How to create a binary matrix with some given condition below:

Question

For a given list of tuples L whose elements are taken from range(n), I want to create A binary matrix A of order n in the following way:

If (i,j)  or (j,i) in L then A[i][j]=1 otherwise A[i][j]=0.

Let us consider the following example:

L=[(2,3),(0,1),(1,3),(2,0),(0,3)]
A=[[0]*4]*4
for i in range(4):
    for j in range(4):
        if (i,j) or (j,i) in L:
            A[i][j]=1
        else:
            A[i][j]=0

print A

This program does not give the accurate result. Where is the logical mistake occurred?

Possible duplicate of [How to test multiple variables against a value?](https://stackoverflow.com/questions/15112125/how-to-test-multiple-variables-against-a-value) — Aran-Fey, Apr 24 '18 at 11:37
https://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly — Aran-Fey, Apr 24 '18 at 11:37
Change `A=[[0]*4]*4` to `A=[[0]*4 for _ in range(4)]` and `(i,j) or (j,i) in L` to `(i,j) in L or (j,i) in L`. — Aran-Fey, Apr 24 '18 at 11:39
I mean the out put should A=[[0,1,1,1,1],[1,0,0,1],[1,0,0,1],[1,1,1,0]] @zwer — Primo Raj, Apr 24 '18 at 11:41
@Aran-Fey Thank you for your correction. Now this gives the desired result. — Primo Raj, Apr 24 '18 at 11:45
@Aran-Fey Can you explain please. When I use A=[[0]*4]*4, the outcome gives wrong but when I use A=[[0]*4 for _ in range(4)], the outcome is correct, Why? — Primo Raj, Apr 24 '18 at 11:47
It's already explained [here](https://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly). — Aran-Fey, Apr 24 '18 at 11:48

jpp · Answer 1 · 2018-04-24T11:51:09.647

You should use a 3rd party library, numpy, for matrix calculations.

Python lists of lists are inefficient for numeric arrays.

import numpy as np

L = [(2,3),(0,1),(1,3),(2,0),(0,3)]

A = np.zeros((4, 4))

idx = np.r_[L].T
A[idx[0], idx[1]] = 1

Result:

array([[ 0.,  1.,  0.,  1.],
       [ 0.,  0.,  0.,  1.],
       [ 1.,  0.,  0.,  1.],
       [ 0.,  0.,  0.,  0.]])

Related: Why NumPy instead of Python lists?

score 0 · Answer 2 · answered Apr 24 '18 at 11:48

According to Aran-Fey's correction the answer is :

L=[(2,3),(0,1),(1,3),(2,0),(0,3)]
#A=[[0]*4]*4
A=[[0]*4 for _ in range(4)]
for i in range(4):
    for j in range(4):
        if (i,j) in L or (j,i) in L:
            A[i][j]=1
        else:
            A[i][j]=0

print A

How to create a binary matrix with some given condition below:

2 Answers2