choose.cpp

   1 // For example, the number of five card hands is choose(52,5)
   2 //
   3 //                          n!
   4 //      choose(n,k) = -------------
   5 //                     k! (n - k)!
   6
   7 #include "stdafx.h"
   8 #include "defs.h"
   9
  10 void
  11 eval_choose(void)
  12 {
  13         push(cadr(p1));
  14         eval();
  15         push(caddr(p1));
  16         eval();
  17         choose();
  18 }
  19
  20 // Result vanishes for k < 0 or k > n. (A=B, p. 19)
  21
  22 #define N p1
  23 #define K p2
  24
  25 void
  26 choose(void)
  27 {
  28         save();
  29
  30         K = pop();
  31         N = pop();
  32
  33         if (choose_check_args() == 0) {
  34                 push_integer(0);
  35                 restore();
  36                 return;
  37         }
  38
  39         push(N);
  40         factorial();
  41
  42         push(K);
  43         factorial();
  44
  45         divide();
  46
  47         push(N);
  48         push(K);
  49         subtract();
  50         factorial();
  51
  52         divide();
  53
  54         restore();
  55 }
  56
  57 int
  58 choose_check_args(void)
  59 {
  60         if (isnum(N) && lessp(N, zero))
  61                 return 0;
  62         else if (isnum(K) && lessp(K, zero))
  63                 return 0;
  64         else if (isnum(N) && isnum(K) && lessp(N, K))
  65                 return 0;
  66         else
  67                 return 1;
  68 }