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## [igraph] Question on erdos_renyi random graph generator

**From**: |
Richard Geddes |

**Subject**: |
[igraph] Question on erdos_renyi random graph generator |

**Date**: |
Sat, 01 Dec 2007 13:35:44 -0500 |

**User-agent**: |
Thunderbird 2.0.0.6 (X11/20071022) |

Hello,
I'm attempting to generate random graph data and took an example in the
tutorial and modified it a bit to generate an edgelist output file:
#include <cstdio>
#include <string>
#include <igraph.h>
int main(void) {
FILE* ofile;
ofile = fopen("erdos_renyi.pairs", "w");
igraph_t graph;
igraph_erdos_renyi_game(&graph, IGRAPH_ERDOS_RENYI_GNP, 1000,
5.0/1000, IGRAPH_UNDIRECTED,
IGRAPH_NO_LOOPS);
igraph_write_graph_edgelist(&graph, ofile);
fclose(ofile);
printf("Erdos Renyi random graph generated\n");
igraph_destroy(&graph);
return 0;
}
I expected 1000 different vertices because the manual says:
--- snip ---
int igraph_erdos_renyi_game(igraph_t *graph, igraph_erdos_renyi_t type,
igraph_integer_t n, igraph_real_t p_or_m,
igraph_bool_t directed, igraph_bool_t loops);
....
n:
The number of vertices in the graph.
--- snip ---
I notice highest vertex value is consistently 999... ie the 1000th
element starting from 0. But, the number of vertices generated is not
consistent... I ran the test 10 times and got the following number of
vertices for each run: 988, 997, 992, 992, 995, 994, 991, 994, 993, 990
I'm using a home-grown program to count the edges/vertices. I've
checked it using standard known datasets, and it produces consistent
results.
I've also verified these results using the netstats program by Aaron
Clauset :
http://arxiv.org/abs/cond-mat/0408187v2
1) Is this function supposed to generate a random number of vertices?
2) Is there an igraph function to verify the number of vertices generated?
Thank you
Richard

**[igraph] Question on erdos_renyi random graph generator**,
*Richard Geddes* **<=**