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;
}

You need three loops: it should be `C[i][j] += A[i][k] * B[k][j]`. — Louis Wasserman, Jul 12 '13 at 21:00
If you use double notice the lowercase D! you don't have to initialize the array first — morpheus05, Jul 12 '13 at 21:00
To answer your question, I suggest that you use a debugger to step through your code to find out what it is doing. Also I suggest using `double` rather than `Double`. — Code-Apprentice, Jul 12 '13 at 21:04
@LouisWasserman Thank you so much. In retrospect, that was so obvious. I'm dumb. Thank you. — , Jul 12 '13 at 21:05
@morpheus05 It was giving me an error for some reason, but it does seem weird that I would have to initialize it, since it's my understanding the array is automatically filled with 0's, correct? I was probably messing up somewhere else. Thank you so much for answering. — , Jul 12 '13 at 21:08
If you need help figuring out the cause of an error, you should copy and paste the exact error message here. — Code-Apprentice, Jul 12 '13 at 21:12
@MonadNewb This is the error I get. Keep in mind all I did to get this was remove the part where I fill the C array with 0's, everything else is the same and the method runs properly if I don't remove it (even if it does not provide the correct values): Exception in thread "main" java.lang.NullPointerException at clase.Ejecutable.multiplicar(Ejecutable.java:29) at clase.Ejecutable.main(Ejecutable.java:11) Java Result: 1 — , Jul 12 '13 at 21:17
The Dobules with a big D are objects, so you are creating an array of objects which initializes to null. An array of small doubles is a primtive and initializes to 0. — morpheus05, Jul 13 '13 at 07:51

score 34 · Answer 1 · edited Sep 27 '17 at 15:22

34

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();
        }
    }
}

edited Sep 27 '17 at 15:22

Carlos Souza

35
6

answered Jul 12 '13 at 21:22

amatellanes

3,645
2
17
19

Thank you, that works, it was that K index. I notice you left in the part where the C array is filled with 0's. Should this really be necessary? Another user told me it shouldn't, but if I remove it I get a nullPointerException. Thank you. – Jul 12 '13 at 21:44
7

The part where the C array is filled with zeros is necessary if you use `Double`. In this line: C[i][j] += A[i][k] * B[k][j]; you would add the value of the multiplication to a `null` reference and you'd get a `NullPointerException`. Instead of using `Double` you use the primitive `double` this part wouldn't be necessary. – amatellanes Jul 12 '13 at 22:32
That makes sense. Thank you very much for taking the time to answer. – Jul 13 '13 at 03:00
Probably a good idea to do a complete check of inner dimensions, i.e., change the check line `if (aColumns != bRows)` to `if (aColumns != bRows || aRows != bColumns)`. It won't fail as currently stated, but changing it will help pick up any bugs when doing matrix math. – James B Jan 29 '14 at 18:13
@JamesB: `(aRows!=bColumns)` is not essential. for example you can multiply `3X3 matrix with 3X3 matrix` – brain storm Feb 14 '14 at 01:18
Thanks. Note error in filling the array with zeros still contains hard-code size of 2. Change i<2 to i – Synco Apr 06 '16 at 00:52
1

Actually, it can be optimised by removing the initialisation section and move C[i][j] = 0 to just before the for (int k..) loop. – Synco Apr 06 '16 at 01:04

score 12 · Answer 2 · answered May 22 '14 at 22:01

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

you didn't test any illegal matrix sizes – Someone Somewhere Apr 17 '20 at 22:55 — Someone Somewhere, Apr 17 '20 at 22:55

score 4 · Answer 3 · answered Jul 12 '13 at 22:12

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");
    }
}

Dulanga · Answer 4 · 2013-07-12T22:00:20.563

2

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;
}

edited Jul 12 '13 at 22:00

answered Jul 12 '13 at 21:38

Dulanga

1,326
7
15

There's actually an error, it has to be B[k][j], not [k][i]. Not that I would have figured that out myself, you were much closer than I was. Thank you for answering, I'd upvote you if I could. – Jul 12 '13 at 21:42
ugghh sorry that was actually a mistake. Sorry for that – Dulanga Jul 12 '13 at 22:00

score 1 · Answer 5 · answered Aug 28 '15 at 14:03

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");
    }
}
}

score 1 · Answer 6 · answered Mar 14 '16 at 12:06

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

score 0 · Answer 7 · edited Mar 30 '22 at 07:05

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();
        }
    }
}

score 0 · Answer 8 · edited Oct 16 '20 at 17:34

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");
    }
}

score 0 · Answer 9 · answered Jul 28 '21 at 20:20

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;
}

score 0 · Answer 10 · answered Nov 27 '22 at 10:17

#include <stdio.h>
int main()
{
int row = 2;
int col = 3;
int a[row][col];
int count = 1;
printf("Array A \n");
for (int i = 0; i < row; i++)
{
    for (int j = 0; j < col; j++)
    {
        a[i][j] = count;
        count++;
        printf(" %d ", a[i][j]);
    }
    printf("\n");
}
int b[col][row];
printf("\nArray B \n");
for (int i = 0; i < col; i++)
{
    for (int j = 0; j < row; j++)
    {
        b[i][j] = count;
        count++;
        printf(" %d ", b[i][j]);
    }
    printf("\n");
}
printf("\n  A  * B \n");
int c[row][col];
int mul = 1, plus = 0;
for (int i = 0; i < row; i++)
{
    for (int j = 0; j < row; j++)
    {
        plus = 0;
        for (int k = 0; k < col; k++)
        {
            mul = a[i][k] * b[k][j];
            plus += mul;
        }
        c[i][j] = plus;
        printf(" %d ", c[i][j]);
    }
    printf("\n");
}
}

Inputs are given through loop you can also take inputs from user. Make sure to check is given matrix is square matrix or row of matrix A and column of matrix b must be equal and column of matrix A and row of matrix B must be equal (Basic rule of matrix multiply). — pankaj singh, Nov 27 '22 at 10:24

Matrix multiplication using arrays

10 Answers10

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