Question

I want to read a graph, whose vertexes are numbered 0..n-1. I tried the following code:

myfile = open('test.in', 'r')
n = int(myfile.readline())
graph = [[]]*n
for line in myfile:
    u, v, w = map(int, line.strip().split(' '))
    graph[u].append((v, w))
print graph

but it doesn’t work well. I found that all elements in graph are the same! Then I use the following code to fix it:

road = [[] for i in xrange(n)]

It does work, but looks not so pretty.

I want to know if there is a better solution to this situation and why all elements are the same in the first code?

Answer 1

You could use

from collections import defaultdict
graph = defaultdict(list)

The reason that graph = [[]]*n doesn’t work is that you are creating a single empty list and then making many references to that same list