using System using System Collections Generic using System Linq using

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

namespace ConsoleApplication23
{
    class Program
    {
        const double alpha0 = 3.1;
        const double alpha1 = -1.4;
        const double A = 8;

        const double beta0 = 2.1;
        const double beta1 = 5;
        const double B = 1;

        public static double f(double x)
        {
            return 3 * Math.Pow(x, 2);
        }

        public static double p()
        {
            return -4;
        }

        public static double q()
        {
            return 7;
        }

        public static double Exact(double x)
        {
            return 0.428571 * (x*x +1.14286*x + 0.154905 * Math.Pow(2.71828,(2*x))*Math.Sin(1.73205*x) + 0.204613 * Math.Pow(2.71828,(2*x))*Math.Cos(1.73205*x)+0.367347);
        }

        //public static double[] Gauss(double[,] M)
        //{
        //    int rows = M.GetLength(0);
        //    int cols = M.GetLength(1);

        //    double[] a = new double[rows];
        //    double[] b = new double[rows];
        //    double[] c = new double[rows];
        //    double[] f = new double[rows];
           
        //    double m;

        //    //for (int i = 0; i < rows; i++)
        //    //{
        //    //    for (int j = 0; j < cols; j++)
        //    //    {
        //    //        Console.Write("{0:f0} ", M[i, j]);
        //    //    }
        //    //    Console.WriteLine();
        //    //}
        //    for (int i = 1; i < rows + 1; i++)
        //    {
        //        b[i - 1] = M[i - 1, i - 1];
        //        c[i - 1] = M[i - 1, i];

        //    }
        //    for (int i = 0; i < rows; i++)
        //    {
        //        f[i] = M[i, rows];
        //    }
        //    for (int i = 0; i < rows - 1; i++)
        //    {
        //        a[i+1] = M[i + 1, i];
        //    }
        //    a[0] = 0;
        //    c[b.Length-1] = 0;
        //    //for (int i = 0; i < rows; i++)
        //    //{
        //    //    Console.WriteLine("{0:f2} ", a[i]);
        //    //}
            
        //    for (int i = 1; i < rows; i++)
        //    {
        //        double mm = a[i] / b[i - 1];
        //        b[i] = b[i] - mm * c[i - 1];
        //        f[i] = f[i] - mm * f[i - 1];
        //    }

        //    double[] ans = new double[rows];
        //    ans[rows - 1] = f[rows - 1] / b[rows - 1];
        //    for (int i = rows - 2; i >= 0; i--)
        //        ans[i] = (f[i] - c[i] * ans[i + 1]) / b[i];
            
        //    return ans;
        //}

        public static double[] Gauss(double[,] M)
        {
            int rows = M.GetLength(0);
            int cols = M.Length / M.GetLength(0);
            for (int x = 0; x < cols - 1; ++x)
            {
                int index = -1;
                for (int y = x; y < rows; ++y)
                {
                    if (Math.Abs(M[y, x]) < 1e-6)
                    {
                        continue;
                    }
                    if (index == -1)
                    {
                        index = y;
                    }
                    else
                    {
                        double k = -1 * M[y, x] / M[index, x];
                        for (int x1 = x; x1 < cols; ++x1)
                        {
                            M[y, x1] = M[y, x1] + M[index, x1] * k;
                        }
                    }
                }
            }

            for (int x = cols - 2; x >= 0; --x)
            {
                int index = -1;
                for (int y = rows - 1; y >= 0; --y)
                {
                    if (Math.Abs(M[y, x]) < 1e-6)
                    {
                        continue;
                    }
                    if (index == -1)
                    {
                        index = y;
                        double k = 1 / M[y, x];
                        M[y, x] = M[y, x] * k;
                        M[y, cols - 1] = M[y, cols - 1] * k;
                    }
                    else
                    {
                        double k = -1 * M[y, x] / M[index, x];
                        M[y, x] = M[y, x] + M[index, x] * k;
                        M[y, cols - 1] = M[y, cols - 1] + M[index, cols - 1] * k;
                    }
                }
            }

            double[] ans = new double[rows];
            for (int i = 0; i < ans.Length; ++i)
            {
                ans[i] = M[i, cols - 1];
            }
            return ans;
        }

        public static double[] Finite_differences(double a, double b, double h, int n)
        {
            double[,] M = new double[n + 1, n + 2];
            for (int i = 1; i < n; ++i)
            {
                double x = a + i * h;
                M[i, i + 1] = 1 + h * p() / 2;
                M[i, i] = -(2 - h * h * q());
                M[i, i - 1] = 1 - h * p() / 2;
                M[i, n + 1] = h * h * f(x);
            }


            //O(h)
            //M[0, 0] = h * alpha0 - alpha1;
            //M[0, 1] = alpha1;
            //M[0, n + 1] = A * h;

            //M[n, n - 1] = -beta1;
            //M[n, n] = h * beta0 + beta1;
            //M[n, n + 1] = B * h;

            //O(h^2)
            M[0, 0] = 2 * h * alpha0 - 3 * alpha1;
            M[0, 1] = 4 * alpha1;
            M[0, 2] = -alpha1;
            M[0, n + 1] = 2 * A * h;

            M[n, n - 2] = beta1;
            M[n, n - 1] = -4 * beta1;
            M[n, n] = 2 * h * beta0 + 3 * beta1;
            M[n, n + 1] = 2 * B * h;

            double[] gauss = Gauss(M);
            double[] res = new double[n + 1];
            int index = 0;
            for (double x = a; x <= b; x += h)
            {
                res[index] = gauss[index];
                ++index;
            }

            return res;
        }
        static void Main(string[] args)
        {
            const double a = 1.5;
            const double b = 2.7;
            const double h = 0.05;
            int n = (int)((b - a) / h);
            double[] x = new double[n + 1];
            x = Finite_differences(a, b, h, n);
            Console.WriteLine("X \tТочное значение\t\tY");
            Console.WriteLine("---------------------------------------");
            for (int i = 0; i < n + 1; i++)
            {
                string firstValue = string.Format("{0}", (a + i * h));
                string secondValue = string.Format("{0:f4}", Math.Round(Exact(a + i * h), 4));
                string thirdValue = string.Format("{0:f4}", Math.Round(x[i], 4));

                Console.Write(firstValue);
                if (firstValue.Length > 7)
                {
                    Console.Write("\t");
                }
                else
                {
                    Console.Write("\t\t");
                }

                Console.Write(secondValue);
                if (secondValue.Length > 7)
                {
                    Console.Write("\t");
                }
                else
                {
                    Console.Write("\t\t");
                }

                Console.WriteLine(thirdValue);
            }
        }
    }
}