Question

I’ve got some code that takes repeated local neighbourhoods from a large array. I can set the code to give me 3×3 neighbourhoods, or any other sized neighbourhoods such as 5×5 or 11×11. I then need to check various things about the array, as detailed below. I’ve started writing it with loads of nested if statements, but thought there must be a better way!

   0  1  2  3  4
   --------------
0 |1  2  3  4  5
1 |6  7  8  9  10
2 |11 12 13 14 15
3 |16 17 18 19 20
4 |21 22 23 24 25

Given the array above I want to check if the values in [0, 2] and [1, 2] are less than a threshold, and the values in [3, 2] and [4, 2] are greater than the threshold. I then want to do the same thing with a vertical line through the centre (rather than the horizontal line in the example I gave), and the same for both diagonals.

At the moment I can’t see any way to do this without loads of if statements which will get very confusing and difficult to maintain very quickly. I’m sure there must be a better way to do it – any ideas?

Answer 1

You can use an array of function pointers to specify what needs to be checked for each square.

int a = ...;
int b = ...;
bool bigger_than_a(int x) { return x > a; }
bool smaller_than_b(int x) { return x < b; }

bool (*)(int) check_functions[5][5] = {
    NULL, bigger_than_a, bigger_than_b, NULL, NULL,
    ...
}

int x_start = ...;
int y_start = ...;
for (int x = 0; x < 5; x++) {
  for (int y = 0; y < 5; y++) {
    if (check_functions[x][y] != NULL &&
        !check_functions[x][y](array[x_start + x][y_start + y]) {
           ...check failed...
    }
  }
}

If you need different sizes, just make a different check_functions array for each size.