# Cycling through iterables diagonally

Pablo Lucena plucena24 at gmail.com
Fri Feb 26 02:44:05 EST 2016

Hello,

I am trying to accomplish the following:

Say I have a group of 4 lists as follows:

l1 = ['a1', 'a2', 'a3', 'a4']
l2 = ['b1', 'b2', 'b3', 'b4']
l3 = ['c1', 'c2', 'c3', 'c4']
l4 = ['d1', 'd2', 'd3', 'd4']

I would like to cycle through these lists "diagonally" in groups of
len(list) (in this example, each list has 4 items).

cycle1: a1, b2, b3, b4
cycle2: a2, b3, c4, d1
cycle3: a3, b4, c1, d2
cycle4: a4, b1, c2, d3

The way I thought about doing this is as follows:

from collections import deque
from itertools import cycle

l1 = deque(['a1', 'a2', 'a3', 'a4'])
l2 = deque(['b1', 'b2', 'b3', 'b4'])
l3 = deque(['c1', 'c2', 'c3', 'c4'])
l4 = deque(['d1', 'd2', 'd3', 'd4'])

l1.rotate(-0)
l2.rotate(-1)
l3.rotate(-2)
l4.rotate(-3)

groups = cycle([l1, l2, l3, l4])

In [115]: for group in groups:
.....:     if not group:
.....:         break
.....:     print(group.popleft())
.....:
a1
b2
c3
d4
a2
b3
c4
d1
a3
b4
c1
d2
a4
b1
c2
d3

Prior to this I was mucking around with index counting while looping, and
popping lists out of a deque, popping an item out of the list, and
appending the list back into the deque during each iteration.

Is there a better/cleaner way to do this? I was hoping for some cool
itertools logic =)

Thanks!

--
*Pabl​o​*