Convert list to matrix (python)

Question

I would like to convert a list to a matrix so that I can use the .append function the way I need it. list = ["1", "2", "3"] should become [["1", "2", "3"]].

Then when I use list.append(["4", "5", "6"]) it will become [["1", "2", "3"], ["4", "5", "6"]] instead of ["1", "2", "3", ["4", "5", "6"]].

How can I do this?

Solution 1

This can be done very easily:

def list_to_matrix(lst):
    return [lst]

Usage:

>>> lst = [1,2,3,4]
>>> lst = list_to_matrix(lst)
>>> lst
[[1,2,3,4]]

If we have cases where we might get things that are already in list-of-list form, we can add a little more robustness by doing this:

from collections import Iterable

def list_to_matrix(lst):
    if lst:
        return lst if all(isinstance(item, Iterable) and not isinstance(item, str) for item in lst) else [lst]
    else:
        # returns empty matrix if input list was empty
        return [[]]

Note that this method is pretty fragile, but at least allows you to safely pass either a base list or a list of list form blindly and get back the correct thing.

Solution 2

Do you mean this (a matrix as a list of lists):

In [1]: matrix = [[1,2,3]]

In [2]: matrix
Out[2]: [[1, 2, 3]]

In [3]: matrix.append([4,5,6])

In [4]: matrix
Out[4]: [[1, 2, 3], [4, 5, 6]]

Solution 3

This works for any objects in list. It gives to every object its coordinates in pseudo 2d list:

def to_matrix(l, n):
    return [(x[0] / n, x[0] % n , x) for x in l]

The output for to_matrix([objects], 3) would be:

[(0, 0, some_object),
 (0, 1, some_object),
 (0, 2, some_object),
 (1, 0, some_object),
 (1, 1, some_object),
 (1, 2, some_object),
 (2, 0, some_object),
 (2, 1, some_object),
 (2, 2, some_object)]

If you need a real 2d array, this code will do the job:

def toMatrix(l, n):
    matrix = []
    for i in range (0, len(l), n):
        matrix.append(l[i:i+n])

    return matrix

Comments

[itzMEonTV]

list =[ ["1", "2", "3"]], then you can append lists. like list.append(["4", "5", "6"]) which gives you expected result

[SecondLemon]

Thanks got it. Tried this myself in the json.dump([list],file) but it didn't work here (which I understand now). Simply added a definition (list2 = [list] before the son.dump and now it works. Thank!

[itzMEonTV]

no need to create new list2. Simply list = [list] and then you can append.

[SecondLemon]

Well I can't change my 'list'. I get a huge list every 30 min via my code and wish to append it. I check if the file is empty or not so it can first print a list and then use the append function to add further list resulting in a matrix.

[Ramces Gonzalez]

thanks so much, this helped me solve my issue of converting a 100 item list into a matrix of 10x10, now i'm going to see how to convert this matrix into a gray-scale image