How to create new lists in python

Question

I want to create new lists from one list. This the example list I am working on:

matrixlist = [['Matrix', '1'], ['1', '4', '6'], ['5', '2', '9'], ['Matrix', '2'], ['2', '6'], ['1', '3'], ['8', '6'], ['Matrix', '3'], ['5', '6', '7', '9'], ['1', '4', '2', '3'], ['8', '7', '3', '5'], ['9', '4', '5', '3'], ['Matrix', '4'], ['7', '8'], ['4', '6'], ['2', '3']]

I split them like this with for loop:

matrix1 = [['1', '4', '6'], ['5', '2', '9']]
matrix2 = [['2', '6'], ['1', '3'], ['8', '6']]
matrix3 = [['5', '6', '7', '9'], ['1', '4', '2', '3'], ['8', '7', '3', '5'], ['9', '4', '5', '3']]
matrix4 = [['7', '8'], ['4', '6'], ['2', '3']]

But I want to give the long list to program and it create lists and append the relevant elements in it. Like matrix 1 elements in matrix1 list.

Edit: I can't use any advanced built-in function. I can only use simple ones (like append, pop, reverse, range) and my functions in code.

What is wrong with the for loop approach? Are you going to have multiple matrices or there are always 4? — Dani Mesejo
– Dani Mesejo, Commented Oct 20, 2019 at 14:29
There is maybe more than 4. I edited the question maybe it is now clear. — Mrrelaxed
– Mrrelaxed, Commented Oct 20, 2019 at 14:35
If you can change the structure of the input list, I would highly recommend using a 2D array of matrices instead of the current structure. With a 2d array, your first matrix will simply be matrixList[0], the second matrixList[1], and so on. — Anis R.
– Anis R., Commented Oct 20, 2019 at 14:36

Ajax1234 · Accepted Answer · 2019-10-20 14:29:57Z

1

You can use itertools.groupby:

from itertools import groupby
matrixlist = [['Matrix', '1'], ['1', '4', '6'], ['5', '2', '9'], ['Matrix', '2'], ['2', '6'], ['1', '3'], ['8', '6'], ['Matrix', '3'], ['5', '6', '7', '9'], ['1', '4', '2', '3'], ['8', '7', '3', '5'], ['9', '4', '5', '3'], ['Matrix', '4'], ['7', '8'], ['4', '6'], ['2', '3']]
result = [list(b) for a, b in groupby(matrixlist, key=lambda x:x[0] == 'Matrix') if not a]

Output:

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

answered Oct 20, 2019 at 14:29

Ajax1234

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Mrrelaxed Over a year ago

Thanks but I can't use it. I want them in different lists to use them.

Dev Khadka · Accepted Answer · 2019-10-20 14:37:37Z

0

you can do it using list comprehension like below

indx = [i  for i, mat in enumerate(matrixlist )if mat[0]=='Matrix']

matrixes = {matrixlist[i][1]: matrixlist[i+1: j] for i, j in zip(indx, indx[1:])}

# access matrix with its id
matrixes["1"]

answered Oct 20, 2019 at 14:37

Dev Khadka

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1 Comment

Mrrelaxed Over a year ago

Thanks, it is useful for me.

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