Rotating a 2 dimensional array in python

Question

I need to rotate this 2 dimensional array clockwise and counter clockwise.

LIST = [['.', '.', '.', '.', '.', '.'],
        ['.', 'O', 'O', '.', '.', '.'],
        ['O', 'O', 'O', 'O', '.', '.'],
        ['O', 'O', 'O', 'O', 'O', '.'],
        ['.', 'O', 'O', 'O', 'O', 'O'],
        ['O', 'O', 'O', 'O', 'O', '.'],
        ['O', 'O', 'O', 'O', '.', '.'],
        ['.', 'O', 'O', '.', '.', '.'],
        ['.', '.', '.', '.', '.', '.']]

I tried this to rotate the list clockwise but it doesn't give me any results:

rotated_list = []

i = 0
j = 0
x = len(LIST)
y = len(LIST[0])

while i < x:
    while j < y:
        rotated_list[j][i] = LIST[i][j]
        j += 1
    i += 1

The result I need should look something like this:

..OO.OO..
.OOOOOOO.
.OOOOOOO.
..OOOOO..
...OOO...
....O....

stackoverflow.com/questions/65004193/… additional shape if interested — ombk
– ombk, Commented Dec 1, 2020 at 17:17

rizerphe · Accepted Answer · 2020-12-01 17:11:59Z

0

The problem is that you're trying to set a value of what you can imagine being a non-existent place. The list is not an array, it doesn't have any empty cells, instead, you have to l.append(value) to it. Also, you can use for loops instead of while - they do the same thing. Here's my solution:

rotated_list = []

x = len(LIST[0])
y = len(LIST)

for i in range(x):
    rotated_list.append([])
    for j in range(y):
        rotated_list[-1].append(LIST[j][i])

Hope that's helpful!

answered Dec 1, 2020 at 17:11

rizerphe

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1 Comment

Tibo.Smet Over a year ago

Sorry to get back to you so late, I tried this and it worked. Thank you!

Alain T. · Accepted Answer · 2020-12-01 22:42:17Z

You can use zip in a list comprehension to do the rotation:

LIST = [['.', '.', '.', '.', '.', '.'],
        ['.', 'O', 'O', '.', '.', '.'],
        ['O', 'O', 'O', 'O', '.', '.'],
        ['O', 'O', 'O', 'O', 'O', '.'],
        ['.', 'O', 'O', 'O', 'O', 'O'],
        ['O', 'O', 'O', 'O', 'O', '.'],
        ['O', 'O', 'O', 'O', '.', '.'],
        ['.', 'O', 'O', '.', '.', '.'],
        ['.', '.', '.', '.', '.', 'X']]

Rotate once:

LIST = [ [*r][::-1] for r in zip(*LIST) ]

for line in LIST: print(line)

['.', '.', 'O', 'O', '.', 'O', 'O', '.', '.']
['.', 'O', 'O', 'O', 'O', 'O', 'O', 'O', '.']
['.', 'O', 'O', 'O', 'O', 'O', 'O', 'O', '.']
['.', '.', 'O', 'O', 'O', 'O', 'O', '.', '.']
['.', '.', '.', 'O', 'O', 'O', '.', '.', '.']
['X', '.', '.', '.', 'O', '.', '.', '.', '.']

Rotate again:

LIST = [ [*r][::-1] for r in zip(*LIST) ]

for line in LIST: print(line)

['X', '.', '.', '.', '.', '.']
['.', '.', '.', 'O', 'O', '.']
['.', '.', 'O', 'O', 'O', 'O']
['.', 'O', 'O', 'O', 'O', 'O']
['O', 'O', 'O', 'O', 'O', '.']
['.', 'O', 'O', 'O', 'O', 'O']
['.', '.', 'O', 'O', 'O', 'O']
['.', '.', '.', 'O', 'O', '.']
['.', '.', '.', '.', '.', '.']

Rotate again:

LIST = [ [*r][::-1] for r in zip(*LIST) ]

for line in LIST: print(line)

['.', '.', '.', '.', 'O', '.', '.', '.', 'X']
['.', '.', '.', 'O', 'O', 'O', '.', '.', '.']
['.', '.', 'O', 'O', 'O', 'O', 'O', '.', '.']
['.', 'O', 'O', 'O', 'O', 'O', 'O', 'O', '.']
['.', 'O', 'O', 'O', 'O', 'O', 'O', 'O', '.']
['.', '.', 'O', 'O', '.', 'O', 'O', '.', '.']

For counterclockwise rotation you can use this:

LIST = [ [*r] for r in zip(*map(reversed,LIST)) ]

Without list comprehensions:

LIST = list(map(list,map(reversed,zip(*LIST)))) # clockwise
LIST = list(map(list,zip(*map(reversed,LIST)))) # counter clockwise

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