Category : C Source Code
Archive   : SNIP1292.ZIP
Filename : COMBIN.C

 
Output of file : COMBIN.C contained in archive : SNIP1292.ZIP
/*
** Compute C(n,m) = the number of combinations of n items,
** taken m at a time.
**
** Written by Thad Smith III, Boulder County, CO.
** Released to the Public Domain 10/14/91.
**
** The def of this function is
** C(n,m) = n! / (m! * (n-m)!).
** Computing this formula can cause overflow for large values of n,
** even when C(n,m) would be defined.
**
** The first version will not overflow if C(n,m) * (n-m+1) < ULONG_MAX.
** The second version will not overflow if C(n,m) < ULONG_MAX, but
** is slightly more complex.
** Function domain: n >= 0, 0 <= m <= n.
**
** Both versions work by reducing the product as it is computed. It
** relies on the property that the product on n consecutive integers
** must be evenly divisible by n.
**
** The first version can be changed to make cnm and the return value
** double to extend the range of the function.
*/

unsigned long ncomb1 (int n, int m)
{
unsigned long cnm = 1UL;
int i;

if (m*2 >n) m = n-m;
for (i=1 ; i <= m; n--, i++)
cnm = cnm * n / i;
return cnm;
}

unsigned long ncomb2 (int n, int m)
{
unsigned long cnm = 1UL;
int i, f;

if (m*2 >n) m = n-m;
for (i=1 ; i <= m; n--, i++)
{
if ((f=n) % i == 0)
f /= i;
else cnm /= i;
cnm *= f;
}
return cnm;
}

#ifdef TEST

#include
#include

main (int argc, char *argv[]) {
int n,m;
n = atoi (argv[1]);
m = atoi (argv[2]);
printf ("ncomb1 = %lu, ncomb2 = %lu\n", ncomb1(n,m), ncomb2(n,m));
return 0;
}

#endif


  3 Responses to “Category : C Source Code
Archive   : SNIP1292.ZIP
Filename : COMBIN.C

  1. Very nice! Thank you for this wonderful archive. I wonder why I found it only now. Long live the BBS file archives!

  2. This is so awesome! 😀 I’d be cool if you could download an entire archive of this at once, though.

  3. But one thing that puzzles me is the “mtswslnkmcjklsdlsbdmMICROSOFT” string. There is an article about it here. It is definitely worth a read: http://www.os2museum.com/wp/mtswslnk/