Java Array - Calculate average of surrounding numbers

Question

Ok, so I have an array like so:

1 2 3
4 5 6
7 8 9

Doesn't necessarily have to be any specific size, just showing it so you guys have something to look at. So I have to go through each number, and calculate the average of the numbers that are surrounding it. For example, for the number 1 I have to calculate the average of 2, 5, and 4. I have to compare that average to the number itself, but that's not my issue. My issue is running through an array of any size (determined by the user) and examining the numbers around it. Now, I could program a separate method for each section, meaning a separate method for each corner, a separate method for each side, and a method for the center. But that sounds like a lot more work and a lot less efficient. So, I've decided to make a sort of border around the outside. This border will be filled with 0's, assuming that 0 will never be an input value. This is so that I will not run out of my array, but i can run one coded function on the inside of this border, so that it calculates the average of the 8 surrounding numbers.

So I'm wondering how I could go about running through an array and ignore any 0's it comes across, but take the average of the other numbers around it?

Example:
0 0 0 0 0
0 1 2 3 0
0 4 5 6 0
0 7 8 9 0
0 0 0 0 0

so in the first spot, it'd take the average of the numbers 2, 4, and 5, but ignore the 0's. the reason I need to ignore them is because:
1. I cannot include them in the number count because even though adding 0 won't affect the total, it'll affect the amount of numbers to divide by in obtaining the average
2. I won't actually be using 0 since 0 is a possible input, it'll actually be -1. I put 0 here for the ease of typing and readability.

Suggested best method of going about this?

EDIT2:

So now, I'm getting an output that's close to what I should be getting. But I've thoroughly looked through and I can't see where this error is coming about.

public static int calculate(int[][] array, int row, int col){
    double num = 0;//Used to find the total of real numbers to be divided by
    double total = 0;
    double average;

    for (int a = -1; a <= 1; a++) 
    {
        for (int b = -1; b <= 1; b++) 
        {
            if(row + a < 0 || col + b < 0)
            {
                continue;
            }
            if(a == 0 && b == 0)//if(row + a == row || col + b == col)
            {
                continue;
            }
            if(row + a >= r || col + b >= c)
            {
                continue;
            }
            else
            {                   
                num+=1;
                total+= array[row + a][b + col];
                System.out.println("row+a = " + (row + a) + " and col+b = " + (col + b));
            }
        }
     }

    average = (double)(total/num);
    count++;
    System.out.print(count + ". Average = " + total + " divided by " + num + " = " + average + "\n");

    if (average < array[row][col])
        return 255;
    else
        return 0;
}

Into this function, I put each value in the array (going through each row) to be changed to either 0 or 255. Apparently, the correct final result should be: 0 255 0 255 0

0 0 255 0 0

255 0 255 0 255

255 0 0 0 0

0 255 0 0 255

But I'm getting

0 255 0 255 0

0 0 0 0 0

255 0 255 0 255

0 0 0 0 0

255 0 255 0 255

So I'm looking into those specific differences and I'm going to see where the issue is.

Answer 1

So, I've decided to make a sort of border around the outside.

Trivia - Dan and Kathy Spacklen (Spacken?) did this wit their chessboard when they wrote Sargon 30 years ago.

You can do the border but I think it isn't going to help much. Tell me, in your first example

1 2 3
4 5 6
7 8 9

Assuming #1 is at [0][0], what would be the indexes for the 8 cells surrounding #2?

Answer 2

It seems to me that the simplest solution is to go through all possible cells in a loop and for each to see if it's inside the matrix. It's not exactly short, but there's no ugly unreadable conditions involved and you don't have to surround matrix with anything.

int sum = 0;
int amount = 0;
for (int di = -1; di <= 1; ++di) {
    for (int dj = -1; dj <= 1; ++dj) {
        // check that cell (i0 + di, j0 + dj) is inside the matrix
        // and not the original cell (i0, j0)
        // increase 'sum' and 'amount' if necessary
    }
}

return (double) sum / amount;

edit
Removed part of the code to leave some challenge in the problem