Matrix Multiplication Returing Array Index out of bound error

Question

This is one first post here, Pardon me if my question doesn't meet required standards here.

I have written a piece of code which takes input for two matrix from two separate files and performs multiplication and output the data to a new file.

It gives perfect output for 2x3 or 3x3 matrix. If i give input of 4x4 matrix i get array index out of bound runtime exception. I don't understand the reason as i dynamically create index

I get an array index out of bound exception at line 40.

I get an error.

![Snipet][2]

1. List item

   public class MM {
   
   
   
   
   private BufferedReader br;
   private int sum = 0;
   private final static String matrixA="matrixA.txt"; 
   private final static String matrixB="matrixB.txt";
   
   public static void main(String[] args) {
       new MM().MathMultiplicationValues(matrixA, matrixB);
   }
   
   private void MathMultiplicationValues(String mat1, String mat2) {
       try {
           br = new BufferedReader(new FileReader(mat1));          
   
           String line;
           int mat1rows = 0, mat1cols = 0, mat2rows = 0, mat2cols = 0;
           while ((line = br.readLine()) != null) {
               mat1cols = line.split(" ").length + 1;
               mat1rows++;
           }
           br.close(); // To close file
           br = new BufferedReader(new FileReader(mat2)); // to read input from file.
           while ((line = br.readLine()) != null) {
               mat2cols = line.split(" ").length + 1;
               mat2rows++;
           }
           int[][] mat1vals = new int[mat1rows ][mat1cols ]; 
       int[][] mat2vals = new int[mat2rows ][mat2cols ]; 
           br.close();
           br = new BufferedReader(new FileReader(mat1));
           for (int i = 1; i < mat1rows + 1; i++) {
               line = br.readLine();
               String[] colvals = line.split(" ");
               for (int j = 1; j < mat1cols; j++) {
                   mat1vals[i][j] = Integer.parseInt(colvals[j - 1]);
   
               }
   
           }
   
   
           br.close();
           br = new BufferedReader(new FileReader(mat2));
           for (int i = 1; i < mat2rows + 1; i++) {
               line = br.readLine();
               String[] colvals = line.split(" ");
               for (int j = 1; j < mat2cols; j++) {
                   mat2vals[i][j] = Integer.parseInt(colvals[j - 1]);
   
               }
   
   
           }
   
           br.close();
           if ((mat1cols-1) == mat2rows) { 
               int[][] resltmat = new int[mat1rows + 1][mat2cols + 1];
               for (int i = 1; i < mat1rows + 1; i++) { //Loop does matrix multiplication. 
                   for (int j = 1; j < mat1cols; j++) {
                       for (int k = 0; k < mat2rows + 1; k++)
                           sum = sum + mat1vals[i][k] * mat2vals[k][j];
                       resltmat[i][j] = sum;
                       sum = 0;
                   }
               }
   
               final PrintWriter pw = new PrintWriter("Answer.txt"); //Creates a new file called Matrix Answer. 
               for (int i = 1; i < mat1rows + 1; i++) 
               {
   
                   for (int j = 1; j < mat2cols; j++) {
                       pw.print(resltmat[i][j] + " "); // Writes the output to file the file called MatrixAnswer
   
                   }
                   pw.println();
   
               }
               pw.close();
           } else // If no of columns not equal to rows control passes to else block. 
               System.out.println("Multiplication of Matrix is not possible because columns are not equal to rows");
   
       } catch (Exception e) {
           e.printStackTrace();
   
       }
   }
   
   }
   

Answer 1

As you used in the allocation, the dimension of the resulting matrix are mat1rows x mat2cols. Thus in the computation of resltmat[i][j] the index i has bound mat1rows (check) and the index j has the upper bound mat2cols (fail). Thus change the range of j from mat1cols to mat2cols.

Answer 2

Might be because of this

for (int i = 1; i < mat1rows + 1; i++) {
    line = br.readLine();
    String[] colvals = line.split(" ");
    for (int j = 1; j < mat1cols; j++) {
        mat1vals[i][j] = Integer.parseInt(colvals[j - 1]);

    }

}

i = mat1rows on the last iteration which is OOB. Change for (int i = 1; i < mat1rows + 1; i++) to for (int i = 1; i < mat1rows; i++)