Question

I’m learning Python and for practicing purposes I’m writing a script that reads a file (containing a graph in Trivial Graph Format) and runs a couple of graph algorithms on the graph.

I thought about storing the graph in a list of n dictionaries, where n is the number of vertexes and all the edges of a vertex would be stored in a dictionary.

I tried this

edges = [{} for i in xrange(num_vertexes)]
for line in file:
    args = line.split(' ')
    vertex1 = int(args[0])
    vertex2 = int(args[1])
    label = int(args[2])
    edges[vertex1][vertex2] = label

but I’m getting this error for the last line:

IndexError: list index out of range

Answer 1

It looks like vertex1 is probably greater than num_vertexes. Given that python indexes from 0 and the example on the wiki of the format goes from 1, the last line’s vertex number is probably 1 higher than the length of the index (I’d need to see the file to know for sure, of course). So in the python case lst[0] is the first element, and lst[n-1] is the last element where for the vertexes 1 is the first element and n is the last element.

So the fix here is to use vertex1 = int(args[0])-1