Using list comprehension and extend

Question

I have this

rows = self.rows()
aaa = []
for r in range(0, 9, 3):
    bbb = []
    for c in range(0, 9, 3):
        ccc = []
        for s in range(3):
            ccc.extend(rows[r+s][c:c+3])
        bbb.append(ccc)
    aaa.append(bbb)

and it returns this

[
    [
        [5, 0, 0, 0, 6, 0, 0, 2, 9],
        [3, 8, 0, 4, 9, 2, 0, 0, 6],
        [0, 6, 2, 1, 0, 0, 3, 0, 4]
    ],
    [
        [0, 7, 6, 0, 0, 8, 0, 4, 0],
        [0, 4, 0, 2, 0, 5, 0, 3, 1],
        [0, 3, 1, 0, 4, 9, 0, 0, 0]
    ],
    [
        [4, 0, 0, 6, 0, 3, 0, 0, 1],
        [0, 0, 0, 7, 0, 0, 0, 0, 0],
        [0, 5, 3, 2, 0, 8, 0, 0, 6]
    ]
]

which is correct. rows is just a list containing 9 other nested lists, each with exactly 9 integers ranging 0 through 9.

When I try to use list comprehension

[[rows[r+s][c:c+3] for s in range(3) for c in range(0, 9, 3)] for r in range(0, 9, 3)]

I get this

[
    [
        [5, 0, 0],
        [3, 8, 0],
        [0, 6, 2],
        [0, 6, 0],
        [4, 9, 2],
        [1, 0, 0],
        [0, 2, 9],
        [0, 0, 6],
        [3, 0, 4]
    ],
    [
        [0, 7, 6],
        [0, 4, 0],
        [0, 3, 1],
        [0, 0, 8],
        [2, 0, 5],
        [0, 4, 9],
        [0, 4, 0],
        [0, 3, 1],
        [0, 0, 0]
    ],
    [
        [4, 0, 0],
        [0, 0, 0],
        [0, 5, 3],
        [6, 0, 3],
        [7, 0, 0],
        [2, 0, 8],
        [0, 0, 1],
        [0, 0, 0],
        [0, 0, 6]
    ]
]

Clearly I'm doing something wrong, but I can't see what? I've checked other SO questions and they allude to structuring the LC a certain way to stop the innermost list splitting into 9 separate lists, but so far, it's not happening.

I always find [this](https://stackoverflow.com/questions/18072759/list-comprehension-on-a-nested-list/45079294#45079294) helpful when converting a nested for loop to a list comprehension. — quamrana, Jul 16 '21 at 11:04

score 2 · Answer 1 · answered Jul 16 '21 at 11:18

2

Try:

import itertools

[[list(itertools.chain(*[rows[r+s][c:c+3] for s in range(3)])) for c in range(0, 9, 3)] for r in range(0, 9, 3)]

answered Jul 16 '21 at 11:18

JMA

803
4
9

Perfect, thank you! Guess now is as good a time as any to start using itertools. – StevieW Jul 16 '21 at 11:25

Tom Karzes · Accepted Answer · 2021-07-16T11:40:30.187

1

This will give you what you want. This version uses bare Python with no libraries, but seems to be the most concise (of the solutions that work):

[[[rows[r+s][c+i] for s in range(3) for i in range(3)] for c in range(0, 9, 3)] for r in range(0, 9, 3)]

Formatted more readably, this is just:

[[[rows[r+s][c+i] for s in range(3)
                  for i in range(3)]
  for c in range(0, 9, 3)]
 for r in range(0, 9, 3)]

This replaces the slice in the original loop nest with an inner i loop, eliminating the need to concatenate intermediate slices (or to generate them).

edited Jul 16 '21 at 11:40

answered Jul 16 '21 at 11:27

Tom Karzes

22,815
2
22
41

Thank you! I can see where I was going wrong now, appreciated :) – StevieW Jul 16 '21 at 11:32

score 0 · Answer 3 · answered Jul 16 '21 at 11:15

0

You can do something like this,

print([[rows[r+s][c:c+3] * 3 for c in range(0, 9, 3)] for r in range(0, 9, 3)])

answered Jul 16 '21 at 11:15

starboy_jb

899
1
6
13

Gives me `NameError: name 's' is not defined`, I believe because you replaced `s in range(3)` with *3. – StevieW Jul 16 '21 at 11:28

Using list comprehension and extend

3 Answers3