betacf.c

Numerical Recipes in C的源代码

#include <math.h>
#define MAXIT 100
#define EPS 3.0e-7
#define FPMIN 1.0e-30

float betacf(float a, float b, float x)
{
	void nrerror(char error_text[]);
	int m,m2;
	float aa,c,d,del,h,qab,qam,qap;

	qab=a+b;
	qap=a+1.0;
	qam=a-1.0;
	c=1.0;
	d=1.0-qab*x/qap;
	if (fabs(d) < FPMIN) d=FPMIN;
	d=1.0/d;
	h=d;
	for (m=1;m<=MAXIT;m++) {
		m2=2*m;
		aa=m*(b-m)*x/((qam+m2)*(a+m2));
		d=1.0+aa*d;
		if (fabs(d) < FPMIN) d=FPMIN;
		c=1.0+aa/c;
		if (fabs(c) < FPMIN) c=FPMIN;
		d=1.0/d;
		h *= d*c;
		aa = -(a+m)*(qab+m)*x/((a+m2)*(qap+m2));
		d=1.0+aa*d;
		if (fabs(d) < FPMIN) d=FPMIN;
		c=1.0+aa/c;
		if (fabs(c) < FPMIN) c=FPMIN;
		d=1.0/d;
		del=d*c;
		h *= del;
		if (fabs(del-1.0) < EPS) break;
	}
	if (m > MAXIT) nrerror("a or b too big, or MAXIT too small in betacf");
	return h;
}
#undef MAXIT
#undef EPS
#undef FPMIN