birthday.c

// gcc -Wall -g birthday.c -o birthday -lm

#include <math.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv){
  double no_match = 1;
  double matches_me;
  double odds = 364./365;
  int ct;
  if(argc != 2){
    printf("Function accepts only one argument, maximum number of persons\n");
    exit(1);
  }
  int num_persons = atoi(argv[1]);
  if(num_persons < 2){
    printf("Number of persons need to be at least 2\n");
    exit(1);
  }

  printf("Calculating for %i persons\n", num_persons);
  printf("People\tMatches me\tAny match\n");

  for(ct = 2; ct <= num_persons; ct++)
  {
    matches_me = 1 - pow(odds, ct-1);
    no_match *= (1 - (ct-1)/365.);
    printf("%i\t %.4f\t\t%.3f\n", ct, matches_me, (1-no_match));
  }
  return 0;
}

Return solution to "birthday" problem, code snippet from http://ben.klemens.org/pdfs/gsl_stats.pdf [p. 24]