TheAlgorithms-C-Plus-Plus/numerical_methods/gaussian_elimination.cpp

/**
 * \file
 * \brief [Gaussian elimination
 * method](https://en.wikipedia.org/wiki/Gaussian_elimination)
 */
#include <iostream>

/** Main function */
int main() {
    int mat_size, i, j, step;

    std::cout << "Matrix size: ";
    std::cin >> mat_size;

    // create a 2D matrix by dynamic memory allocation
    double **mat = new double *[mat_size + 1], **x = new double *[mat_size];
    for (i = 0; i <= mat_size; i++) {
        mat[i] = new double[mat_size + 1];
        if (i < mat_size)
            x[i] = new double[mat_size + 1];
    }

    // get the matrix elements from user
    std::cout << std::endl << "Enter value of the matrix: " << std::endl;
    for (i = 0; i < mat_size; i++) {
        for (j = 0; j <= mat_size; j++) {
            std::cin >>
                mat[i][j];  // Enter (mat_size*mat_size) value of the matrix.
        }
    }

    // perform Gaussian elimination
    for (step = 0; step < mat_size - 1; step++) {
        for (i = step; i < mat_size - 1; i++) {
            double a = (mat[i + 1][step] / mat[step][step]);

            for (j = step; j <= mat_size; j++)
                mat[i + 1][j] = mat[i + 1][j] - (a * mat[step][j]);
        }
    }

    std::cout << std::endl
              << "Matrix using Gaussian Elimination method: " << std::endl;
    for (i = 0; i < mat_size; i++) {
        for (j = 0; j <= mat_size; j++) {
            x[i][j] = mat[i][j];
            std::cout << mat[i][j] << " ";
        }
        std::cout << std::endl;
    }
    std::cout << std::endl
              << "Value of the Gaussian Elimination method: " << std::endl;
    for (i = mat_size - 1; i >= 0; i--) {
        double sum = 0;
        for (j = mat_size - 1; j > i; j--) {
            x[i][j] = x[j][j] * x[i][j];
            sum = x[i][j] + sum;
        }
        if (x[i][i] == 0)
            x[i][i] = 0;
        else
            x[i][i] = (x[i][mat_size] - sum) / (x[i][i]);

        std::cout << "x" << i << "= " << x[i][i] << std::endl;
    }

    for (i = 0; i <= mat_size; i++) {
        delete[] mat[i];
        if (i < mat_size)
            delete[] x[i];
    }
    delete[] mat;
    delete[] x;

    return 0;
}