Adjacency Matrix not populating correctly python

Question

I tried to implement a simple adjacency matrix to keep track of which nodes are connected to which nodes in an undirected graph. However, my adjacency matrix keeps screwing up by changing entire columns instead of individual cells. Here's my code:

def setup_adj_matrix(size, edges):
    # initialize matrix with zeros
    adj_matrix = [[0] * size] * size
    # edges is a list of tuples, representing 2 nodes connected by an edge
    for edge in edges:
        v1 = edge[0]
        v2 = edge[1]
        adj_matrix[v1][v2] = 1
        adj_matrix[v2][v1] = 1
    for row in adj_matrix:
        print row

for a graph with 3 nodes (0, 1, 2) and edges [(0,1),(0,2),(1,2)], I should be getting

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

However, I am getting all 1's. Any ideas where the problem might be?

Answer 1

The multiplication operator with a list and an int returns multiple references to the same list, not multiple copies. Your array contains the same object nine times.

You can create the correct array with a double list comprehension:

def init_matrix(x, y):
    return [[[0] for i in range(x)] for j in range(y)]

Answer 2

The lists are all shallow copies of each other, so when you edit one you're actually editing each row. Try this for initializing the matrix:

adj_matrix = [[0] * size for i in range(size)]