For loop producing java.lang.ArrayIndexOutOfBoundsException: 3

Question

I'm writing a program that reads a file and sees if the information make up a magic square, but I am getting this error: "java.lang.ArrayIndexOutOfBoundsException: 3" in my sumCol method. Specifically the line below: for (int row = 0; row < square[row].length; row++) {

public int sumCol(int col) {
    int sum = 0;
    for (int row = 0; row < square[row].length; row++) {
        sum += square[row][col];
    }
    return sum;
}

I'm not sure if it is necessary, but here is the rest of my class as well:

// ****************************************************************

// Square.java
//
// Define a Square class with methods to create and read in
// info for a square matrix and to compute the sum of a row,
// a col, either diagonal, and whether it is magic.
//         
// ****************************************************************
import java.util.Scanner;

public class Square {

int[][] square;

//--------------------------------------
//create new square of given size
//--------------------------------------
public Square(int size) {

    square = new int[size][size];

    for (int row = 0; row < square.length; row++) {
        for (int col = 0; col < square.length; col++) {
            square[row][col] = row * 10 + col;
        }
    }

}

//--------------------------------------
//return the sum of the values in the given row
//--------------------------------------
public int sumRow(int row) {
    int sum = 0;
    for (int col = 0; col < square.length; col++) {
        sum += square[row][col];
    }
    return sum;
}

//--------------------------------------
//return the sum of the values in the given column
//--------------------------------------
public int sumCol(int col) {
    int sum = 0;
    for (int row = 0; row < square[row].length; row++) {
        sum += square[row][col];
    }
    return sum;
}

//--------------------------------------
//return the sum of the values in the main diagonal
//--------------------------------------
public int sumMainDiag() {
    int sum = 0;
    for (int j = 0; j < square.length; j++) {
        sum += square[j][j];    //you can do this because a square's diagonals have the same coordinate points
    }
    return sum;
}

//--------------------------------------
//return the sum of the values in the other ("reverse") diagonal
//--------------------------------------
public int sumOtherDiag() {
    int sum = 0;
    for (int j = 0; j < square.length; j++) {
        sum += square[j][square.length - 1 - j];
    }
    return sum;
}

//--------------------------------------
//return true if the square is magic (all rows, cols, and diags have
//same sum), false otherwise
//--------------------------------------
public boolean magic() {
    boolean answer = true;
    int sum = sumMainDiag();
    if (sumOtherDiag() != sum) {
        answer = false;
    } else {
        for (int col = 0; col < square.length; col++) {
            if (sum != sumCol(col)) {
                answer = false;
            }
        }
        for (int row = 0; row < square.length; row++) {
            if (sum != sumRow(row)) {
                answer = false;
            }
        }
    }
    return answer;
}

//--------------------------------------
//read info into the square from the input stream associated with the
//Scanner parameter
//--------------------------------------
public void readSquare(Scanner scan) {
    for (int[] square1 : square) {
        for (int col = 0; col < square.length; col++) {
            square1[col] = scan.nextInt();
        }
    }
}

//--------------------------------------
//print the contents of the square, neatly formatted
//--------------------------------------
public void printSquare() {
    for (int[] square1 : square) {
        for (int col = 0; col < square1.length; col++) {
            System.out.print(square1[col] + "\t");
        }
        System.out.println();
    }

}

}

Nir Alfasi · Accepted Answer · 2013-11-27 02:03:00Z

1

change

row < square[row].length

to:

row < square.length

answered Nov 27, 2013 at 2:03

Nir Alfasi

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Comments

Óscar López · Accepted Answer · 2013-11-27 02:03:06Z

1

Perhaps you meant this?

public int sumCol(int col) {
    int sum = 0;
    for (int row = 0; row < square.length; row++) {
        sum += square[row][col];
    }
    return sum;
}

Notice that the number of rows is given by square.length, whereas the number of columns in a given row is given by square[row].length.

answered Nov 27, 2013 at 2:03

Óscar López

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2 Comments

user2923395 Over a year ago

That produced an infinite loop... Well not infinite, but quite close

Óscar López Over a year ago

@user2923395 no, it doesn't. The problem must be elsewhere, use a debugger to find the real culprit.

nanofarad · Accepted Answer · 2013-11-27 02:03:09Z

1

You're doing the check wrong. Use:

public int sumCol(int col) {
    int sum = 0;
    for (int row = 0; row < square.length; row++) {
        sum += square[row][col];
    }
    return sum;
}

You're checking the length of the row, not of the overall array.

answered Nov 27, 2013 at 2:03

nanofarad

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2 Comments

user2923395 Over a year ago

That produced an infinite loop... Well not infinite, but quite close

nanofarad Over a year ago

@user2923395 Then the loop is elsewhere in the logic. I suggest singlestepping your code with a debugger.

Elliott Frisch · Accepted Answer · 2013-11-27 02:11:18Z

1

I think you want

// square.length not square[row].length
for (int row = 0; row < square.length; row++) {
  // Here's where we might check square[row].length to be safe.
  if (col < square[row].length) {
    sum += square[row][col];
  }
}

edited Nov 27, 2013 at 2:11

answered Nov 27, 2013 at 2:05

Elliott Frisch

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