example5_4.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_4 extends Applet
{
 public double eelg3(double x,double h,int n,double[] y,double t)
 {int i,j,k,l;
  double s,f;
  if(t<x)return (y[0]+(y[1]-y[0])/h*(t-x));
  if(t>x+n*h-h)return ( y[n-1]+ (y[n-1]-y[n-2])/h*(t-x-n*h+h));
  for(i=0;i<n;i++)
  {if(t==x+i*h)return y[i];
   if(t<x+i*h)break;}
  if(i==1){k=0;l=3;}
  else if(i==n-1){k=n-3;l=n;}
  else
  { if(t>=x+(i-0.5)*h){k=i-1;l=i+2;}
    else {k=i-2;l=i+1;}
  }
  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 void paint(Graphics g)
 {double[] y=new double[10];
  double f;
  int i;
  for(i=0;i<10;i++)y[i]=Math.exp(-0.1-0.1*i);
  f=eelg3(0.1,0.1,10,y,0.33);
  g.drawString("t="+0.33+"   f(t)="+f+"  实际值="+Math.exp(-0.33),10,20);
  f=eelg3(0.1,0.1,10,y,0.67);
  g.drawString("t="+0.67+"   f(t)="+f+"  实际值="+Math.exp(-0.67),10,40);
  f=eelg3(0.1,0.1,10,y,0.83);
  g.drawString("t="+0.83+"   f(t)="+f+"  实际值="+Math.exp(-0.83),10,60);
  }
}