Java - Adding Matrices - Error

Question

Code:

  public static void main(String[] args) {
  int m, n, p, q, c, d, k;
  int sum1 = 0;

  Scanner scan = new Scanner(System.in); 

  System.out.println("Please enter the number of rows and columns for the first     matrix.");
  m = scan.nextInt();
  n = scan.nextInt();

  int first[][] = new int[m][n];

  System.out.println("Enter the elements of first matrix:"); 

  for ( c = 0 ; c < m ; c++ ) // The numbers are put into the matrix.
     for ( d = 0 ; d < n ; d++ )
        first[c][d] = scan.nextInt();

  System.out.println("Please enter the number of rows and columns for the second matrix."); 
  p = scan.nextInt();
  q = scan.nextInt();


  int second[][] = new int[p][q];
  int multiply[][] = new int[m][q];
  int sum[][] = new int[m][n]; 

  System.out.println("Enter the elements of second matrix:");

  for ( c = 0 ; c < p ; c++ )    
     for ( d = 0 ; d < q ; d++ )
        second[c][d] = scan.nextInt();


  for ( c = 0 ; c < m ; c++ )  
  {
     for ( d = 0 ; d < q ; d++ )
     {   
        for ( k = 0 ; k < p ; k++ )
        {
           sum1 = sum1 + first[c][k]*second[k][d]; 
        }
        multiply[c][d] = sum1;
        sum1 = 0;
     }
  }

  System.out.println("The product of the two matrices is: ");  
  for ( c = 0 ; c < m ; c++ ) {
     for ( d = 0 ; d < q ; d++ ) {
        System.out.print(multiply[c][d]+"\t");
     }

     System.out.print("\n");

  }

  for ( c = 0 ; c < m ; c++ )   { 
     for ( d = 0 ; d < n ; d++ ) {
        sum[c][d] = first[c][d] + second[c][d];    


        System.out.println("The sum of the two matrices is: "); 
        for ( c = 0 ; c < m ; c++ )
        {
           for ( d = 0 ; d < n ; d++ )
              System.out.print(sum[c][d]+"\t");

           System.out.println();

        }
     }
  }
  }
  }

The Output is incorrect for the sum. It prints like this:
The sum of the two matrices is:
0 0
0 0

Anyone know how to fix this?

I've been trying to figure this out, but ended up getting more errors. Sorry for all the questions, I'm a beginner. Thank you :)

Just for future reference, most people who look at this are only seeing "Here's a ton of code that doesn't work, fix it for me." Try using a debugger/print statements to step through your code and see where the variables are no longer what you expect them to be. — takendarkk
– takendarkk, Commented Jan 17, 2014 at 3:25

Ted Hopp · Accepted Answer · 2014-01-17 03:27:31Z

One problem with your code is that the loops for printing the sum matrix are inside the loops for summing the matrices. Note that when printing loops exit, c and d will be m and n respectively, so the outer loops will immediately exit. Move the printing loops later in the code.

So instead of this:

for ( c = 0 ; c < m ; c++ )   { 
    for ( d = 0 ; d < n ; d++ ) {
        sum[c][d] = first[c][d] + second[c][d];    


        System.out.println("The sum of the two matrices is: "); 
        for ( c = 0 ; c < m ; c++ )
        {
           for ( d = 0 ; d < n ; d++ )
              System.out.print(sum[c][d]+"\t");

           System.out.println();

        }
    }
}

use this:

for ( c = 0 ; c < m ; c++ )   { 
    for ( d = 0 ; d < n ; d++ ) {
        sum[c][d] = first[c][d] + second[c][d];    
    }
}
System.out.println("The sum of the two matrices is: "); 
for ( c = 0 ; c < m ; c++ ) {
    for ( d = 0 ; d < n ; d++ )
        System.out.print(sum[c][d]+"\t");
    System.out.println();
}

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