraction" efficiently in a single operation.

Thanks for reading.

Answer 1

I think that slices cannot do it, unfortunately. I'd solve the problem using list comprehensions

>>> a = range(100)
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 
 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
    ...
 90, 91, 92, 93, 94, 95, 96, 97, 98, 99]
>>> [e for i, e in enumerate(a) if i % 30 < 10]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 
 30, 31, 32, 33, 34, 35, 36, 37, 38, 39,
 60, 61, 62, 63, 64, 65, 66, 67, 68, 69,
 90, 91, 92, 93, 94, 95, 96, 97, 98, 99]

Answer 2

I'd use a loop:

#!/usr/bin/env python


def magicslicer(l, stepsize, stepgap):
    output = []
    i = 0
    while i<len(l):
        output += l[i:i+stepsize]
        i += stepsize + stepgap
    return output


mylist = range(100)
print magicslicer(mylist,10,20)