Gaus

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

namespace GaussAndSeidel
{
    class Program
    {

        const double epsilon = 0.00001;

        static double[,] A =
        {
            {7.4, 2.2, -3.1, -0.7 },
            {1.6, 4.8, -8.5, 4.5 },
            {4.7, 7.0, -6.0, 6.6 },
            {5.9, 2.7, 4.9, -6.3 }
        };

        static double[] B = { 2, 4, 6, 8 };

        static int N = A.GetLength(0);

        static void Gauss(double[] X)
        {
            double[] Q = new double[N - 1];

            for (int k = 0; k < N - 1; k++)
            {
                for (int i = 0; i < N - 1; i++)
                    Q[i] = A[i + 1, k] / A[k, k];
                for (int i = 0, j = k; j < N - 1;)
                {
                    A[j + 1, i] = A[j + 1, i] - Q[j] * A[k, i];
                    if (i == N - 1)
                    {
                        B[j + 1] = B[j + 1] - Q[j] * B[k];
                        j++;
                        i = 0;
                    }
                    else i++;
                }
            }

            X[N - 1] = B[N - 1] / A[N - 1, N - 1];
            double s;
            for (int i = N - 2; ; i--)
            {
                s = 0;
                for (int j = i + 1; j < N; j++)
                    s += A[i, j] * X[j];
                X[i] = (B[i] - s) / A[i, i];
                if (i == 0)
                    break;
            }
            Console.WriteLine("Метод Гаусса.");
            for (int i = 0; i < N; i++)
                Console.WriteLine(X[i]);
            Console.WriteLine();
        }

        static void Seidel()
        {
            Random r = new Random();

            double[] X = { r.Next(4), r.Next(4), r.Next(4), r.Next(4) };
            double[] newX = { 0, 0, 0, 0 };
            bool first = true;
            int i = 0;

            do
            {
                i++;
                if (!first)
                {
                    X[0] = newX[0];
                    X[1] = newX[1];
                    X[2] = newX[2];
                    X[3] = newX[3];
                }
                else
                    first = false;

                newX[0] = 1 / A[0, 0] * (-A[0, 1] * X[1] - A[0, 2] * X[2] - A[0, 3] * X[3] + B[0]);

                newX[1] = 1 / A[1, 1] * (-A[1, 0] * newX[0] - A[1, 2] * X[2] - A[1, 3] * X[3] + B[1]);

                newX[2] = 1 / A[2, 2] * (-A[2, 0] * newX[0] - A[2, 1] * newX[1] - A[2, 3] * X[3] + B[2]);

                newX[3] = 1 / A[3, 3] * (-A[3, 0] * newX[0] - A[3, 1] * newX[1] - A[3, 2] * newX[2] + B[3]);

            } while (Math.Sqrt(Math.Pow((newX[0] - X[0]), 2) + Math.Pow((newX[1] - X[1]), 2)
                        + Math.Pow((newX[2] - X[2]), 2) + Math.Pow((newX[3] - X[3]), 2)) > epsilon);

            Console.WriteLine("Метод Зейделя:");
            Console.WriteLine("{0}\n{1}\n{2}\n{3}\nКоличество итераций:{4}", newX[0], newX[1], newX[2], newX[3], i);
        }

        static void Main(string[] args)
        {
            Console.WriteLine(N+"\n");
            double[] X = new double[N];

            Gauss(X);
            Seidel();
            Console.ReadKey();
        }
    }
}