Assign random numbers in double list of zeros

Question

I have the following code in python:

#-*-coding:cp1252-*-
from random import randint

matriz = [[0] * 5] * 5
print matriz
for i in range(5):
    for j in range(5):
        numero = randint(0,10)
        matriz[i][j] = numero

for  i in matriz:
     print i

Why the all rows are the same???

by example, a random double list generated:

[0, 5, 1, 0, 10]
[0, 5, 1, 0, 10]
[0, 5, 1, 0, 10]
[0, 5, 1, 0, 10]
[0, 5, 1, 0, 10]

Thanks.

This is a duplicate but I don't have time to find it. It is because you are creating 5 copies of the initial list [0] * 5 is a list that then gets duplicated 5 times — PyNEwbie, Jun 09 '16 at 15:00
a hint to the answer would be to run `for i in matriz: print id(i)`... all the entries in matriz are references to the same object — Simon Fraser, Jun 09 '16 at 15:00

score 2 · Answer 1 · answered Jun 09 '16 at 15:00

This is attributed to how Python treats variables, when you do matriz = [[0] * 5] * 5, each element of the list matriz is a pointer to the same list in memory.

To accomplish what you want to do, use this matriz = [ [0]*5 for n in xrange(5)] so that each sub-list is its own list in memory.

score 1 · Accepted Answer · answered Jun 09 '16 at 15:11

Your matrix consists of 5 references to the same list. The shortest way to avoid it would be some nested comprehension along the lines of:

> m = [[random.randint(1,10) for _ in xrange(5)] for _ in xrange(5)]
> m
[[9, 4, 4, 10, 10], [1, 2, 4, 3, 5], [3, 4, 8, 9, 10], [5, 9, 4, 3, 3], [3, 3, 8, 6, 7]]

Assign random numbers in double list of zeros

2 Answers2