Question

I’d like to know the best way to read a disconected undirected graph using igraph for python. For instance, if I have the simple graph in which 0 is linked to 1 and 2 is a node not connected to any other. I couldn’t get igraph to read it from a edgelist format(Graph.Read_Edgelist(…)), because every line must be an edge, so the following is not allowed:

0 1
2

I’ve been just wondering if adjacency matrix is my only/best option in this case (I could get it to work through this representation)? I’d rather a format in which I could understand the data by looking it (something really hard when it comes to matrix format).

Thanks in advance!

Answer 1

There’s the LGL format which allows isolated vertices (see Graph.Read_LGL). The format looks like this:

# nodeID
nodeID2
nodeID3
# nodeID2
nodeID4
nodeID5
nodeID
# isolatedNode
# nodeID5

I think you get the basic idea; lines starting with a hash mark indicate that a new node is being defined. After this, the lines specify the neighbors of the node that has just been defined. If you need an isolated node, you just specify the node ID prepended by a hash mark in the line, then continue with the next node.

More information about the LGL format is to be found here.

Another fairly readable format that you might want to examine is the GML format which igraph also supports.