Question

Is there any way I can prevent javascript from dropping an error if I try to go into a non existing array index?

Example: array[-1] would return error and eventually break all my code. How can I let it just return ‘undefined’ and let my script go on? I can implement an if statement before checking the array (so that if the index is minor than zero or major than the array size it would skip it) but this would be very tedious!

this is my code:

if (grid[j-1][i])
n++;
if (grid[j+1][i])
n++;
if (grid[j][i+1])
n++;
if (grid[j][i-1])
n++;
if (grid[j-1][i-1])
n++;
if (grid[j+1][i+1])
n++;
if (grid[j-1][i+1])
n++;
if (grid[j+1][i-1])
n++;

It is inside of two loops which both sees J and I starting from zero. I don’t want to change them and neither writing another if statement (as you can see, there are already too much of them!). Is there any solution?

Thanks!

Answer 1

If you know the measures of your grid, you can put “sentinel cells” around it.

If you add a -1st index to an array x, it does not count to x.length. Putting an additional last element into the list would increment x.length.

I daresay using sentinel cells combined with the arithmetic counting algorithms mentioned by d_inevitable would be the fastest solution, since it would not involve branches. You even can omit the !! because true will evaluate to 1 and false to 0 in an equalization.

Update:

Do not use index -1. Its an awful lot slower that normal array indexes. See http://jsperf.com/index-1.