[igraph] Question about igraph_linegraph

[Giuseppe G]

Hi,

I've just started using the C API for a number of purposes and I have a 
question about the igraph_linegraph function.

I need to compute the line graph of a network similar to the following

---
a b
a c
a d
b d
---

It is my understanding that igraph does not deal with string entries so 
I convert the network into numbers

--
1 2
1 3
1 4
2 4
--
and store the id->number map elsewhere.

Now, I need to compute the line graph of this graph, which is stored in 
a textfile. I wrote the following code

int main(int argc, char **argv) {
    if ( argc != 2 ){
        fprintf(stderr, "usage: %s filename.in\n", argv[0] );
        exit(1);
    }
    FILE *instream;
    igraph_t graph;
    igraph_t linegraph;
    int result;

    instream = fopen(argv[1], "r");
        if(instream == 0){
                fprintf(stderr, "Can't open input file.\n" );
                exit(1);
        }
	result = igraph_read_graph_edgelist(&graph, instream, 0, 
IGRAPH_UNDIRECTED);
        if (result != 0) {
                fprintf(stderr, "Problems reading the inputstream. Code %d\n", 
result);
                exit(1);
        }
        result = igraph_linegraph(&graph, &linegraph);
        if (result != 0) {
                fprintf(stderr, "Problems obtaining the linegraph. Code %d\n", 
result);
                exit(1);
        }
        result = igraph_write_graph_edgelist(&linegraph, stdout);
        if (result != 0) {
		fprintf(stderr, "Problems writing in the outputstream. Code %d\n", 
result);
                exit(1);
        }
        igraph_destroy(&graph);
        igraph_destroy(&linegraph);
        return 0;
}

this code will print the linegraph on the stdout as follows:

---
0 1
0 2
0 3
1 2
2 3
---

Now I was expecting something like

12 24
12 14
12 13
14 24
13 14

instead I'm getting a redefined set of vertices and the mapping between 
the line graph vertices (0,1,2) and the graph edges is not provided.

I need to cluster the line graph and then find out which line graph 
vertices cluster together (ie which of the original ab, bc etc edges 
cluster together) I was wondering if there's a way to get a different 
representation of the line graph or another way to achieve this. Thank you!

Best
Giuseppe

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