Neighbours does not yield true-CodePudding

I am writing a small program of how to calculate sum of neighbours of a given position in a grid. for some reason the program does not recognize the right value as correct. I was wondering if that may be because i am using try catch to restrict out of bounds or if it something else i have missed?

i am using a simple 3x3 grid numbered 1 - 9. i have used the same matrix for many other tests so assume there is nothing wrong with the grid. This even though i get 11 when debugging and checking step by step. i dont quite understand, does anyone have an idea?

the -1 in sum was simply to force it to 11 (2 4 5) but the program still yields false when running the input positions. either there is something ive missed or something i have not understood

{1, 2, 3},
{4, 5, 6},
{7, 8, 9}


out.println(sumNeighbours(matrix, 0, 0) == 11);

int sumNeighbours(int[][] matrix, int row, int col) {
    int sum = -1;
    try {
        for (int i = row - 1; i <= row; i  ) {
            for (int j = col - 1; j <= col; j  ) {
                sum = sum   matrix[i][j];
            }
        }
    } catch (ArrayIndexOutOfBoundsException e) {
    } return sum;

CodePudding user response：

Simple printouts (or better: using a debugger ) show you what is happening:

int sumNeighbours(int[][] matrix, int row, int col) {

    int sum = 1;
    try {
        for (int i = row - 1; i <= row; i  ) {
            for (int j = col - 1; j <= col; j  ) {
                System.out.println("Step 1 invalid indecies: i "   i  " j " j);
                sum = sum   matrix[i][j];
            }
        }
    } catch (ArrayIndexOutOfBoundsException e) {
        System.out.println("Step 2 exiting loop, muted exception");
    }

    System.out.println("Step 3 returning sum " sum);
    return sum;
}

Side note: exceptions are just that. They are meant to handle exceptional situations. Avoid using exceptions to control your program.

CodePudding user response：

I'm going to post this here as a community wiki, so as not to detract from cOder's answer (1 to it, by the way), but the key is to write your code to avoid running into AIOOBE, to create loop start and end values that prevent this from happening, something like:

int sumNeighbours(int[][] matrix, int row, int col) {
    int sum = 0;
    
    int iStart = Math.max(0, row - 1);
    int iEnd = Math.min(matrix.length, row   2);
    for (int i = iStart; i < iEnd; i  ) {
        int jStart = Math.max(0, col -1);
        int jEnd = Math.min(matrix[i].length, col   2);
        for (int j = jStart; j < jEnd; j  ) {
            sum  = (i == row && j == col) ? 0 : matrix[i][j];
        }
    }
    return sum;
}

CodePudding user response：

In the first iteration i and j is equal to -1 and you are trying to get value from matrix[-1][-1] and then exception is thrown. Catch block is empty so no operation is done and finnaly your programme returns sum (it is -1).

If you want to continue iteration after exception you should do something like this:

        for (int i = row - 1; i <= row; i  ) {
            for (int j = col - 1; j <= col; j  ) {
                try {
                   sum = sum   matrix[i][j];
                }
                catch (Exception e) {
                // do nothing
                }
            }
        }
    } return sum;

In this case only adding is in the loop, so error would dont stop it.

PS: try to avoid handling situations like this with try catches