InterLagrange

#include <iostream>
#include <conio.h>

using namespace std;

double my_lagrange(double x, int n, double x_arr[], double y_arr[]);

int main()
{

    int n = 5;

    double x_arr[5] = {0.079, 0.637, 1.345, 2.095, 2.782};
    double y_arr[5] = {-4.308, -0.739, 1.697, 4.208, 6.203};

    for (double x = 0; x < 3; x += 1) {
        cout << "f(" << x << ") = "
            << my_lagrange(x, n, x_arr, y_arr) << endl;

    }

    system("pause");
    return 0;
}

double my_lagrange(double x, int n, double x_arr[], double y_arr[]) {
    //Пусть точки отсортированы по возрастанию координаты x

    if (n > 5)
        return (x <= x_arr[n / 2]) ? my_lagrange(x, (n + 1) / 2, x_arr, y_arr) :
        my_lagrange(x, (n + 1) / 2, &x_arr[n / 2], &y_arr[n / 2]);

    double sum = 0;
    for (int i = 0; i < n; ++i) {

        double l = 1;
        for (int j = 0; j < n; ++j)
            if (j != i)
                l *= (x - x_arr[j]) / (x_arr[i] - x_arr[j]);
        sum += y_arr[i] * l;
    }

    return sum;
}