//==================================================================================================
// Double factorial n!!
//==================================================================================================
double log10doublefactorial(int n)
{
if(n<=1) return log10(1.0);
// here the useful relation is (2m-1)!!=(2m)! / m! / 2^m
int m;
//
if ( n % 2 == 0 ) m=n/2;
else m=(n+1)/2;
return log10factorial(2*m) - log10factorial(m) - m*log10(2.0);
}
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;
}