Hi,
The first line replaces a subset of Ubc, with indices being replaced (2:41, 2:41), where matrix U is of the same size. You can make a nested for loop to do this with counters going from 1 to 40 in C because arrays start from 0 in C.
Ubc(2:Nx+1,2:Ny+1) = U; % Copy U into Ubc
These lines are replacing rows or columns in Ubc,
Ubc( 1,2:Ny+1) = U(Nx, :); % Periodic bc
Ubc(Nx+2,2:Ny+1) = U( 1, :); % Periodic bc
Ubc(2:Nx+1, 1) = U( :,Ny); % Periodic bc
Ubc(2:Nx+1,Ny+2) = U( :, 1); % Periodic bc
Indexing in MATLAB is simple, when you have colon it means take all elements in that row or column, depending where the colon is in the code
U(a,b) % a - row, b - column
U(Nx, :) % Take a subset of U(Nxth row, all elements in that row)
MATLAB part for first line, you can do others similarly with only one for loop
for ii = 1 : Nx
for jj = 1 : Ny
Ubc(ii+1, jj+1) = U(ii, jj);
end
end
