Iterative equation and difficulties to pick values in a table for 2D FDTD

Hello,

I am trying to program a system of 2D finite differences in time domain. I need to split the program into several parts. The cells on the edges of my table are defined by others equations since they carry boundary conditions.

My problem is the following : 

I use a double "for" loop in order to calculate the value of the temperature in the central cells. However, when I run the program, calculation has been made only for my jmax-1 row, I don't understand why I only have value in that row.

Another problem is that I can't manage to calculate temperature in a specific cell by adding the value of a neighbor cell.

If anyone has an idea of what's happeing, you're are more than welcome.

Thanks in advance