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From: Pablo Lucena <plucena24@gmail.com>
Newsgroups: comp.lang.python
Subject: Cycling through iterables diagonally
Date: Thu, 25 Feb 2016 23:44:05 -0800
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Hello,

I am trying to accomplish the following:

Say I have a group of 4 lists as follows:

l1 =3D ['a1', 'a2', 'a3', 'a4']
l2 =3D ['b1', 'b2', 'b3', 'b4']
l3 =3D ['c1', 'c2', 'c3', 'c4']
l4 =3D ['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 =3D deque(['a1', 'a2', 'a3', 'a4'])
l2 =3D deque(['b1', 'b2', 'b3', 'b4'])
l3 =3D deque(['c1', 'c2', 'c3', 'c4'])
l4 =3D deque(['d1', 'd2', 'd3', 'd4'])

l1.rotate(-0)
l2.rotate(-1)
l3.rotate(-2)
l4.rotate(-3)

groups =3D 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 =3D)

Thanks!


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