Question

I feel like I’m doing something very dumb here. Maybe I’m just too tired.

I’m trying to make a truth table with a size depending on “count” – the number of variables for the table.

    table = [[None] * int(pow(2, count))] * count
    in_a_row = pow(2, count) / 2
    iterator = 0
    for i in range(count):
        for j in range(int(pow(2, count))):
            print(str(i) + ' ' + str(j), end = '')
            if iterator < in_a_row:
                table[i][j] = 'T'
                print(' T')
            elif iterator == 2 * in_a_row:
                table[i][j] = 'T'
                iterator = 0
                print(' T')
            else:
                table[i][j] = 'F'
                print(' F')
            iterator += 1

        print(table)
        in_a_row /= 2
        iterator = 0

Which outputs this:

0 0 T
0 1 T
0 2 F
0 3 F
[['T', 'T', 'F', 'F'], ['T', 'T', 'F', 'F']]
1 0 T
1 1 F
1 2 T
1 3 F
[['T', 'F', 'T', 'F'], ['T', 'F', 'T', 'F']]

You can see what whatever I set in one iteration is echoed in all “rows”. Can anyone show me what’s wrong here?

I’m of course expecting this:

[['T', 'T', 'F', 'F'], ['T', 'F', 'T', 'F']]

Answer 1

Change the first line to:

table = [[None] * int(pow(2, count)) for _ in range(count) ]

as you’ve no doubt noticed, in the first version, each row shares a reference to the same list, whereas in this new version, we create a new list for each row.