main.cpp

#include <iostream>
#include "main.h"

using std::cin;
using std::cout;


double L(double x, int k, double *xi, int d) {
  int i;
  double ans = 1.0;

  for (i = 0; i <= d; i++) {
    if (i == k) continue;
    ans *= (x - xi[i]) / (xi[k] - xi[i]);
  }
  return ans;
}


double p(double x, double *xi, double *fi, int d) {
  int i;
  double ans = 0.0;

  for (i = 0; i <= d; i++)
    ans += fi[i] * L(x, i, xi, d);
  return ans;
}


int main() {
  char s[256];
  int d, i;
  double *xi, *fi, x;

  cout << "Number of cases: ";
  cin >> d;
  xi = new double[d];
  fi = new double[d];
  d--;

  for (i = 0; i <= d; i++) {
    cout << "\nEnter x" << i << ": ";
    cin >> xi[i];
    cout << "Enter f" << i << ": ";
    cin >> fi[i];
  }
  while (1) {
    cout << "\nFind f(x) at x: ";
    cin >> x;
    cout << "f(" << x << ") = " << p(x, xi, fi, d) << "\n";
    cout << "Stop? ";
    cin >> s;
    if (s[0] == 'y' || s[0] == 'Y')break;
  }

  delete fi;
  delete xi;
  return 0;
}