Finding inverse of a matrix using Gauss – Jordan Method | Set 2

Given a Matrix, the task is to find the inverse of this Matrix using the Gauss-Jordan method.

What is matrix?

Matrix is an ordered rectangular array of numbers.  

Operations that can be performed on a matrix are: Addition, Subtraction, Multiplication or Transpose of matrix etc.

Inverse of a matrix:

Given a square matrix A, which is non-singular (means the Determinant of A is nonzero); Then there exists a matrix 

which is called inverse of matrix A.

The inverse of a matrix is only possible when such properties hold: 

1. The matrix must be a square matrix.
2. The matrix must be a non-singular matrix and,
3. There exist an Identity matrix I for which

 In general, the inverse of n X n matrix A can be found using this simple formula:   

where, Adj(A) denotes the adjoint of a matrix and, Det(A) is Determinant of matrix A.  

Methods for finding Inverse of Matrix:

Finding the inverse of a 2×2 matrix is a simple task, but for finding the inverse of larger matrix (like 3×3, 4×4, etc) is a tough task, So the following methods can be used: 

1. Elementary Row Operation (Gauss-Jordan Method) (Efficient)
2. Minors, Cofactors and Ad-jugate Method (Inefficient)

Elementary Row Operation (Gauss – Jordan Method):

Gauss-Jordan Method is a variant of Gaussian elimination in which row reduction operation is performed to find the inverse of a matrix.

Steps to find the inverse of a matrix using Gauss-Jordan method:
In order to find the inverse of the matrix following steps need to be followed:  

1. Form the augmented matrix by the identity matrix.
2. Perform the row reduction operation on this augmented matrix to generate a row reduced echelon form of the matrix.
3. The following row operations are performed on augmented matrix when required: 
   • Interchange any two row.
   • Multiply each element of row by a non-zero integer.
   • Replace a row by the sum of itself and a constant multiple of another row of the matrix.

Example:

• Augmented Matrix is formed as A:B

• After applying the Gauss-Jordan elimination method:

Below is the C++ program to find the inverse of a matrix using the Gauss-Jordan method:  

C++

// C++ program to find the inverse of Matrix.

#include <iostream>
#include <vector>
using namespace std;

// Function to Print matrix.
void PrintMatrix(float** ar, int n, int m)
{
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            cout << ar[i][j] << "  ";
        }
        printf("\n");
    }
    return;
}

// Function to Print inverse matrix
void PrintInverse(float** ar, int n, int m)
{
    for (int i = 0; i < n; i++) {
        for (int j = n; j < m; j++) {
            printf("%.3f  ", ar[i][j]);
        }
        printf("\n");
    }
    return;
}

// Function to perform the inverse operation on the matrix.
void InverseOfMatrix(float** matrix, int order)
{
    // Matrix Declaration.

    float temp;

    // PrintMatrix function to print the element
    // of the matrix.
    printf("=== Matrix ===\n");
    PrintMatrix(matrix, order, order);

    // Create the augmented matrix
    // Add the identity matrix
    // of order at the end of original matrix.
    for (int i = 0; i < order; i++) {

        for (int j = 0; j < 2 * order; j++) {

            // Add '1' at the diagonal places of
            // the matrix to create a identity matrix
            if (j == (i + order))
                matrix[i][j] = 1;
        }
    }

    // Interchange the row of matrix,
    // interchanging of row will start from the last row
    for (int i = order - 1; i > 0; i--) {

        // Swapping each and every element of the two rows
        // if (matrix[i - 1][0] < matrix[i][0])
        // for (int j = 0; j < 2 * order; j++) {
        //
        //        // Swapping of the row, if above
        //        // condition satisfied.
        // temp = matrix[i][j];
        // matrix[i][j] = matrix[i - 1][j];
        // matrix[i - 1][j] = temp;
        //    }

        // Directly swapping the rows using pointers saves
        // time

        if (matrix[i - 1][0] < matrix[i][0]) {
            float* temp = matrix[i];
            matrix[i] = matrix[i - 1];
            matrix[i - 1] = temp;
        }
    }

    // Print matrix after interchange operations.
    printf("\n=== Augmented Matrix ===\n");
    PrintMatrix(matrix, order, order * 2);

    // Replace a row by sum of itself and a
    // constant multiple of another row of the matrix
    for (int i = 0; i < order; i++) {

        for (int j = 0; j < order; j++) {

            if (j != i) {

                temp = matrix[j][i] / matrix[i][i];
                for (int k = 0; k < 2 * order; k++) {

                    matrix[j][k] -= matrix[i][k] * temp;
                }
            }
        }
    }

    // Multiply each row by a nonzero integer.
    // Divide row element by the diagonal element
    for (int i = 0; i < order; i++) {

        temp = matrix[i][i];
        for (int j = 0; j < 2 * order; j++) {

            matrix[i][j] = matrix[i][j] / temp;
        }
    }

    // print the resultant Inverse matrix.
    printf("\n=== Inverse Matrix ===\n");
    PrintInverse(matrix, order, 2 * order);

    return;
}

// Driver code
int main()
{
    int order;

    // Order of the matrix
    // The matrix must be a square a matrix
    order = 3;
    /*
float matrix[20][20] = { { 5, 7, 9 },
                         { 4, 3, 8 },
                         { 7, 5, 6 },
                         { 0 } };
*/
    float** matrix = new float*[20];
    for (int i = 0; i < 20; i++)
        matrix[i] = new float[20];

    matrix[0][0] = 5;
    matrix[0][1] = 7;
    matrix[0][2] = 9;
    matrix[1][0] = 4;
    matrix[1][1] = 3;
    matrix[1][2] = 8;
    matrix[2][0] = 7;
    matrix[2][1] = 5;
    matrix[2][2] = 6;

    // Get the inverse of matrix
    InverseOfMatrix(matrix, order);

    return 0;
}

Output:

=== Matrix ===
5  7  9  
4  3  8  
7  5  6  

=== Augmented Matrix ===
7  5  6  0  0  1  
5  7  9  1  0  0  
4  3  8  0  1  0  

=== Inverse Matrix ===
-0.210  0.029  0.276  
0.305  -0.314  -0.038  
-0.010  0.229  -0.124