using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace D_19_INT1_SzałataSzymon
{
    class Program
    {
        static void GJ(int n, double[,] A)
        {
            int r;
            int variable = n + 1;
            double max, p;

            for (int k = 0; k < n; k++)
            {
                max = A[k, k];
                r = k;

                for (int i = k; i < n; i++)
                {
                    if (Math.Abs(A[i, k]) > Math.Abs(max))
                    {
                        max = A[i, k];
                        r = i;
                    }
                }

                if (max == 0)
                {
                    Console.WriteLine("Macierz układu osobliwa...");
                    return;
                }
                else
                {
                    for (int j = k; j < variable; j++)
                    {
                        double pomoc = A[k, j];
                        A[k, j] = A[r, j];
                        A[r, j] = pomoc;
                    }

                    for (int j = k; j < variable; j++)
                    {
                        A[k, j] = A[k, j] / max;
                    }

                    for (int i = 0; i < n; i++)
                    {
                        if (i != k)
                        {
                            p = A[i, k];

                            for (int j = k; j < variable; j++)
                            {
                                A[i, j] = A[i, j] - p * A[k, j];
                            }
                        }
                    }
                }

            }
        }
        static double Sum1(double[,] t, int n, double x)
        {
            double suma = 0;
            for (int i = 0; i < n; i++)
            {
                suma += t[i, n] * Math.Pow(x, i);
            }
            return suma;
        }

        static double Sum2(int n, double x, double[] An)
        {
            double p = An[n - 1];
            for (int i = n - 2; i >= 0; i--)
            {
                p = p * x + An[i];
            }
            return p;
        }
        static void Main(string[] args)
        {
            int n = 5;
            double[] Xi = { 1.5, 2, 2.5, 3.5, 3.8, 4.1 };
            double[] Yi = { 2, 5, -1, 0.5, 3, 7 };
            double[,] tab = new double[n + 1, n + 2];

            for (int i = 0; i < n + 1; i++)
            {
                tab[i, 0] = 1;
                tab[i, n + 1] = Yi[i];
            }

            for (int i = 0; i < n + 1; i++)
            {
                for (int j = 1; j < n + 1; j++)
                {
                    tab[i, j] = Xi[i] * tab[i, j - 1];
                }
            }


            GJ(n + 1, tab);


            double x = 1;

            double[] An = new double[n + 1];

            for (int i = 0; i < An.Length; i++)
            {
                An[i] = tab[i, n + 1];
            }


            string help;
            do
            {
                Console.WriteLine("Wpisz wartość od 1.5 do 4.1 ( 0 = wyjście z pętli)");
                help = Console.ReadLine();
                x = Convert.ToDouble(help);
                Console.WriteLine("Wartość: " + Sum2(n + 1, x, An));

            } while (x != 0);
            Console.ReadLine();
        }
    }
}