Exclude calculation in a part of array

I have created 300 by 300 array and I am interested to perform calculation for the space as shown in attached image. How can I exclude some part of array from main calculation?

[Adam]

Assuming you have the area define by a logical mask then just apply the mask to your data. It will collapse it to 1d, but that is fine if it is a per-point calculation. You can then put the results back into the original shape after.