Finding valid neighbors in 2D array

Question

So, I have a 4x4 2D array (it will always be these dimensions). Starting with a location on the array, some row and column, I want to find all of its valid neighbors. So far, I

Answer 1

Here's how I'd do it: a method that gets a list of x,y pairs for valid neighbors, given an arbitrary [x,y] point and generalized to any array size:

public List<int[]> getNeighbors(x, y, maxX, maxY) {
    neighbors = new ArrayList<int[]>;
    if x > 0:
        neighbors.add({x-1, y});
    if y > 0:
        neighbors.add({x, y-1});
    if x < maxX:
        neighbors.add({x+1, y});
    if x < maxY:
        neighbors.add({x, y+1});
    return neighbors;
}

[...]

for (int[] coords : getNeighbors(x, y, 4, 4)) {
    // do stuff
}

Answer 2

public class FindingNeighboursInMatrix {

public static void main(String[] args) {
    int array[][] = { { 1, 2, 3, 4 }, 
                      { 5, 6, 7, 8 }, 
                      { 9, 10, 11, 12 } };

    for (int i = 0; i < array.length; i++) {

        for (int j = 0; j < array[0].length; j++) {

            System.out.println("neightbours of " + array[i][j]);
            int neb[] = findneighbours(i, j, array);
            for (int k = 0; k < neb.length; k++) {
                if (neb[k] != -1) {
                    System.out.print(" " + neb[k] + ",");
                }
            }
            System.out.println();
        }

    }

}

public static int[] findneighbours(int i, int j, int matrix[][]) {
    int neb[] = new int[8];
    // top row
    neb[0] = getvalue(i - 1, j - 1, matrix);
    neb[1] = getvalue(i - 1, j, matrix);
    neb[2] = getvalue(i - 1, j + 1, matrix);

    // left element

    neb[3] = getvalue(i, j - 1, matrix);

    // right element

    neb[4] = getvalue(i, j + 1, matrix);

    // bottom row
    neb[5] = getvalue(i + 1, j - 1, matrix);
    neb[6] = getvalue(i + 1, j, matrix);
    neb[7] = getvalue(i + 1, j + 1, matrix);

    return neb;

}

public static int getvalue(int i, int j, int matrix[][]) {
    int rowSize = matrix.length;
    int colSize = matrix[0].length;

    if (i < 0 || j < 0 || i > rowSize - 1 || j > colSize - 1) {
        return -1;
    }
    return matrix[i][j];
}}

Answer 3

This is my solution:

public int[4][4] array2d;
//don't forget to fill it!

private void adjustNeighbors(int xCoord, int yCoord) {

    for (int yi = y-1; yi <= yCoord+1; yi++) {         //loop through the neighbors

        for (int xi = x-1; xi <= xCoord+1; xi++) {

            try {

                if (!(xCoord != xi && yCoord != yi)) {
                    array2d[y][x]++;  //do whatever you want to all the neighbors!
                } 

            } catch (Exception e) {
                // something is out of bounds
            }

        }

    }

}

Answer 4

Unfortunately by writing code you are telling a computer what to do and the computer doesn't know anything more than what you tell it.

You can automate this kind of thing a little with nonstandard loop logic though I guess:

for (int coff = -1; coff < 3; coff += 2) {
    for (int roff = -1; roff < 3; roff += 2) {

        if (    col + coff >= 0 &&
                col + coff < array.length &&
                row + roff >= 0 &&
                row + roff < array[row].length) {

            // do stuff with array[col + coff][row + roff]

        }
    }
}

That loop structure will flip the column and row offset from -1 to 1 and then break when they become 3 on the 3rd iteration.

But note that in your code, checking against !(stuff) > 4 will give you an ArrayIndexOutOfBounds exception because remember the last index is 4 - 1.

Answer 5

For any 2D array cellValues[][] of (x,y) dimensions below code can be used for getting all 8 neighbors for any cell (i,j). Code will return 0 by default.

public static ArrayList<Integer> getNeighbors(int i, int j, int x, int y, int[][] cellValues) {
    ArrayList<Integer> neighbors = new ArrayList<>();

    if(isCabin(i, j, x, y)) {
        if(isCabin(i + 1, j, x, y))
            neighbors.add(cellValues[i+1][j]);
        if(isCabin(i - 1, j, x, y))
            neighbors.add(cellValues[i-1][j]);
        if(isCabin(i, j + 1, x, y))
            neighbors.add(cellValues[i][j+1]);
        if(isCabin(i, j - 1, x, y))
            neighbors.add(cellValues[i][j-1]);
        if(isCabin(i - 1, j + 1, x, y))
            neighbors.add(cellValues[i-1][j+1]);
        if(isCabin(i + 1, j - 1, x, y))
            neighbors.add(cellValues[i+1][j-1]);
        if(isCabin(i + 1, j + 1, x, y))
            neighbors.add(cellValues[i+1][j+1]);
        if(isCabin(i - 1, j - 1, x, y))
            neighbors.add(cellValues[i-1][j-1]);
    }
    return neighbors;
}

public static boolean isCabin(int i, int j, int x, int y) {
    boolean flag = false;
    if (i >= 0 && i <= x && j >= 0 && j <= y) {
        flag = true;
    }
    return flag; 
}

Answer 6

The way I would do it would be to have a seperate method.

public void example(int changeSign, boolean shouldCheckRow,boolean shouldCheckColumn){
    int num = 4;
    if(changeSign < 0)
        num = 0;
    if(shouldCheckRow)
        //adding a negative is the same as subtracting so if you add -1, you're really subtracting by one.

        if(!((row + changeSign) < num))
            //do stuff
    else
        if(!((col + changeSign) < num))
            //do stuff
}

And the method calls would be

public static void main(String args[]){
    int shouldTestRight = 1;
    int shouldTestLeft = -1;
    int shouldTestUp = 1;
    int shouldTestDown = -1;
    // so if you want to test up or right, the first parameter should be positive
    // if you want to test for down or left, the first parameter should be negative
    // this is because the negative will flip the sign.
    // if you should change the row, the second parameter should be true
    // if you should change the column, the third parameter should be true.
    example(shouldTestRight,true,false);
    example(shouldTestLeft,true,false);
    example(shouldTestUp,false,true);
    example(shouldTestDown,false,true);
}

Of course, you don't have to include the extra ints in the method you're calling from but I did it for extra code readability.