example5_2.java

清华大学2002年出版的《科学与工程数值计算算法Java》配套源码

import java.applet.*;
import java.awt.*;
public class Example5_2 extends Applet
{
 public double eelgr(double x,double h,int n,double[] y,double t)
 {int i,j,k,l;
  double s,f;
  for(i=0;i<n;i++)
  {if(t==x+i*h)return y[i];
   if(t<x+i*h)break;}
  if(i<4){k=0;l=i+4;}
  else if(i>=n-4){k=i-4;l=n;}
  else {k=i-4;l=i+4;}
  f=0;
  for(i=k;i<l;i++)
  { s=1;
	for(j=k;j<i;j++)s*=(t-x-j*h)/(i-j)/h;
	for(j=i+1;j<l;j++)s*=(t-x-j*h)/(i-j)/h;
	f+=s*y[i];
  }
  return f;
  }

 public double eelgr2(double x,double h ,int n,double[] y,double t)
 {int i,j;
  double s,g=0;
  for(i=0;i<n;i++)
  {s=1;
   for(j=0;j<i;j++)s*=(t-x-j*h)/(i-j)/h;
   for(j=i+1;j<n;j++)s*=(t-x-j*h)/(i-j)/h;
   g+=s*y[i];}
  return g;}

 public void paint(Graphics g)
 {double[] y=new double[10];
  double[] t={0,0.25,0.63,0.95,1.05};
  double f,h;
  int i;
  for(i=0;i<10;i++)y[i]=Math.exp(-0.1-i*0.1);
  for(i=0;i<5;i++)
  {f=eelgr(0.1,0.1,10,y,t[i]);h=eelgr2(0.1,0.1,10,y,t[i]);
   g.drawString("t="+t[i]+"   f(t)="+f+"  h(t)="+h+"    实际值="+Math.exp(-t[i]),10,20*i+20);}
  }
}