Question

I would like to find node connectivity between node 1 and rest of the nodes in a graph. The input text file format is as follows:

1 2 1
1 35 1
8 37 1

and so on for 167 lines. First column represents source node, second column represents destination node while the last column represents weight of the edge.

I’m trying to read the source, destination nodes from input file and forming an edge between them. I need to then find out if it is a connected network (only one component of graph and no sub-components). Here is the code

from numpy import*
import networkx as nx
G=nx.empty_graph()

for row in file('out40.txt'):
    row = row.split()
    src = row[0]
    dest = row[1]
    #print src
    G.add_edge(src, dest)
    print src, dest

for i in range(2, 41):
    if nx.bidirectional_dijkstra(G, 1, i): print "path exists from 1 to ", i

manually adding the edges using

G.add_edge(1, 2)

works but is tedious and not suitable for large input files such as mine. The if loop condition works when I add edges manually but throws the following error for the above code:

in neighbors_iter
raise NetworkXError("The node %s is not in the graph."%(n,))
networkx.exception.NetworkXError: The node 2 is not in the graph.

Any help will be much appreciated!

Answer 1

In your code, you’re adding nodes "1" and "2" et cetera (since reading from a file is going to give you strings unless you explicitly convert them).

However, you’re then trying to refer to nodes 1 and 2. I’m guessing that networkx does not think that 2 == "2".

Try changing this…

G.add_edge(src, dest)

to this:

G.add_edge(int(src), int(dest))