How to import multiple matrices from a file in Python

Question

I'm looking for a way to import N matrices from one file to a single 3D array. I know the number of rows m and the number of columns n of each matrix (respectively listed in two lists m[N] and n[N]).

For example I have N=3 matrices like these:

1  2   3   4
5  6   7   8
9  10  11  12
1   2   3
4   5   6
7   8   9
10  11  12
1  2  3  4   5   6 
7  8  9  10  11  12

They are matrices of dimension 3x4, 4x3, 2x6 respectively, so in this case m = [3,4,2] and n = [4,3,6]. I would store them in an object of the type M[i][j][k] where i,j,k are the indeces that identify the matrix, the row and the column respectively (in this case M[1][2][0]=7).

For example in C++ this can be easily done:

ifstream file ("filename.txt");
  for(int i = 0; i < N; i++)
    for(int j = 0; j < m[i]; j++)
      for(int k = 0; k < n[i]; k++)
        file >> M[i][j][k];

file.close();

Thanks in advance.

@Epsi95, I usually use numpy.genfromtxt but here the problem is the different number of columns. Nested python list could be the right way, but i don't know how to import the data. — Wil
– Wil, Commented Feb 4, 2021 at 13:15
"I know the number of rows m and the number of columns n of each matrix." how do you have these m and n? — python_user
– python_user, Commented Feb 4, 2021 at 13:27

python_user · Accepted Answer · 2021-02-04 14:21:51Z

1

You can use a list comprehension with itertools.islice to get what you want. islice allows you to read lines from the file without having to read the whole file into the memory.

from itertools import islice

m = [3, 4, 2]
n = [4, 3, 6]

M = []

with open('matrix_data.txt') as file:
    for row_count in m:
        rows = islice(file, row_count) # read `row_count` lines from the file
        rows = [row.split() for row in rows] # split every line in the file on whitespace
        rows = [[int(cell) for cell in row] for row in rows] # convert to int
        M.append(rows) # add to `M`

for matrix in M:
    print(matrix)

Output

[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]
[[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]]
[[1, 2, 3, 4, 5, 6], [7, 8, 9, 10, 11, 12]]

edited Feb 4, 2021 at 14:21

answered Feb 4, 2021 at 14:15

python_user

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6 Comments

Wil Over a year ago

Thank you very much!! So the n[N] is not needed.

python_user Over a year ago

not for the way I suggested, m is enough, you can upvote the answer as well :) if it helped

Wil Over a year ago

how can I convert M to a np.ndarray? thank you

python_user Over a year ago

you mean a 3d numpy array?

Wil Over a year ago

Exactly! Because in this way, for example the first row of the first matrix [1, 2, 3, 4] is a list.

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