example5_3.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_3 extends Applet
{
 public double enlg3(double[] x,double[] y,double t)
 {int i,j,n=x.length,k,l;
  double s,f;
  if(t<x[0])return (y[0]+(y[1]-y[0])/(x[1]-x[0])*(t-x[0]));
  if(t>x[n-1])return (y[n-1]+(y[n-1]-y[n-2])/(x[n-1]-x[n-2])*(t-x[n-1]));
  for(i=0;i<n;i++)
  {if(t==x[i])return y[i];
   if(t<x[i])break;}
  if(i==1){k=0;l=3;}
  else if(i==n-1){k=n-3;l=n;}
  else
  { if((x[i]-t)>(t-x[i-1])){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])/(x[i]-x[j]);
	for(j=i+1;j<l;j++)s*=(t-x[j])/(x[i]-x[j]);
	f+=s*y[i];
  }
  return f;
  }

 public void paint(Graphics g)
 {double[] x={1.615,1.634,1.702,1.828,1.921};
  double[] y={2.41450,2.46459,2.65271,3.030335,3.34066};
  double f;
  f=enlg3(x,y,1.682);
  g.drawString("t="+1.682+"   f(t)="+f,10,20);
  f=enlg3(x,y,1.813);
  g.drawString("t="+1.813+"   f(t)="+f,10,40);
  }
}