Java Program to multiply two matrices - GeeksforGeeks

26 Dec, 2021
Given two matrices, the task to multiply them. Matrices can either be square or rectangular.
Examples: 
Input : mat1[][] = {{1, 2}, 
                   {3, 4}}
        mat2[][] = {{1, 1}, 
                    {1, 1}}
Output : {{3, 3}, 
          {7, 7}}
Input : mat1[][] = {{2, 4}, 
                    {3, 4}}
        mat2[][] = {{1, 2}, 
                    {1, 3}}       
Output : {{6, 16}, 
          {7, 18}}
Multiplication of Square Matrices : The below program multiplies two square matrices of size 4*4, we can change N for different dimensions. 
Java
// Java program to multiply two square// matrices.import java.io.*;  class GFG {      static int N = 4;      // This function multiplies mat1[][]    // and mat2[][], and stores the result    // in res[][]    static void multiply(int mat1[][],                         int mat2[][], int res[][])    {        int i, j, k;        for (i = 0; i < N; i++) {            for (j = 0; j < N; j++) {                res[i][j] = 0;                for (k = 0; k < N; k++)                    res[i][j] += mat1[i][k]                                 * mat2[k][j];            }        }    }      // Driver code    public static void main(String[] args)    {        int mat1[][] = { { 1, 1, 1, 1 },                         { 2, 2, 2, 2 },                         { 3, 3, 3, 3 },                         { 4, 4, 4, 4 } };          int mat2[][] = { { 1, 1, 1, 1 },                         { 2, 2, 2, 2 },                         { 3, 3, 3, 3 },                         { 4, 4, 4, 4 } };          // To store result        int res[][] = new int[N][N];        int i, j;        multiply(mat1, mat2, res);          System.out.println("Result matrix"                           + " is ");        for (i = 0; i < N; i++) {            for (j = 0; j < N; j++)                System.out.print(res[i][j]                                 + " ");            System.out.println();        }    }}  // This code is contributed by anuj_67.
Result matrix is 
10 10 10 10 
20 20 20 20 
30 30 30 30 
40 40 40 40
Time complexity: O(n3). It can be optimized using Strassen’s Matrix Multiplication
Auxiliary Space: O(n2)
Multiplication of Rectangular Matrices : We use pointers in C to multiply to matrices. Please refer to the following post as a prerequisite of the code.How to pass a 2D array as a parameter in C? 
Java
// Java program to multiply two matrices.  public class GFG {      /**     * to find out matrix multiplication     *     * @param matrix1 First matrix     * @param rows1   Number of rows in matrix 1     * @param cols1   Number of columns in matrix 1     * @param matrix2 Second matrix     * @param rows2   Number of rows in matrix 2     * @param cols2   Number of columns in matrix 2     * @return the result matrix (matrix 1 and matrix 2     * multiplication)     */    public static int[][] matrixMultiplication(        int[][] matrix1, int rows1, int cols1,        int[][] matrix2, int rows2, int cols2)        throws Exception    {          // Required condition for matrix multiplication        if (cols1 != rows2) {            throw new Exception("Invalid matrix given.");        }          // create a result matrix        int resultMatrix[][] = new int[rows1][cols2];          // Core logic for 2 matrices multiplication        for (int i = 0; i < resultMatrix.length; i++)         {            for (int j = 0;                  j < resultMatrix[i].length;                 j++)             {                for (int k = 0; k < cols1; k++)                 {                    resultMatrix[i][j]                        += matrix1[i][k] * matrix2[k][j];                }            }        }        return resultMatrix;    }      // Driver code    public static void main(String[] args) throws Exception    {          // Initial matrix 1 and matrix 2        int matrix1[][] = { { 2, 4 }, { 3, 4 } };        int matrix2[][] = { { 1, 2 }, { 1, 3 } };          // Function call to get a matrix multiplication        int resultMatrix[][] = matrixMultiplication(            matrix1, 2, 2, matrix2, 2, 2);          // Display result matrix        System.out.println("Result Matrix is:");        for (int i = 0; i < resultMatrix.length; i++)         {            for (int j = 0;                  j < resultMatrix[i].length;                 j++)            {                System.out.print(resultMatrix[i][j] + "    ");            }            System.out.println();        }    }    // This code is contributed by darshatandel1998 (Darshan    // Tandel)}
6 16 
7 18
Time complexity: O(n3). It can be optimized using Strassen’s Matrix Multiplication
Auxiliary Space: O(m1 * n2)
Please refer complete article on Program to multiply two matrices for more details!
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