Creating a matrix of equations for counter phased flow - matlab

Question

I am modelling flow through a tube in tube heat exchanger using MATLAB using the nodal port method. I need to populate a matrix with differential equations in order to solve for the enthalpies at each point. I have divided my pipe into multiple sections, each with 3 nodes. 1 node for the inner fluid, 1 node for the outer fluid, and 1 node for the pipe. However, due to the fact that there is counter flow I need to populate the matrix in a way that the outer fluid counts in reverse to the inner fluid. For example, if I have 9 nodes, my column vector will be [Eq 1 Eq 2 Eq 9 Eq 4 Eq 5 Eq6 Eq 7 Eq 8 Eq 3]. My code seems to work, but it doesn't enter anything for the 3rd position of the vector. Thank you in advance for the help.
NXP = 5; %Number of Divisions
HX = zeros(NXP,1);
cntr=2; %To see which number equation is being input
for j=1:NXP
if j==1
HX(1+(j-1)*3,1) = 125; %Boundary Condition
else
HX(1+(j-1)*3,1) = cntr;
cntr = cntr+1;
end
HX(2+(j-1)*3,1) = cntr;
cntr = cntr+1;
if j==NXP
HX(3+(j-1)*3,1) = 40; %Boundary Condition
else
HX(3*NXP-3*(j-1),1) = cntr;
cntr = cntr+1;
end
end
'''

Answer 1

I changed a bit your attempt, most notably moving the boundary conditions outside of the loop, but it now returns what I guess you were looking for:
NXP = 5;
HX = zeros(3*NXP, 1);
for j = 1:NXP
idx = (j - 1)*3;
HX(idx + 1, 1) = idx + 1;
HX(idx + 2, 1) = idx + 2;
HX(length(HX) - idx, 1) = idx + 3;
end
% Boundary conditions
HX(1) = 125;
HX(end) = 40;
EDIT:
Even better, just initialize the array as
HX = (1:3*NXP)';
and then just change the order for your 3rd nodes
HX(3:3:length(HX)) = flipud(HX(3:3:length(HX)));
and finally set your boundary conditions
HX(1) = 125;
HX(end) = 40;