Returning matrix columns as lists in Python

Question

If I have the following matrix:

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

how do I return a list that looks like this

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

without using built-in functions?

Edit: I can use len() and range()

Yes, I can use range, sorry haha, I meant if there are any other built-ins that are specifically made for this type of problem — lilai, Oct 25 '21 at 10:33
Does this answer your question? [Matrix Transpose in Python](https://stackoverflow.com/questions/4937491/matrix-transpose-in-python) — sushanth, Oct 25 '21 at 10:34

score 2 · Answer 1 · answered Oct 25 '21 at 10:34

2

You can use a nested comprehension, à la:

[[row[i] for row in m] for i in range(len(m[0]))]

If the typical transpositioning idiom zip(*m) (or [*map(list, zip(*m))] if exact types matter) is to be avoided as too purpose-built.

answered Oct 25 '21 at 10:34

user2390182

72,016
6
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other version [[m[j][i] for j, _ in enumerate(m[i])] for i,_ in enumerate(m)] with minimal builtins – doze Oct 25 '21 at 10:57
True, however, I would consider `enumerate` a "higher level" util than `range` and `len` :) – user2390182 Oct 25 '21 at 11:01

score 1 · Answer 2 · answered Oct 25 '21 at 10:38

1

Weird approach:

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

res = map(lambda *args: args, *m)
print(list(res))

Output

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

If list are required:

res = list(map(lambda *args: list(args), *m))
print(res)

Alternative:

res = [*map(lambda *args: list(args), *m)]

answered Oct 25 '21 at 10:38

Dani Mesejo

61,499
6
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1

Very creative `zip` implementation: `zip = lambda *iterables: map(lambda *args: args, *iterables)` One does have to stare down what's happening though :D +1 – user2390182 Oct 25 '21 at 11:06

score 0 · Answer 3 · answered Oct 25 '21 at 10:35

0

try this

m = [[1, 2, 3],
     [4, 5, 6], 
     [7, 8, 9]]
result=[[0 for i in range(len(m))] for j in range(len(m[0]))]
for i in range(len(m)):
    for j in range(len(m[0])):
        result[j][i]=m[i][j]
print(result)

answered Oct 25 '21 at 10:35

Prasad Darshana

186
2
14

Returning matrix columns as lists in Python

3 Answers3