Testing a .txt file for a Magic Square Java

Question

I didn't want to have to ask, but I can not figure out this assignment, and neither could the TA when I asked for help.

I have to take input from a text file, feed the integers in the file into an array list, and test to see if it is a n x n magic square. n is equal to the square root of the array list's length. If it isn't a perfect square it immediately fails the magic square test.

Anyway I have it almost completed; I just don't seem to understand what my professor is telling/asking us to do in the final step of the magic square test.

All the testing before these final four steps works flawlessly. I'll post my current code after the steps.

4. Let rowSums and colSums be two arrays of length n with entries all zeros. Also, let sumDiagMajor and sumDiagMinor, representing the sum of the entries along the top-left to bottom-right and top-right to bottom-left diagonals, respectively, of the table.

5. Let index = 0

6. Repeat until index = n2 (a) Increment rowSums[row] by ArrayList{index} (b) increment colSums[col] by ArrayList{index} (c) If row = col, then increment sumDiagMajor by ArrayList{index}. (d) If row + col = n−1, then increment sumDiagMinor by ArrayList{index} (e) Increment index by 1

7. If sumDiagMajor is equal to sumDiagMinor and each entry of rowSums and colSums, the the table is a magic square; otherwise, it is not.

   int rowSums[] = new int[_n];
   int colSums[] = new int[_n];
   int sumDiagMajor = 0;
   int sumDiagMinor = 0;

   int row, col;
   row = col = 0;

   for (int index = 0; index < (n*n); index++)
   {          
       rowSums[row] = rowSums[row] + magicSquare.get(index);
       colSums[col] = colSums[col] + magicSquare.get(index);

       if (row == col)
       {       
           sumDiagMajor = sumDiagMajor + magicSquare.get(index);   
       }

       if ((row + col) == (n - 1))
       {
           sumDiagMinor = sumDiagMinor + magicSquare.get(index);   
       }

   }

   System.out.println(sumDiagMajor);
   System.out.println(sumDiagMinor);

My questions include, am I properly incrementing the arrays rowSums, and rowCols? He never actually states what to do with rows or cols, so is initializing them to zero the best option?

If I did everything correct so far, how can sumDiagMajor ever equal sumDiagMinor because rows will always equal cols, so the second nested if statement will never run. Therefore it will rule out everything test as being a magic square?

Sorry for the long post, but this is very confusing.

Answer 1

As per your updated requirements. A full example.

public static void main(String[] args) {
    List<Integer> magicSquare = Arrays.asList(2,7,6,9,5,1,4,3,8);

    int n = (int) Math.sqrt(magicSquare.size());
    int rowSums[] = new int[n];
    int colSums[] = new int[n];
    int sumDiagMajor = 0;
    int sumDiagMinor = 0;

    int row = -1;
    int col = -1;

    for (int index = 0; index < n*n; index++) {

        col++;
        if (col % n == 0) {
            row++;
            col = 0;
        }

        rowSums[row] = rowSums[row] + magicSquare.get(index);
        colSums[col] = colSums[col] + magicSquare.get(index);


        if (row == col)
        {
            sumDiagMajor += magicSquare.get(index);
        }

        if ((row + col) == (n - 1))
        {
            sumDiagMinor += magicSquare.get(index);
        }

    }

    boolean isMagicSquare = true;
    for (int i = 0; i < n && isMagicSquare; i++) {
        isMagicSquare = sumDiagMajor == rowSums[i] && sumDiagMajor == colSums[i];
    }
    isMagicSquare = isMagicSquare && sumDiagMajor == sumDiagMinor;

    System.out.println(isMagicSquare); // true
}

Answer 2

You never change row and col in your for-loop which is a bug. For easier understanding, I suggest using two nested for-loops over rowIndex and colIndex and using a little helper to get the value from the 1-D Array like so:

int getValue(int row, int col){
    return magicSquare.get( row * _n + col );
}

Assuming the 1-D Array is a "flattened" square that is built up like (0,0), (0,1), ... (0,_n), (1, 0) , ... (_n,_n)

To make it clearer: Iterate over the square instead of using the index:

for( int row = 0 ; row < _n ; row++){
    for( int col = 0 ; col < _n ; col++){
       // Your stuff here.
    }
}