/// <summary>
        ///
        /// </summary>
        /// <param name="theta"></param>
        /// <param name="x"></param>
        /// <param name="y"></param>
        /// <param name="lambda"></param>
        /// <returns>Item1 - cost, Item2 - grad</returns>
        public Tuple <double, double[]> CostFunction()
        {
            int m = y.Length;
            int n = Theta.Length;

            double[] one_minus_y   = MLMath.ItemCalculate(1, y, (a, b) => a - b);
            double[] h             = H();
            double[] log_h         = MLMath.ItemCalculate(h, i => Math.Log(i));
            double[] one_minus_h   = MLMath.ItemCalculate(1, h, (a, b) => a - b);
            double[] log_1_minus_h = MLMath.ItemCalculate(one_minus_h, i => Math.Log(i));
            double   cost          = (1.0 / m) * (-MLMath.ItemMultiplyAndSum(y, log_h) - MLMath.ItemMultiplyAndSum(one_minus_y, log_1_minus_h));

            //Regularization
            double[] theta_reg = new double[n];
            theta_reg[0] = 0;
            for (int i = 1; i < n; i++)
            {
                theta_reg[i] = Theta[i];
            }
            double reg = Lambda / (2 * m) * (MLMath.ItemMultiplyAndSum(theta_reg, theta_reg));

            cost += reg;
            //Gradient
            double[] grad = new double[n];
            for (int i = 0; i < n; i++)
            {
                double[] x_i_vector = MLMath.GetVector(x, 1, i);
                grad[i] = -Alpha * ((1.0 / m) * MLMath.ItemMultiplyAndSum(x_i_vector, MLMath.Minus(h, y)) + Lambda / m * theta_reg[i]);
            }
            return(new Tuple <double, double[]>(cost, grad));
        }
        public double[] H()
        {
            int m = x.GetLength(0);

            double[] z = new double[m];
            for (int i = 0; i < m; i++)
            {
                double[] x_vector = MLMath.GetVector(x, 0, i);
                z[i] = MLMath.ItemMultiplyAndSum(Theta, x_vector);
            }
            return(MLMath.ItemCalculate(z, c => MLMath.Sigmoid(c)));
        }