Rotating a two-dimensional array in Python

Question

In a program I\'m writing the need to rotate a two-dimensional array came up. Searching for the optimal solution I found this impressive one-liner that does the job:

Answer 1

That's a clever bit.

First, as noted in a comment, in Python 3 zip() returns an iterator, so you need to enclose the whole thing in list() to get an actual list back out, so as of 2020 it's actually:

list(zip(*original[::-1]))

Here's the breakdown:

• [::-1] - makes a shallow copy of the original list in reverse order. Could also use reversed() which would produce a reverse iterator over the list rather than actually copying the list (more memory efficient).
• * - makes each sublist in the original list a separate argument to zip() (i.e., unpacks the list)
• zip() - takes one item from each argument and makes a list (well, a tuple) from those, and repeats until all the sublists are exhausted. This is where the transposition actually happens.
• list() converts the output of zip() to a list.

So assuming you have this:

[ [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9] ]

You first get this (shallow, reversed copy):

[ [7, 8, 9],
  [4, 5, 6],
  [1, 2, 3] ]

Next each of the sublists is passed as an argument to zip:

zip([7, 8, 9], [4, 5, 6], [1, 2, 3])

zip() repeatedly consumes one item from the beginning of each of its arguments and makes a tuple from it, until there are no more items, resulting in (after it's converted to a list):

[(7, 4, 1), 
 (8, 5, 2), 
 (9, 6, 3)]

And Bob's your uncle.

To answer @IkeMiguel's question in a comment about rotating it in the other direction, it's pretty straightforward: you just need to reverse both the sequences that go into zip and the result. The first can be achieved by removing the [::-1] and the second can be achieved by throwing a reversed() around the whole thing. Since reversed() returns an iterator over the list, we will need to put list() around that to convert it. With a couple extra list() calls to convert the iterators to an actual list. So:

rotated = list(reversed(list(zip(*original))))

We can simplify that a bit by using the "Martian smiley" slice rather than reversed()... then we don't need the outer list():

rotated = list(zip(*original))[::-1]

Of course, you could also simply rotate the list clockwise three times. :-)