The way I'd do it if I didn't want to use ifs is to loop n times, and for each row fill the first cell and the last one. So now I have the left and right side a

The way I'd do it if I didn't want to use ifs is to loop n times, and for each row fill the first cell and the last one. So now I have the left and right side at O(n). Then i'd loop n-1 times starting at the second column and for each column fill the first and last cell again. O(n) all in all without repeating yourself, and no tests. Another way to look at this problem altogether would be to change the data structure, so instead of using a classical 2d array the problem becomes trivial if you use a spiral array. Filling the edge in this case is just a matter of filling the first 4*(n-1) cells. Obviously this solution's acceptability depends entirely on what you're gonna do with your data afterwards.