Function example_1 

pub fn example_1() -> Result<RandomGraphStats>

Lesson 1: the diameter and mean degree of a random graph.

Seeds the calling thread’s default random number generator with 42 (as the C program does, “to ensure identical results across runs”), generates a simple undirected Erdős–Rényi G(n, m) graph with n = 1000 vertices and m = 1000 edges, and computes its diameter (over all components, the graph being disconnected) and its mean degree 2m / n = 2.

It translates examples/tutorial/tutorial1.c of the first lesson, and computes exactly the values printed by the C program with igraph 1.0.1: Diameter of a random graph with average degree 2: 23.

C code

#include <igraph.h>

int main(void) {
    igraph_int_t num_vertices = 1000;
    igraph_int_t num_edges = 1000;
    igraph_real_t diameter, mean_degree;
    igraph_t graph;

    /* Initialize the library. */
    igraph_setup();

    /* Ensure identical results across runs. */
    igraph_rng_seed(igraph_rng_default(), 42);

    igraph_erdos_renyi_game_gnm(
            &graph, num_vertices, num_edges,
            IGRAPH_UNDIRECTED, IGRAPH_SIMPLE_SW, IGRAPH_EDGE_UNLABELED);

    igraph_diameter(
        &graph, /* weights = */ NULL,
        &diameter,
        /* from = */ NULL, /* to = */ NULL,
        /* vertex_path = */ NULL, /* edge_path = */ NULL,
        IGRAPH_UNDIRECTED, /* unconn= */ true);

    igraph_mean_degree(&graph, &mean_degree, IGRAPH_LOOPS);
    printf("Diameter of a random graph with average degree %g: %g\n",
           mean_degree, diameter);

    igraph_destroy(&graph);

    return 0;
}

Rust translation

The body of this function, step by step (no igraph_setup() and no igraph_destroy() are needed):

use igraph::prelude::*;

let (num_vertices, num_edges) = (1000, 1000);

// Ensure identical results across runs.
rng::seed(42)?;

let graph = Graph::erdos_renyi_game_gnm(
    num_vertices,
    num_edges,
    false, // undirected
    EdgeTypeSw::Simple,
    false, // unlabeled edges
)?;

let diameter = graph.diameter()?; // unweighted, over all components
let mean_degree = graph.mean_degree(true)?; // loops counted
println!("Diameter of a random graph with average degree {mean_degree}: {diameter}");

assert_eq!(diameter, 23.0);
assert_eq!(mean_degree, 2.0);

// ... which is what `example_1` returns.
let stats = igraph::tutorial::example_1()?;
assert_eq!((stats.diameter, stats.mean_degree), (diameter, mean_degree));
assert_eq!(
    stats.to_string(),
    "Diameter of a random graph with average degree 2: 23"
);

Errors

Propagates the errors of the igraph calls (none are expected).