# Matlab Array indexing and slicing

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Evan McGrane on 28 Feb 2021
Commented: Evan McGrane on 28 Feb 2021
Im writing currently rewriting a Matlab script in C. When i get to the last few lines of the Matlab script a for loop is executed and it iterates through an array. Since i am trying to rewrite the program in C the slicing notation in the Matlab script is confusing me. I have attached the line of code that is troubling me below.
How would i write this line of code in a nested for loop indexing with i and j only, since you cant slice in c obviously. just for reference U and Ubc are 2D arrays of size (NX+2, NY+2). Where NX = NY = 40.
Below is the line of code in Matlab i need to translate to for loop indexing. i want this code translated to matlab for loope style of indexing. I can do the rest in C myself.
% The following implement the bc's by creating a larger array
% for U and putting the appropriate values in the first and last
% columns or rows to set the correct bc's
Ubc(2:Nx+1,2:Ny+1) = U; % Copy U into 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

Mario Malic on 28 Feb 2021
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
Evan McGrane on 28 Feb 2021
thatnk you so much. Very helpful!!