The functions generating graph objects are called graph generators. Stochastic (=randomized) graph generators are called “games”.

igraph can handle directed and undirected graphs. Most graph generators are able to create both types of graphs and most other functions are usually also capable of handling both. Eg. igraph_shortest_paths() which (surprisingly) calculates shortest paths from a vertex to another vertices can calculate directed or undirected paths.

igraph has sophisticated ways for creating graphs. The simplest graphs are deterministic regular structures like star graphs (igraph_star()), ring graphs (igraph_ring()), lattices (igraph_lattice()) or trees (igraph_tree()).

The following example creates an undirected regular circular lattice, adds some random edges to it and calculates the average length of shortest paths between all pairs of vertices in the graph before and after adding the random edges. (The message is that some random edges can reduce path lengths a lot.)

#include <igraph.h>

int main(void) {
  igraph_real_t avg_path;
  igraph_t graph;
  igraph_vector_t dimvector;
  igraph_vector_t edges;
  int i;
  
  igraph_vector_init(&dimvector, 2);
  VECTOR(dimvector)[0]=30;
  VECTOR(dimvector)[1]=30;
  igraph_lattice(&graph, &dimvector, 0, IGRAPH_UNDIRECTED, 0, 1);

  srand(100);
  igraph_vector_init(&edges, 20);
  for (i=0; i<igraph_vector_size(&edges); i++) {
    VECTOR(edges)[i] = rand() % (int)igraph_vcount(&graph);
  }

  igraph_average_path_length(&graph, &avg_path, IGRAPH_UNDIRECTED, 1);
  printf("Average path length (lattice):            %f\n", (double) avg_path);

  igraph_add_edges(&graph, &edges, 0);
  igraph_average_path_length(&graph, &avg_path, IGRAPH_UNDIRECTED, 1);
  printf("Average path length (randomized lattice): %f\n", (double) avg_path);
  
  igraph_vector_destroy(&dimvector);
  igraph_vector_destroy(&edges);
  igraph_destroy(&graph);

  return 0;
}

This example illustrates some new points. igraph uses igraph_vector_t instead of plain C arrays. igraph_vector_t is superior to regular arrays in almost every sense. Vectors are created by the igraph_vector_init() function and like graphs they should be destroyed if not needed any more by calling igraph_vector_destroy() on them. A vector can be indexed by the VECTOR() function (right now it is a macro). Vectors can be resized, eg. most igraph functions returning the result in a vector resize it to the size of the result.

igraph_lattice() takes a vector argument specifying the dimensions of the lattice, in this example we generate a 30x30 two dimensional lattice. See the documentation of igraph_lattice() in the reference manual for the other arguments.

The vertices in a graph are identified by an integer number between 0 and N-1, N is the number of vertices in the graph (this can be obtained by igraph_vcount(), as in the example).

The igraph_add_edges() function simply takes a graph and a vector of vertex ids defining the new edges. The first edge is between the first two vertex ids in the vector, the second edge is between the second two, etc. This way we add ten random edges to the lattice.

Note that in the example it is possible to add loop edges, edges pointing to the same vertex and multiple edges, more than one edge between the same pair of vertices. igraph_t can of course represent loops and multiple edges, although some routines expect simple graphs, ie. graphs without loop and multiple edges, because for example some structural properties are ill-defined for non-simple graphs. Loop edges can be removed by calling igraph_simplify().