The following short example program demonstrates the basic usage of the igraph library. Save it into a file named igraph_test.c.

#include <igraph.h>

int main(void) {
  igraph_integer_t num_vertices = 1000;
  igraph_integer_t num_edges = 1000;
  igraph_real_t diameter;
  igraph_t graph;

  igraph_rng_seed(igraph_rng_default(), 42);

  igraph_erdos_renyi_game_gnm(
    &graph, num_vertices, num_edges,
    IGRAPH_UNDIRECTED, IGRAPH_NO_LOOPS
  );

  igraph_diameter(
    &graph, &diameter,
    /* from = */ NULL, /* to = */ NULL,
    /* vertex_path = */ NULL, /* edge_path = */ NULL,
    IGRAPH_UNDIRECTED, /* unconn= */ true
  );
  printf("Diameter of a random graph with average degree %g: %g\n",
          2.0 * igraph_ecount(&graph) / igraph_vcount(&graph),
          (double) diameter);

  igraph_destroy(&graph);

  return 0;
}

This example illustrates a couple of points:

For compiling this program you need a C compiler. Optionally, CMake can be used to automate the compilation.

cmake_minimum_required(VERSION 3.18)
project(igraph_test)

find_package(igraph REQUIRED)

add_executable(igraph_test igraph_test.c)
target_link_libraries(igraph_test PUBLIC igraph::igraph)

mkdir build
cd build

cmake ..

cmake --build .

cc igraph_test.c -I/usr/local/include/igraph -L/usr/local/lib -ligraph -o igraph_test

pkg-config --libs --cflags igraph

pkg-config --static --libs --cflags igraph

./igraph_test

The functions generating graph objects are called graph generators. Stochastic (i.e. 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. E.g., igraph_get_shortest_paths(), which calculates shortest paths from a vertex to other 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_square_lattice()) or trees (igraph_kary_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_t graph;
  igraph_vector_int_t dimvector;
  igraph_vector_int_t edges;
  igraph_vector_bool_t periodic;
  igraph_real_t avg_path_len;

  igraph_vector_int_init(&dimvector, 2);
  VECTOR(dimvector)[0]=30;
  VECTOR(dimvector)[1]=30;

  igraph_vector_bool_init(&periodic, 2);
  igraph_vector_bool_fill(&periodic, true);
  igraph_square_lattice(&graph, &dimvector, 0, IGRAPH_UNDIRECTED, /* mutual= */ false, &periodic);

  igraph_average_path_length(&graph, &avg_path_len, NULL, IGRAPH_UNDIRECTED, /* unconn= */ true);
  printf("Average path length (lattice):            %g\n", (double) avg_path_len);

  igraph_rng_seed(igraph_rng_default(), 42); /* seed RNG before first use */
  igraph_vector_int_init(&edges, 20);
  for (igraph_integer_t i=0; i < igraph_vector_int_size(&edges); i++) {
    VECTOR(edges)[i] = RNG_INTEGER(0, igraph_vcount(&graph) - 1);
  }

  igraph_add_edges(&graph, &edges, NULL);
  igraph_average_path_length(&graph, &avg_path_len, NULL, IGRAPH_UNDIRECTED, /* unconn= */ true);
  printf("Average path length (randomized lattice): %g\n", (double) avg_path_len);

  igraph_vector_bool_destroy(&periodic);
  igraph_vector_int_destroy(&dimvector);
  igraph_vector_int_destroy(&edges);
  igraph_destroy(&graph);

  return 0;
}

This example illustrates some new points. igraph uses igraph_vector_t and its related types (igraph_vector_int_t, igraph_vector_bool_t and so on) 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). The elements of a vector are of type igraph_real_t for igraph_vector_t, and of type igraph_integer_t for igraph_vector_int_t. As you might expect, igraph_vector_bool_t holds igraph_bool_t values. Vectors can be resized and most igraph functions returning the result in a vector automatically resize it to the size they need.

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

The vertices in a graph are identified by a vertex ID, an integer between 0 and n - 1, where n is the number of vertices in the graph. The vertex count can be retrieved using 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 this example program may add loop edges, edges pointing a vertex to itself, or 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, i.e. graphs which contain neither of these. This is because some structural properties are ill-defined for non-simple graphs. Loop and multi-edges can be removed by calling igraph_simplify().

In our next example we will calculate various centrality measures in a friendship graph. The friendship graph is from the famous Zachary karate club study. (Do a web search on "Zachary karate" if you want to know more about this.) Centrality measures quantify how central is the position of individual vertices in the graph.

#include <igraph.h>

int main(void) {
  igraph_t graph;
  igraph_vector_int_t v;
  igraph_vector_int_t result;
  igraph_vector_t result_real;
  igraph_integer_t edges[] = { 0,1, 0,2, 0,3, 0,4, 0,5, 0,6, 0,7, 0,8,
                               0,10, 0,11, 0,12, 0,13, 0,17, 0,19, 0,21, 0,31,
                               1, 2, 1, 3, 1, 7, 1,13, 1,17, 1,19, 1,21, 1,30,
                               2, 3, 2, 7, 2,27, 2,28, 2,32, 2, 9, 2, 8, 2,13,
                               3, 7, 3,12, 3,13, 4, 6, 4,10, 5, 6, 5,10, 5,16,
                               6,16, 8,30, 8,32, 8,33, 9,33, 13,33, 14,32, 14,33,
                               15,32, 15,33, 18,32, 18,33, 19,33, 20,32, 20,33,
                               22,32, 22,33, 23,25, 23,27, 23,32, 23,33, 23,29,
                               24,25, 24,27, 24,31, 25,31, 26,29, 26,33, 27,33,
                               28,31, 28,33, 29,32, 29,33, 30,32, 30,33, 31,32,
                               31,33, 32,33 };

  igraph_vector_int_view(&v, edges, sizeof(edges) / sizeof(edges[0]));
  igraph_create(&graph, &v, 0, IGRAPH_UNDIRECTED);

  igraph_vector_int_init(&result, 0);
  igraph_vector_init(&result_real, 0);

  igraph_degree(&graph, &result, igraph_vss_all(), IGRAPH_ALL, IGRAPH_LOOPS);
  printf("Maximum degree is      %10" IGRAPH_PRId ", vertex %2" IGRAPH_PRId ".\n",
         igraph_vector_int_max(&result),
         igraph_vector_int_which_max(&result));

  igraph_closeness(&graph, &result_real, NULL, NULL, igraph_vss_all(), IGRAPH_ALL,
                   /* weights= */ NULL, /* normalized= */ false);
  printf("Maximum closeness is   %10g, vertex %2" IGRAPH_PRId ".\n",
         (double) igraph_vector_max(&result_real),
         igraph_vector_which_max(&result_real));

  igraph_betweenness(&graph, &result_real, igraph_vss_all(),
                     IGRAPH_UNDIRECTED, /* weights= */ NULL);
  printf("Maximum betweenness is %10g, vertex %2" IGRAPH_PRId ".\n",
         (double) igraph_vector_max(&result_real),
         igraph_vector_which_max(&result_real));

  igraph_vector_int_destroy(&result);
  igraph_vector_destroy(&result_real);
  igraph_destroy(&graph);

  return 0;
}

This example demonstrates some new operations. First of all, it shows a way to create a graph a list of edges stored in a plain C array. Function igraph_vector_view() creates a view of a C array. It does not copy any data, which means that you must not call igraph_vector_destroy() on a vector created this way. This vector is then used to create the undirected graph.

Then the degree, closeness and betweenness centrality of the vertices is calculated and the highest values are printed. Note that the vector result, into which these functions will write their result, must be initialized first, and also that the functions resize it to be able to hold the result.

Notice that in order to print values of type igraph_integer_t, we used the IGRAPH_PRId format macro constant. This macro is similar to the standard PRI constants defined in stdint.h, and expands to the correct printf format specifier on each platform that igraph supports.

The igraph_vss_all() argument tells the functions to calculate the property for every vertex in the graph. It is shorthand for a vertex selector, represented by type igraph_vs_t. Vertex selectors help perform operations on a subset of vertices. You can read more about them in one of the following chapters.