How to access a column in a list of lists in python

Question

I have a 2D array in python modeled by a list of lists and I want to extract the column. I made a quick research and I found a way that uses numpy arrays. The problem is that I do not want to use numpy so I don't want to convert my list of lists into a numpy array and then use [:,1] syntax. I tried using it on a normal list of lists but it shows an error so it's not possible. I am asking for a similar thing for list of lists without having to go through each element (In numpy arrays, it's faster to access a column by using [:,1] syntax than iterating over the elements of the array).

I found this link but again it suggests iterating over elements without a shortcut.

Unless you use Numpy, you must use a loop and iterate through all sublists. There is no other way. In fact, Numpy also iterates through the "sublists", but in a more efficient way. — DYZ, Jun 05 '17 at 00:27
@DYZ So I am obliged to "hardcode" it in a function right? Using numpy gives me a hard time for another function that's why I want to avoid it. — Oussama Boussif, Jun 05 '17 at 00:28
@DYZ it's not really hardcoding(I put it between quotes :p), I just thought there might exist a quicker way than having to write a function. Anyways, if that is the only way, I guess I have no choice. — Oussama Boussif, Jun 05 '17 at 00:34
"Using numpy gives me a hard time for another function that's why I want to avoid it." doesn't seem like a valid reason. What exactly gives you a hard time in numpy? I'm sure you can fix that instead... — Julien, Jun 05 '17 at 00:41
@Julien well explaining that is a whole question on stackoverflow so... — Oussama Boussif, Jun 05 '17 at 01:02

score 91 · Accepted Answer · answered Jun 05 '17 at 00:44

91

List comprehensions are your friend when working with lists of lists:

In [111]: alist
Out[111]: 
[[0, 1, 2, 3, 4, 5],
 [6, 7, 8, 9, 10, 11],
 [12, 13, 14, 15, 16, 17],
 [18, 19, 20, 21, 22, 23]]
In [112]: [row[1] for row in alist]
Out[112]: [1, 7, 13, 19]

There's also a handy 'idiom' for transposing a nested list, turning 'columns' into 'rows':

In [113]: tlist = list(zip(*alist))
In [114]: tlist
Out[114]: 
[(0, 6, 12, 18),
 (1, 7, 13, 19),
 (2, 8, 14, 20),
 (3, 9, 15, 21),
 (4, 10, 16, 22),
 (5, 11, 17, 23)]
In [115]: tlist[1]
Out[115]: (1, 7, 13, 19)

answered Jun 05 '17 at 00:44

hpaulj

221,503
14
230
353

2

How about changing the value of the whole row with a row from another array? – Oussama Boussif Jun 05 '17 at 01:07
1

Isn't the first syntax pointing to columns? [row[1] for row in alist] -------> [col[1] for col in alist] the name is clearly not influencing the result, but it reflecting what is the output. – giammi56 Sep 30 '20 at 09:24
1

`[... for sublist in alist]` would be better. – hpaulj Sep 30 '20 at 10:30

score 0 · Answer 2 · answered Feb 24 '23 at 19:58

If you're interested in a more functional approach, you can also use itemgetter from the standard operator library to fetch items in specific position(s) in each sub-list of a nested list.

from operator import itemgetter
mylist = [[0,1], [2,3], [4,5]]
second_column_vals = list(map(itemgetter(1), mylist))         # [1, 3, 5]

first_and_third_column_vals = list(map(itemgetter(0, 2), mylist))

How to access a column in a list of lists in python

2 Answers2

Linked

Related