拉格朗日插值法

#include"iostream"
#include"math.h"
using namespace std;
int n;
int main()
{
    int N;
    double x[3]={0.2,0.3,0.4}, y[3]={1.2214,1.3499,1.4918}, Y=0, X1=0.285,b,m;
    int i,j,k;
    for(k=0;k<3;k++)
    {
      double p=1;
       for(j=0;j<3;j++)
       {
           if(j==k)  continue;
           else 
           {
               p=p*(X1-x[j])/(x[k]-x[j]);
           }
                  
       }
         Y=Y+p*y[k];
    }
    /*X1=0.285时的近似值*/
    printf("e(%.3lf)=%.6lf\n",X1,Y);
       cout<<"Please Input N  value( 2 or 3 )"<<endl<<"N=";
         while(cin>>N)
        {
          
                if(N==2||N==3)
                {
                    double x[3]={0.2,0.3,0.4}, y[3]={1.2214,1.3499,1.4918}, Y=0, X=0.285;
                   for(k=0;k<N;k++)
                  {
                    double p=1;
                    for(j=0;j<N;j++)
                    { 

                      if(j==k)  continue;
                        else 
                               p=p*(X-x[j])/(x[k]-x[j]);
                    }
                     Y=Y+p*y[k];
                  }
                          printf("L(%d)=%.6lf\n", N-1,Y);
                       double X2;Y=0;

                       m=1;
                      if(N==2)  X2=0.3;
                         else X2=0.4;
                         for(k=0;k<N;k++)
                        {
                          double p=1;
                           for(j=0;j<N;j++)
                           {
                             if(j==k)  continue;
                               else 
                               {
                                p=p*(X2-x[j])/(x[k]-x[j]);
                               }
                  
                           }
                              Y=Y+p*y[k];
                              m*=(X1-x[k]);
                        }
                    /*N==2或N==3的截断误差*/
                        printf("R%d(%.6lf)<%.5lf\n",N-1,X1,fabs(Y*m/6));
                      // cout<<"Please Input N  value(2 or 3)"<<endl<<"N=";
                }
                else
                {
                 cout<<"error!"<<endl;
                }
            cout<<"Please Input N  value( 2 or 3 )"<<endl<<"N=";
      }    
}

同有兴趣的朋友共勉！