Accessing data in neighbor cells - c

Question

I have a matrix M of size m x n which is saved as a one dimensional array N of length m * n. Every cell of this array contains some integer variables which are the ID of data points. The amount of integer variables in every cell changes over time.
N[0] = {1,4,5,7}
N[1] = {2,9,3,1,7,4}
N[2] = {7,1,3,9,8}
N[3] = {6,4,2}
...
I access these elements by using an index function which returns
idx = x + y * n
Given some index idx I want to use all integer variables of the neighbor cells and the central cell with index idx to access an array of data points D of size s. Size s can be very large.
To make my point clear: Instead of such a loop over all data points
for(int i=0; i<s; i++)
// Do something with D[i]
I want something like (but more compact)
// Access central cell
idx = x + y*n;
num_Elements = Number_of_Elements_Cell(x,y);
for(int i=0; i<num_Elements; i++)
// Do something with D[N[idx][i]]
// Access right cell
idx = (x+1) + y*n;
num_Elements = Number_of_Elements_Cell(x+1,y);
for(int i=0; i<num_Elements; i++)
// Do something with D[N[idx][i]]
// Access left cell
idx = (x-1) + y*n;
num_Elements = Number_of_Elements_Cell(x-1,y);
for(int i=0; i<num_Elements; i++)
// Do something with D[N[idx][i]]
and so on. For all cells I have to do that 9 times.
My question: Is there a better way to do that given the structure N?

Answer 1

I'm not sure i understand your question well ....
but you could try :
for (int i=-1 ; i <= 1 ; i++){
for (int j = -1 ; j <=1 ; j++){
idx = (x+i) + (y+j)*n;
// Check if idx is not out of bounds
num_Elements = Number_of_Elements_Cell(x+i,y+j);
for(int k=0; k<num_Elements; k++)
// Do something with D[N[idx][k]]
}
}
Note that your index could very well be out-of-bounds which such a method, so you'll have to implement a test to prevent that.
That's the way to simply iterate on a cell and its 8 neighbors using a double for loop.
If it's not what you expect, let me know, i'll edit/delete.

Answer 2

I'm not sure but maybe you're looking for something like this:
var distinctDataPoints = new List<int>();
for(int z = x - 1, z <= x + 1, z++)
{
if(z < 0 || z > m)
continue;
for(int t = y-1, t <= y + 1, t++)
{
if(t < 0 || t > n)
continue;
idx = z + t * n;
for(int i = 0; i < num_Elements; i++)
{
if(!distinctDataPoints.Contains(N[idx][i])
distinctDataPoints.Add(N[idx][i])
}
}
}
for(int dpIdx = 0; dpIdx < distinctDataPoints.Count; dpIdx++)
{
//Do something with D[dpIdx]
}