6.3

#include <iostream>

using namespace std;

int main(){
    int n; //
    double xp; //
    double a,b;
    int i;
    cout<<"n=";
    cin>>n;
    double x[n];
    double y[n];
    for (int l=0;l<n;l++){
        cout<<"x_"<<l<<" || y_"<<l<<endl;
        cin>>x[l]>>y[l];
    }
    cout<<"xp=";
    cin>>xp;
    if(xp<x[0]){
        cout<<"|backward extrapolation|"<<endl;
        i=1;
    }
    else if(xp>x[n-1]){
        cout<<"|forward extrapoaltion|"<<endl;
        i=n-1;
    }
    else{
        int j = 0;
        while (j<n-1){
            j+=1;
            if(xp>=x[j-1]&&xp<=x[j]){
                i=j;
                j=n-1;
            }
        }
    }
    a = (y[i]-y[i-1])/(x[i]-x[i-1]);
    b = y[i-1] - a*x[i-1];
    double yp = (a*xp + b);
    cout<<"f(xp)="<<yp<<endl;
    return 0;
}