Insert String values into 2d array/matrix

Question

I have a string of 1s and 0s that I need to insert into a [4] by [4] matrix, that I can then use for other things.

This is my attempt at it:

b = '0110000101101000'

m = [[], [], [], []]
for i in range(4):
    for j in range(4):
        m[i].append(b[i * j])

But where I expected to get

[['0', '1', '1', '0'], ['0', '0', '0', '1'], ['0', '1', '1', '0'], ['1', '0', '0', '0']

I got [['0', '0', '0', '0'], ['0', '1', '1', '0'], ['0', '1', '0', '0'], ['0', '0', '0', '1']].

Could someone point me in the right direction here?

John Zwinck · Accepted Answer · 2021-01-23 23:14:56Z

2

Get paper and a pencil and write a table of what you have now vs what you want:

i j i*j desired
0 0  0  0
0 1  0  1
0 2  0  2
0 3  0  3
1 0  0  4
1 1  1  5
... up to i=3, j=3

Now you can see that i * j is not the correct index in b. Can you see what the desired index formula is?

answered Jan 23, 2021 at 23:14

John Zwinck

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Comments

marc_s · Accepted Answer · 2021-01-24 14:08:41Z

0

I'd agree with @John Zwinck that you can easily figure it out but if you hate math simply do

counter = 0
for i in range(4):
    for j in range(4):
        m[i].append(b[counter])
        counter += 1 # keep track of the overall iterations

otherwise you have to find the starting row you are in (i * columns) and add the current column index

m[i].append(b[i * 4 + j]) # i * 4 gives the overall index of the 0th element of the current row

edited Jan 24, 2021 at 14:08

marc_s

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answered Jan 23, 2021 at 23:18

Countour-Integral

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Abner3 · Accepted Answer · 2021-01-23 23:19:55Z

0

Here is a hint: range(4) starts from 0 and ends at 3. See the python documentation: https://docs.python.org/3.9/library/stdtypes.html#typesseq

answered Jan 23, 2021 at 23:19

Abner3

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Comments

Berthin · Accepted Answer · 2021-01-23 23:54:06Z

0

First of all, the rule to convert coordinates to index is index = row * NRO_COLS + col. You should use i * 4 + j.

Second, you can use list comprehension:

m = [[b[i * 4 + j] for j in range(4)] for i in range(4)]

then, it can be rewritten as:

m = [[b[i + j] for j in range(4)] for i in range(0, len(b), 4)]

or

m = [list(b[i:i+4]) for i in range(0, len(b), 4)]

another alternative is to use numpy, which is a great library, specially to handle multidimensional arrays

import numpy as np

m = np.array(list(b)).reshape(4,4)

or:

print(np.array(list(b)).reshape(4, -1))
print(np.array(list(b)).reshape(-1, 4))

answered Jan 23, 2021 at 23:54

Berthin

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