home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
OS/2 Shareware BBS: 10 Tools
/
10-Tools.zip
/
snip9707.zip
/
FACTORYL.C
< prev
next >
Wrap
C/C++ Source or Header
|
1997-07-05
|
3KB
|
105 lines
/* +++Date last modified: 05-Jul-1997 */
/*
** FACTORYL.C
**
** Original Copyright 1992 by Bob Stout as part of
** the MicroFirm Function Library (MFL)
**
** The user is granted a free limited license to use this source file
** to create royalty-free programs, subject to the terms of the
** license restrictions specified in the LICENSE.MFL file.
**
** Uses DBLROUND.C, also in SNIPPETS
*/
#include <float.h>
#include "snipmath.h"
#define dfrac(x) ((x)-dround(x))
#define SQRT2PI sqrt(2 * PI)
#define ONESIXTH (1.0/6.0)
/*
** log10factorial()
**
** Returns the logarithm (base 10) of the factorial of a given number.
** The logarithm is returned since this allows working with extremely
** large values which would otherwise overflow the F.P. range.
**
** Parameters: 1 - Number whose factorial to return.
**
** Returns: log10() of the passed value, -1.0 if error
**
** Limitations: Cannot return 0! since log(0) is undefined.
*/
double log10factorial(double N)
{
double dummy;
if ((N < 1.0) || (0.0 != modf(N, &dummy)))
return -1.0;
if (N < 40) /* Small, explicitly compute */
{
int i;
double f;
if (0.0 == N)
return N;
for (i = 1, f = 1.0; i <= (int)N; ++i)
f *= i;
return log10(f);
}
else /* Large, use approximation */
{
return log10(SQRT2PI)+((N + 0.5) *
(log10(sqrt(N * N + N + ONESIXTH) / exp(1.0))));
}
}
#ifdef TEST
#include <stdio.h>
#include <stdlib.h>
main(int argc, char *argv[])
{
double f, lf;
char *dummy;
while (--argc)
{
f = strtod((const char *)(*(++argv)), &dummy);
if (0.0 == f)
{
puts("0! = 0");
continue;
}
if (-1.0 == (lf = log10factorial(f)))
{
printf(">>> ERROR: %g! is not a valid expression\n", f);
continue;
}
if (171.0 > f)
printf("%.14g! = %.14g\n", f, pow(10.0, lf));
else
{
printf("%.14g! = %.14ge+%ld\n", f,
pow(10.0, dfrac(lf)), (long)dround(lf));
}
}
lf = log10C(1000000L,750000L);
printf("\nJust to dazzle with you with big numbers:\n"
"C(1000000,750000) = %.14ge+%ld\n",
pow(10.0, dfrac(lf)), (long)dround(lf));
lf = log10P(1000000L,750000L);
printf("\n...once more:\n"
"P(1000000,750000) = %.14ge+%ld\n",
pow(10.0, dfrac(lf)), (long)dround(lf));
return EXIT_SUCCESS;
}
#endif /* TEST */