Интерполяция полиномом Лагранжа

#include <iostream>
 
using namespace std;
 
double my_lagrange(double x, int n, double x_arr[], double y_arr[]);
double my_difference_lagrange(double x, int n, double x_arr[], double y_arr[]);
 
int main(){
    
    int n = 3;
    double x_arr[3] = {0, 0.2, 0.4};
    double y_arr[3] = {1.763, 1.917, 2.143};
    
    for (double x = 0.1; x < 0.6; x += 0.2){
        cout << "f("<< x << ") = " 
             << my_lagrange(x, n, x_arr, y_arr) << endl;
        cout << "f'("<< x << ") = "
             << my_difference_lagrange(x, n, x_arr, y_arr) << endl;
    }
    
    
    return 0;
}
 
double my_lagrange(double x, int n, double x_arr[], double y_arr[]){
    
    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;
}
 
double my_difference_lagrange(double x, int n, double x_arr[], double y_arr[]){
    
    double eps = 1e-6;
    double sum;
        
    sum = (my_lagrange(x, n, x_arr, y_arr) 
        - my_lagrange(x - eps, n, x_arr, y_arr)) / eps;
    
    return sum;
}