Rotating a list of lists

Question

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

    rotated_map = []
    for i in range(len(m[0])):
      rotated_map.append([x[i] for x in m]) 
      print(rotated_map)

    """
    my result = [[5, 2, 6, 1], [9, 4, 3, 7], [1, 5, 3, 6], [8, 7, 2, 3]]

    desired result = [[8,7,2,3],
                      [1,5,3,6],
                      [9,4,3,7],
                      [5,2,6,1]]
    """

I am trying to rotate the list by putting all the last elements first from the lists into one list then the second to last element into another and so on until i get to the first element.

Also, i not even sure how to indent it to get the desired result — Naafiu Mohammed, Nov 10 '18 at 20:07

timgeb · Answer 1 · 2018-11-10T20:33:26.527

2

Transpose the list with zip, then reverse it with the [::-1] syntax.

>>> m = [[5, 9, 1, 8], [2, 4, 5, 7], [6, 3, 3, 2], [1, 7, 6, 3]]
>>> list(map(list, zip(*m)))[::-1]
>>> [[8, 7, 2, 3], [1, 5, 3, 6], [9, 4, 3, 7], [5, 2, 6, 1]]

edit:

If you want pretty printing, it's probably easiest to use numpy arrays all the way.

>>> import numpy as np
>>> 
>>> m = [[5, 9, 1, 8], [2, 4, 5, 7], [6, 3, 3, 2], [1, 7, 6, 3]]
>>> m = np.array(m)
>>> m
>>> 
array([[5, 9, 1, 8],
       [2, 4, 5, 7],
       [6, 3, 3, 2],
       [1, 7, 6, 3]])
>>> 
>>> m.T[::-1]
>>> 
array([[8, 7, 2, 3],
       [1, 5, 3, 6],
       [9, 4, 3, 7],
       [5, 2, 6, 1]])

Note that m and m.T[::-1] share the same data, because m.T[::-1] is just another view of m. If you need to duplicate the data, use

result = m.T[::-1].copy()

edited Nov 10 '18 at 20:33

answered Nov 10 '18 at 20:09

timgeb

76,762
20
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hey, i havent learnt zip but do you know how to indent the lists so it may be in columns of 4 – Naafiu Mohammed Nov 10 '18 at 20:16
@NaafiuMohammed you mean how to print it as a square? – timgeb Nov 10 '18 at 20:21
@NaafiuMohammed there's a question about this [here](https://stackoverflow.com/questions/13214809/pretty-print-2d-python-list). – timgeb Nov 10 '18 at 20:26

sacuL · Answer 2 · 2018-11-10T20:29:28.790

1

You could use zip, unpacking your list of lists with the *, and inversing the result with [::-1]:

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

res = [list(i) for i in zip(*m)][::-1]

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

If numpy is an option, transposing is easier:

import numpy as np
>>> np.transpose(m)[::-1]
array([[8, 7, 2, 3],
       [1, 5, 3, 6],
       [9, 4, 3, 7],
       [5, 2, 6, 1]])
# or:
>>> np.flip(np.transpose(m),0)
array([[8, 7, 2, 3],
       [1, 5, 3, 6],
       [9, 4, 3, 7],
       [5, 2, 6, 1]])

edited Nov 10 '18 at 20:29

answered Nov 10 '18 at 20:09

sacuL

49,704
8
81
106

numpy is not an option – Naafiu Mohammed Nov 11 '18 at 13:55

score 0 · Answer 3 · answered Nov 10 '18 at 20:16

0

You can use numpy module to do it. It has the property to transpose the array. Check the below code:

import numpy as np
m = [[5,9,1,8],
    [2,4,5,7],
    [6,3,3,2],
    [1,7,6,3]]
arr = np.array(m).transpose()
new_list = []
for i in range(arr.shape[0]-1,-1,-1):
    new_list.append(list(arr[i]))

print(new_list)

Output:

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

answered Nov 10 '18 at 20:16

Sanchit Kumar

1,545
1
11
19

Hey thank you, i get this. i am trying to figure out how i can indent it so it can look like m originally – Naafiu Mohammed Nov 10 '18 at 20:21

score 0 · Answer 4 · answered Jan 04 '22 at 14:43

0

If you want to rotate the list clockwise:

list(map(list, zip(*m[::-1])))

Else, for anti-clockwise:

list(map(list, zip(*m)))[::-1]

answered Jan 04 '22 at 14:43

Suprateem Banerjee

428
6
13

score 0 · Answer 5 · answered Jan 04 '22 at 14:50

0

use the reverse keyword

result = [[5, 2, 6, 1], [9, 4, 3, 7], [1, 5, 3, 6], [8, 7, 2, 3]]
result.reverse()
print(result)

output:

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

answered Jan 04 '22 at 14:50

Golden Lion

3,840
2
26
35

Rotating a list of lists

5 Answers5

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