gser.cpp

来自「Numerical recipes in C++」· C++ 代码 · 共 34 行

#include <cmath>
#include <limits>
#include "nr.h"
using namespace std;

void NR::gser(DP &gamser, const DP a, const DP x, DP &gln)
{
	const int ITMAX=100;
	const DP EPS=numeric_limits<DP>::epsilon();
	int n;
	DP sum,del,ap;

	gln=gammln(a);
	if (x <= 0.0) {
		if (x < 0.0) nrerror("x less than 0 in routine gser");
		gamser=0.0;
		return;
	} else {
		ap=a;
		del=sum=1.0/a;
		for (n=0;n<ITMAX;n++) {
			++ap;
			del *= x/ap;
			sum += del;
			if (fabs(del) < fabs(sum)*EPS) {
				gamser=sum*exp(-x+a*log(x)-gln);
				return;
			}
		}
		nrerror("a too large, ITMAX too small in routine gser");
		return;
	}
}