Matrix multiplication using arrays

Question

I'm trying to make a simple matrix multiplication method using multidimensional arrays ( [2][2] ). I'm kinda new at this, and I just can't find what it is I'm doing wrong. I'd really appreciate any help in telling me what it is. I'd rather not use libraries or anything like that, I'm mostly doing this to learn how it works. Thank you so much in advance.

I'm declaring my arays in the main method as follows:

Double[][] A={{4.00,3.00},{2.00,1.00}}; 
Double[][] B={{-0.500,1.500},{1.000,-2.0000}};

A*B should return the identity matrix. It doesn't.

public static Double[][] multiplicar(Double[][] A, Double[][] B){
//the method runs and returns a matrix of the correct dimensions
//(I actually changed the .length function to a specific value to eliminate 
//it as a possible issue), but not the correct values

    Double[][] C= new Double[2][2];
    int i,j;

    ////I fill the matrix with zeroes, if I don't do this it gives me an error
    for(i=0;i<2;i++) {
        for(j=0;j<2;j++){
            C[i][j]=0.00000;
        }
    } 
    ///this is where I'm supposed to perform the adding of every element in
    //a row of A multiplied by the corresponding element in the
    //corresponding column of B, for all columns in B and all rows in A
    for(i=0;i<2;i++){
        for(j=0;j<2;j++)
            C[i][j]+=(A[i][j]*B[j][i]);
    }
    return C;
}

Answer 1

You can try this code:

public class MyMatrix {
    Double[][] A = { { 4.00, 3.00 }, { 2.00, 1.00 } };
    Double[][] B = { { -0.500, 1.500 }, { 1.000, -2.0000 } };

    public static Double[][] multiplicar(Double[][] A, Double[][] B) {

        int aRows = A.length;
        int aColumns = A[0].length;
        int bRows = B.length;
        int bColumns = B[0].length;

        if (aColumns != bRows) {
            throw new IllegalArgumentException("A:Rows: " + aColumns + " did not match B:Columns " + bRows + ".");
        }

        Double[][] C = new Double[aRows][bColumns];
        for (int i = 0; i < aRows; i++) {
            for (int j = 0; j < bColumns; j++) {
                C[i][j] = 0.00000;
            }
        }

        for (int i = 0; i < aRows; i++) { // aRow
            for (int j = 0; j < bColumns; j++) { // bColumn
                for (int k = 0; k < aColumns; k++) { // aColumn
                    C[i][j] += A[i][k] * B[k][j];
                }
            }
        }

        return C;
    }

    public static void main(String[] args) {

        MyMatrix matrix = new MyMatrix();
        Double[][] result = multiplicar(matrix.A, matrix.B);

        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 2; j++)
                System.out.print(result[i][j] + " ");
            System.out.println();
        }
    }
}

Answer 2

Java. Matrix multiplication.

Tested with matrices of different size.

public class Matrix {

/**
 * Matrix multiplication method.
 * @param m1 Multiplicand
 * @param m2 Multiplier
 * @return Product
 */
    public static double[][] multiplyByMatrix(double[][] m1, double[][] m2) {
        int m1ColLength = m1[0].length; // m1 columns length
        int m2RowLength = m2.length;    // m2 rows length
        if(m1ColLength != m2RowLength) return null; // matrix multiplication is not possible
        int mRRowLength = m1.length;    // m result rows length
        int mRColLength = m2[0].length; // m result columns length
        double[][] mResult = new double[mRRowLength][mRColLength];
        for(int i = 0; i < mRRowLength; i++) {         // rows from m1
            for(int j = 0; j < mRColLength; j++) {     // columns from m2
                for(int k = 0; k < m1ColLength; k++) { // columns from m1
                    mResult[i][j] += m1[i][k] * m2[k][j];
                }
            }
        }
        return mResult;
    }

    public static String toString(double[][] m) {
        String result = "";
        for(int i = 0; i < m.length; i++) {
            for(int j = 0; j < m[i].length; j++) {
                result += String.format("%11.2f", m[i][j]);
            }
            result += "\n";
        }
        return result;
    }

    public static void main(String[] args) {
        // #1
        double[][] multiplicand = new double[][] {
                {3, -1, 2},
                {2,  0, 1},
                {1,  2, 1}
        };
        double[][] multiplier = new double[][] {
                {2, -1, 1},
                {0, -2, 3},
                {3,  0, 1}
        };
        System.out.println("#1\n" + toString(multiplyByMatrix(multiplicand, multiplier)));
        // #2
        multiplicand = new double[][] {
                {1, 2, 0},
                {-1, 3, 1},
                {2, -2, 1}
        };
        multiplier = new double[][] {
                {2},
                {-1},
                {1}
        };
        System.out.println("#2\n" + toString(multiplyByMatrix(multiplicand, multiplier)));
        // #3
        multiplicand = new double[][] {
                {1, 2, -1},
                {0,  1, 0}
        };
        multiplier = new double[][] {
                {1, 1, 0, 0},
                {0, 2, 1, 1},
                {1, 1, 2, 2}
        };
        System.out.println("#3\n" + toString(multiplyByMatrix(multiplicand, multiplier)));
    }
}

Output:

#1
      12.00      -1.00       2.00
       7.00      -2.00       3.00
       5.00      -5.00       8.00

#2
       0.00
      -4.00
       7.00

#3
       0.00       4.00       0.00       0.00
       0.00       2.00       1.00       1.00

Answer 3

static int b[][]={{21,21},{22,22}};

static int a[][] ={{1,1},{2,2}};

public static void mul(){
    int c[][] = new int[2][2];

    for(int i=0;i<b.length;i++){
        for(int j=0;j<b.length;j++){
            c[i][j] =0;
        }   
    }

    for(int i=0;i<a.length;i++){
        for(int j=0;j<b.length;j++){
            for(int k=0;k<b.length;k++){
            c[i][j]= c[i][j] +(a[i][k] * b[k][j]);
            }
        }
    }

    for(int i=0;i<c.length;i++){
        for(int j=0;j<c.length;j++){
            System.out.print(c[i][j]);
        }   
        System.out.println("\n");
    }
}

Answer 4

Try this,

public static Double[][] multiplicar(Double A[][],Double B[][]){
    Double[][] C= new Double[2][2];
    int i,j,k;
     for (i = 0; i < 2; i++) {
         for (j = 0; j < 2; j++) {
             C[i][j] = 0.00000;
         }
     }
    for(i=0;i<2;i++){
        for(j=0;j<2;j++){
            for (k=0;k<2;k++){
                  C[i][j]+=(A[i][k]*B[k][j]);

            }

        }
    }
    return C;
}

Answer 5

try this,it may help you

import java.util.Scanner;

public class MulTwoArray {

public static void main(String[] args) {

    int i, j, k;
    int[][] a = new int[3][3];
    int[][] b = new int[3][3];
    int[][] c = new int[3][3];



    Scanner sc = new Scanner(System.in);

    System.out.println("Enter size of array a");
    int rowa = sc.nextInt();
    int cola = sc.nextInt();

    System.out.println("Enter size of array b");
    int rowb = sc.nextInt();
    int colb = sc.nextInt();


    //read and b

    System.out.println("Enter elements of array a");
    for (i = 0; i < rowa; ++i) {
        for (j = 0; j < cola; ++j) {

            a[i][j] = sc.nextInt();

        }
        System.out.println();
    }
    System.out.println("Enter elements of array b");
    for (i = 0; i < rowb; ++i) {
        for (j = 0; j < colb; ++j) {

            b[i][j] = sc.nextInt();

        }
        System.out.println("\n");
    }

    //print a and b

    System.out.println("the elements of array a");
    for (i = 0; i < rowa; ++i) {
        for (j = 0; j < cola; ++j) {

            System.out.print(a[i][j]);
            System.out.print("\t");

        }
        System.out.println("\n");
    }
    System.out.println("the elements of array b");
    for (i = 0; i < rowb; ++i) {
        for (j = 0; j < colb; ++j) {

            System.out.print(b[i][j]);
            System.out.print("\t");

        }
        System.out.println("\n");

    }

    //multiply a and b

    for (i = 0; i < rowa; ++i) {

        for (j = 0; j < colb; ++j) {
            c[i][j] = 0;
            for (k = 0; k < cola; ++k) {
                c[i][j] += a[i][k] * b[k][j];
            }
        }
    }


    //print multi result

    System.out.println("result of multiplication of array a and b is ");
    for (i = 0; i < rowa; ++i) {
        for (j = 0; j < colb; ++j) {

            System.out.print(c[i][j]);
            System.out.print("\t");
        }
        System.out.println("\n");
    }
}
}

Answer 6

The method mults is a procedure(Pascal) or subroutine(Fortran)

The method multMatrix is a function(Pascal,Fortran)

import java.util.*;

public class MatmultE
{
private static Scanner sc = new Scanner(System.in);
  public static void main(String [] args)
  {
    double[][] A={{4.00,3.00},{2.00,1.00}}; 
    double[][] B={{-0.500,1.500},{1.000,-2.0000}};
    double[][] C=multMatrix(A,B);
    printMatrix(A);
    printMatrix(B);    
    printMatrix(C);

    double a[][] = {{1, 2, -2, 0}, {-3, 4, 7, 2}, {6, 0, 3, 1}};
    double b[][] = {{-1, 3}, {0, 9}, {1, -11}, {4, -5}};
    double[][] c=multMatrix(a,b);
    printMatrix(a);
    printMatrix(b);    
    printMatrix(c);

    double[][] a1 = readMatrix();
    double[][] b1 = readMatrix();
    double[][] c1 = new double[a1.length][b1[0].length];
    mults(a1,b1,c1,a1.length,a1[0].length,b1.length,b1[0].length);
    printMatrix(c1);
    printMatrixE(c1);
  }

   public static double[][] readMatrix() {
       int rows = sc.nextInt();
       int cols = sc.nextInt();
       double[][] result = new double[rows][cols];
       for (int i = 0; i < rows; i++) {
           for (int j = 0; j < cols; j++) {
              result[i][j] = sc.nextDouble();
           }
       }
       return result;
   }


  public static void printMatrix(double[][] mat) {
  System.out.println("Matrix["+mat.length+"]["+mat[0].length+"]");
       int rows = mat.length;
       int columns = mat[0].length;
       for (int i = 0; i < rows; i++) {
           for (int j = 0; j < columns; j++) {
               System.out.printf("%8.3f " , mat[i][j]);
           }
           System.out.println();
       }
   System.out.println();
  }

  public static void printMatrixE(double[][] mat) {
  System.out.println("Matrix["+mat.length+"]["+mat[0].length+"]");
       int rows = mat.length;
       int columns = mat[0].length;
       for (int i = 0; i < rows; i++) {
           for (int j = 0; j < columns; j++) {
               System.out.printf("%9.2e " , mat[i][j]);
           }
           System.out.println();
       }
   System.out.println();
  }


   public static double[][] multMatrix(double a[][], double b[][]){//a[m][n], b[n][p]
   if(a.length == 0) return new double[0][0];
   if(a[0].length != b.length) return null; //invalid dims

   int n = a[0].length;
   int m = a.length;
   int p = b[0].length;

   double ans[][] = new double[m][p];

   for(int i = 0;i < m;i++){
      for(int j = 0;j < p;j++){
         ans[i][j]=0;
         for(int k = 0;k < n;k++){
            ans[i][j] += a[i][k] * b[k][j];
         }
      }
   }
   return ans;
   }

   public static void mults(double a[][], double b[][], double c[][], int r1, 
                        int c1, int r2, int c2){
      for(int i = 0;i < r1;i++){
         for(int j = 0;j < c2;j++){
            c[i][j]=0;
            for(int k = 0;k < c1;k++){
               c[i][j] += a[i][k] * b[k][j];
            }
         }
      }
   }
}

where as input matrix you can enter

inE.txt

4 4
1 1 1 1
2 4 8 16
3 9 27 81
4 16 64 256
4 3
4.0 -3.0 4.0
-13.0 19.0 -7.0
3.0 -2.0 7.0
-1.0 1.0 -1.0

in unix like cmmd line execute the command:

$ java MatmultE < inE.txt > outE.txt

and you get the output

outC.txt

Matrix[2][2]
   4.000    3.000 
   2.000    1.000 

Matrix[2][2]
  -0.500    1.500 
   1.000   -2.000 

Matrix[2][2]
   1.000    0.000 
   0.000    1.000 

Matrix[3][4]
   1.000    2.000   -2.000    0.000 
  -3.000    4.000    7.000    2.000 
   6.000    0.000    3.000    1.000 

Matrix[4][2]
  -1.000    3.000 
   0.000    9.000 
   1.000  -11.000 
   4.000   -5.000 

Matrix[3][2]
  -3.000   43.000 
  18.000  -60.000 
   1.000  -20.000 

Matrix[4][3]
  -7.000   15.000    3.000 
 -36.000   70.000   20.000 
-105.000  189.000   57.000 
-256.000  420.000   96.000 

Matrix[4][3]
-7.00e+00  1.50e+01  3.00e+00 
-3.60e+01  7.00e+01  2.00e+01 
-1.05e+02  1.89e+02  5.70e+01 
-2.56e+02  4.20e+02  9.60e+01 

Answer 7

import java.util.*; public class Mult {

public static int[][] C;

public static void main(String[] args) {

    Scanner s = new Scanner(System.in);

    System.out.println("Enter Row of Matrix A");
    int Rowa = s.nextInt();

    System.out.println("Enter Column of Matrix A");
    int Cola = s.nextInt();

    System.out.println("Enter Row of Matrix B");
    int Rowb = s.nextInt();

    System.out.println("Enter Column of Matrix B");
    int Colb = s.nextInt();

    int[][] A = new int[Rowa][Cola];
    int[][] B = new int[Rowb][Colb];

    C= new int[Rowa][Colb];
    //int[][] C = new int;
    System.out.println("Enter Values of Matrix A");

    for(int i =0 ; i< A.length ; i++) {
        for(int j = 0 ; j<A.length;j++) {
            A[i][j] = s.nextInt();
        }

    }

    System.out.println("Enter Values of Matrix B");

    for(int i =0 ; i< B.length ; i++) {
        for(int j = 0 ; j<B.length;j++) {
            B[i][j] = s.nextInt();
        }
    }




    if(Cola==Rowb) {
        for(int i = 0;i < A.length;i++){
              for(int j = 0;j < A.length;j++){
                 C[i][j]=0;
                 for(int k = 0;k < B.length;k++){
                    C[i][j] += A[i][k] * B[k][j];
                 }
              }
           }
    }
    else {
        System.out.println("Cannot multiply");
    }









    // Printing matrix A
    /*
    for(int i =0 ; i< A.length ; i++) {
        for(int j = 0 ; j<A.length;j++) {
        System.out.print(A[i][j]+ "\t");
        }
        System.out.println();
    }
    */

    for(int i =0 ; i< A.length ; i++) {
        for(int j = 0 ; j<A.length;j++) {
        System.out.print(C[i][j]+ "\t");
        }
        System.out.println();
    }

}

}

Answer 8

My code is super easy and works for any order of matrix

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.println(" Enter No. of rows in matrix 1 : ");
    int arows = sc.nextInt();
    System.out.println(" Enter No. of columns in matrix 1 : ");
    int acols = sc.nextInt();
    System.out.println(" Enter No. of rows in matrix 2 : ");
    int brows = sc.nextInt();
    System.out.println(" Enter No. of columns in matrix 2 : ");
    int bcols = sc.nextInt();
    if (acols == brows) {
        System.out.println(" Enter elements of matrix 1 ");
        int a[][] = new int[arows][acols];
        int b[][] = new int[brows][bcols];
        for (int i = 0; i < arows; i++) {
            for (int j = 0; j < acols; j++) {
                a[i][j] = sc.nextInt();
            }
        }
        System.out.println(" Enter elements of matrix 2 ");
        for (int i = 0; i < brows; i++) {
            for (int j = 0; j < bcols; j++) {
                b[i][j] = sc.nextInt();
            }
        }
        System.out.println(" The Multiplied matrix is : ");
        int sum = 0;
        int c[][] = new int[arows][bcols];
        for (int i = 0; i < arows; i++) {
            for (int j = 0; j < bcols; j++) {
                for (int k = 0; k < brows; k++) {
                    sum = sum + a[i][k] * b[k][j];
                    c[i][j] = sum;
                }

                System.out.print(c[i][j] + " ");
                sum = 0;
            }
            System.out.println();
        }
    } else {
        System.out.println("Order of matrix in invalid");
    }
}

Answer 9

Using For Loop

import java.util.Scanner;
 
public class MatrixMultiplicationExample{  
  public static void main(String args[]){  
 
    int row1, col1, row2, col2;
    Scanner s = new Scanner(System.in);
    System.out.print("Enter number of rows in first matrix:");
    row1 = s.nextInt();
    System.out.print("Enter number of columns in first matrix:");
    col1 = s.nextInt();
    System.out.print("Enter number of rows in second matrix:");
    row2 = s.nextInt();
    System.out.print("Enter number of columns in second matrix:");
    col2 = s.nextInt();
 
    if (col1 != row2) {
        System.out.println("Matrix multiplication is not possible");
    }
    else {
        int a[][] = new int[row1][col1];
        int b[][] = new int[row2][col2];
        int c[][] = new int[row1][col2];
 
        System.out.println("Enter values for matrix A : \n");
        for (int i = 0; i < row1; i++) {
            for (int j = 0; j < col1; j++) 
                a[i][j] = s.nextInt();
        }
        System.out.println("Enter values for matrix B : \n");
        for (int i = 0; i < row2; i++) {
            for (int j = 0; j < col2; j++) 
                b[i][j] = s.nextInt();
        }
 
        System.out.println("Matrix multiplication is : \n");
        for(int i = 0; i < row1; i++) {    
            for(int j = 0; j < col2; j++){    
              c[i][j]=0;      
              for(int k = 0; k < col1; k++){      
                c[i][j] += a[i][k] * b[k][j];      
              }
              System.out.print(c[i][j] + " ");  
            }
            System.out.println();
        }    
    }
  }
}

Program Output

Enter number of rows in first matrix:2
Enter number of columns in first matrix:2
Enter number of rows in second matrix:2
Enter number of columns in second matrix:2
Enter values for matrix A :
 
3 4
2 1
Enter values for matrix B :
 
1 5
3 7
Matrix multiplication is :
 
15 43
5 17

Reference: Scaler Topics and GeeksforGeeks

Answer 10

multiply 4x4 Matrixes

float[] mul(float[] l, float[] r) {
    float[] res = new float[16];
    for (int i = 0; i < 16; i++) {
        int y = i / 4;
        int x = i % 4;
        res[i] = l[x]      * r[y] +
                 l[x + 4]  * r[y + 4] +
                 l[x + 8]  * r[y + 8] +
                 l[x + 12] * r[y + 12];
    }
    return res;
}