binomial.cpp

   1 //      Binomial coefficient
   2 //
   3 //      Input:          tos-2           n
   4 //
   5 //                      tos-1           k
   6 //
   7 //      Output:         Binomial coefficient on stack
   8 //
   9 //      binomial(n, k) = n! / k! / (n - k)!
  10 //
  11 //      The binomial coefficient vanishes for k < 0 or k > n. (A=B, p. 19)
  12
  13 #include "stdafx.h"
  14 #include "defs.h"
  15 static void ybinomial(void);
  16 static int check_args(void);
  17
  18 void
  19 eval_binomial(void)
  20 {
  21         push(cadr(p1));
  22         eval();
  23         push(caddr(p1));
  24         eval();
  25         binomial();
  26 }
  27
  28
  29 void
  30 binomial(void)
  31 {
  32         save();
  33         ybinomial();
  34         restore();
  35 }
  36
  37 #define N p1
  38 #define K p2
  39
  40 static void
  41 ybinomial(void)
  42 {
  43         K = pop();
  44         N = pop();
  45
  46         if (check_args() == 0) {
  47                 push(zero);
  48                 return;
  49         }
  50
  51         push(N);
  52         factorial();
  53
  54         push(K);
  55         factorial();
  56
  57         divide();
  58
  59         push(N);
  60         push(K);
  61         subtract();
  62         factorial();
  63
  64         divide();
  65 }
  66
  67 static int
  68 check_args(void)
  69 {
  70         if (isnum(N) && lessp(N, zero))
  71                 return 0;
  72         else if (isnum(K) && lessp(K, zero))
  73                 return 0;
  74         else if (isnum(N) && isnum(K) && lessp(N, K))
  75                 return 0;
  76         else
  77                 return 1;
  78 }