不断收到“类型错误：无法设置未定义的属性‘1’”

Question

I'm trying to solve algorithm problems, but I keep getting this error, and it frustrate me:

    var data = [".......", "...O...", "....O..", ".......", "OO.....", "OO....."]
    var R = 6;
    var C = 7;
    var N = 3;
    var cell = new Array(R);
    for (i=0; i<R; i++) {
        cell[i] = new Array(C);
    }
    for (i=0; i<R; i++) {
        var currentRow = data[i].split("");
        for (j=0; j<C; j++) {
            cell[i][j] = currentRow[j];
        }
    }
    for (x=0; x<=N; x++) {
        for (i=0; i<R; i++) {
            for (j=0; j<C; j++) {
                if (cell[i][j] == '.') {
                    cell[i][j] = 0;
                } else if (cell[i][j] == 'O') {
                    cell[i][j] = 1;
                } else if (cell[i][j] == 3) {
                    cell[i][j] = 0;
                    // Why are these following 4 LINES are not working???
                    cell[i + 1][j] = 0;
                    cell[i - 1][j] = 0;
                    cell[i][j + 1] = 0;
                    cell[i][j - 1] = 0;
                } else if (cell[i][j] >= 0) {
                    cell[i][j] += 1;
                }
            }
        }
    }

The culprit is when we are trying to use 2D array with some operation like

cell[i + 1][j] = 0;
cell[i - 1][j] = 0;
cell[i][j + 1] = 0;
cell[i][j - 1] = 0; 

I dont know why but keep getting TypeError: Cannot set property '1' of undefined

Answer 1

If the first horizontal or first vertical cell of your array is equal 3, than you can't do i - 1 or j -1 . Same thing if the last horizontal cell or the last vertical cell equals 3, you can't do i + 1 or j + 1 . So you just have to check wheter is not the first or the last vertical cell of your bidimensional array.

Answer 2

It looks like these lines of code are sometimes accessing out of bounds on your arrays. The code will work without the error message by changing those 4 lines to guarantee the access remains in bounds, like this:

                // Why are these following 4 LINES are not working???
                if (i < R - 1) {
                    cell[i + 1][j] = 0;
                }
                if (i > 0) {
                    cell[i - 1][j] = 0;
                } 
                if (j < C - 1) {
                    cell[i][j + 1] = 0;
                }
                if (j > 0) {
                    cell[i][j - 1] = 0;
                }

Here's a link to a jsfiddle where I have placed the updated code.