Probability Code

Since John Derbyshire has posted code for the probability problem, I thought I'd provide my own, written in C. Typical output is:

Two boys: 2499636 0.249964
One boy, one girl: 4999669 0.499967
Two girls: 2500694 0.250069

Tuesday boys: 714025 0.071402
Their brothers: 357115 0.500144
Their sisters: 356910 0.499856
Enjoy!

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

#define GIRL 1
#define BOY 2
#define TUESDAY 3

/* return a random integer from 1 to max */
int randint(int max)
  {
  return(1+floor(max*(rand()/(1+(double)RAND_MAX))));
  }

int main(void)
  {
  int N=10000000; /* number of trials */

  int n,g[3],d[3],a,b;
  int twogirls=0,twoboys=0,boygirl=0;
  int tuesdayboys=0,sibgirls=0,sibboys=0;

  for (n=1;n<=N;n++)
    {
    g[1]=randint(2);  /* random gender of child 1 (GIRL==1, BOY==2) */
    d[1]=randint(7);  /* random birth day of child 1 (SUNDAY==1, SATURDAY==7) */

    g[2]=randint(2);  /* random gender of child 2 */
    d[2]=randint(7);  /* random birth day of child 2 */
  
    a=randint(2);  /* child parent tells you about (1 or 2), chosen at random */
    b=3-a;         /* child parent tells you nothing about (1 or 2) */

    if ((g[a]==BOY)&&(g[b]==BOY)) twoboys++;   /* increase count of 2-boy families by one */
    if ((g[a]==BOY)&&(g[b]==GIRL)) boygirl++;  /* increase count of 1-boy 1-girl families by one */
    if ((g[a]==GIRL)&&(g[b]==BOY)) boygirl++;  /* increase count of 1-boy 1-girl families by one */
    if ((g[a]==GIRL)&&(g[b]==GIRL)) twogirls++; /* increase of 2-girl families by one */
    
    if ((g[a]==BOY)&&(d[a]==TUESDAY)) /* if the child the parent chooses to tell you about is a Tuesday-boy */
      {
      tuesdayboys++; /* increase count of Tuesday-boys you are told about by one */
      if (g[b]==BOY) sibboys++;  /* increase count of their brothers by one */
      if (g[b]==GIRL) sibgirls++; /* increase count of their sisters by one */
      }
    }

  /* print out counts and ratios */
  printf("         Two boys: %d %f\n",twoboys,twoboys/(double)N);
  printf("One boy, one girl: %d %f\n",boygirl,boygirl/(double)N);
  printf("        Two girls: %d %f\n",twogirls,twogirls/(double)N);
  printf("\n");
  printf("     Tuesday boys: %d %f\n",tuesdayboys,tuesdayboys/(double)N);
  printf("   Their brothers: %d %f\n",sibboys,sibboys/(double)tuesdayboys);
  printf("    Their sisters: %d %f\n",sibgirls,sibgirls/(double)tuesdayboys);

  return(0);
  }

#undef TUESDAY
#undef BOY
#undef GIRL