trapzd.cpp

这是数学计算上常用的计算方法

#include "nr.h"

DP NR::trapzd(DP func(const DP), const DP a, const DP b, const int n)
{
	DP x,tnm,sum,del;
	static DP s;
	int it,j;

	if (n == 1) {
		return (s=0.5*(b-a)*(func(a)+func(b)));
	} else {
		for (it=1,j=1;j<n-1;j++) it <<= 1;
		tnm=it;
		del=(b-a)/tnm;
		x=a+0.5*del;
		for (sum=0.0,j=0;j<it;j++,x+=del) sum += func(x);
		s=0.5*(s+(b-a)*sum/tnm);
		return s;
	}
}