Merge multiple 2d lists considering axis in order

Question

My purpose is to combine multiple 2d list in order such as:

a = [[1,2],[3,1]]
b= [[3,6],[2,9]]
c = [[5,1],[8,10]]
Expected: [[1,2,3,6,5,1],[3,1,2,9,8,10]]

Following other's advice from this site, I tried to use collections module like the code below:

from collections import Counter
a = [[1,2],[3,1]]
b= [[3,6],[2,9]]
c = [[5,1],[8,10]]
d = [[k,v] for k,v in (Counter(dict(a)) + Counter(dict(b))+ Counter(dict(c))).items()]
print d

However, the result is [[1, 2], [3, 1], [3, 6], [2, 9]] which is not what I expected.

Do you have any idea to solve this problem? Maybe if there is function or module to consider the axis to combine the lists.

Answer 1

You can use zip and a list comprehension:

>>> a = [[1,2],[3,1]]
>>> b = [[3,6],[2,9]]
>>> c = [[5,1],[8,10]]
>>> [x+y+z for x,y,z in zip(a, b, c)]
[[1, 2, 3, 6, 5, 1], [3, 1, 2, 9, 8, 10]]
>>>

Answer 2

You could use itertools.chain.from_iterable():

>>> a = [[1, 2], [3, 1]]
>>> b = [[3, 6], [2, 9]]
>>> c = [[5, 1], [8, 10]]
>>> from itertools import chain
>>> [list(chain.from_iterable(x)) for x in zip(a, b, c)]
[[1, 2, 3, 6, 5, 1], [3, 1, 2, 9, 8, 10]]

This might be handy if you have an arbitrary number of 2D lists - for example:

>>> list_of_lists = [
...     [[1, 2], [3, 1]],
...     [[3, 6], [2, 9]],
...     [[5, 1], [8, 10]],
...     # ...
...     [[4, 7], [11, 12]]
... ]
>>> [list(chain.from_iterable(x)) for x in zip(*list_of_lists)]
[[1, 2, 3, 6, 5, 1, ..., 4, 7], [3, 1, 2, 9, 8, 10, ..., 11, 12]]

Note the * before list_of_lists in this last example, which is an example of argument unpacking.

Answer 3

This might be another solution using numpy but this is much slower.

import numpy as np

a = [[1,2],[3,1]]
b = [[3,6],[2,9]]
c = [[5,1],[8,10]]

print np.hstack((np.hstack((a,b)),c))

# [[ 1  2  3  6  5  1]
# [ 3  1  2  9  8 10]]

and if you want it to have a list format then use

np.hstack((np.hstack((a,b)),c)).tolist()