Cycling along a list

This question on Stack Overflow involved the problem of moving along a list and then, once you reach the end, cycling back to the beginning. One very simple way to do that is to use the % (mod) operator. If the length of the list isn't changing (and it isn't a good idea to change a list during iteration), you can cache the length of the list:

>>> L = list('abcde') >>> N = len(L) >>> rL = list() >>> for i in range(12): ... rL.append(L[i%N]) ... >>> print ''.join(rL) abcdeabcdeab

Other answers used several functions from itertools:

from itertools import cycle

We can check out the docs (remember)

$ pydoc -w itertools

>>> L = list('abcde') >>> it = cycle(L) >>> rL = list() >>> for i in range(12): ... rL.append(it.next()) ... >>> print ' '.join(rL) a b c d e a b c d e a b

To get the elements in groups of two (advancing one index at a time):

>>> it = cycle(L) >>> x = it.next() >>> for i in range(12): ... y = it.next() ... rL.append(x+y) ... x = y ... >>> print ' '.join(rL) ab bc cd de ea ab bc cd de ea ab bc

Or, you can do a full itertools solution with the functions tee and izip as well as cycle.

tee makes two independent iterators for the sequence and izip zips them up (of course):

>>> from itertools import izip, cycle, tee >>> >>> def pairwise(seq): ... a, b = tee(seq) ... next(b) ... return izip(a, b) ... >>> L = list('abcde') >>> rL = list() >>> it = pairwise(cycle(L)) >>> for i in range(12): ... rL.append(''.join(it.next())) ... >>> print ' '.join(rL) ab bc cd de ea ab bc cd de ea ab bc

Or use islice:

>>> from itertools import islice >>> it = pairwise(cycle(L)) >>> [''.join(t) for t in islice(it,6)] ['ab', 'bc', 'cd', 'de', 'ea', 'ab']