How can I make this code more efficient?

Hi

This piece of code is so slow due to the big number of nx1 and nye. Could you please tell me how I can make it quicker?

Thanks

nx1=1216;
nye=160;
dx=1.973684;
dy=1.875;
 nm=0;  
for ip=1:nx1
    for iq=1:nye
        nm=nm+1;
   coord(1, 1) = (ip - 1) * dx;
        coord(2, 1) = (ip - 1) * dx;
        coord(3, 1) = (ip - 1) * dx;
        coord(5, 1) = ip * dx;
        coord(6, 1) = ip * dx;
        coord(7, 1) = ip * dx;
        coord(4, 1) = (coord(3, 1) + coord(5, 1)) * 0.5;
        coord(8, 1) = coord(4, 1);
        coord(3, 2) = -(iq - 1) * dy;
        coord(4, 2) = -(iq - 1) * dy;
        coord(5, 2) = -(iq - 1) * dy;
        coord(1, 2) = -iq * dy;
        coord(8, 2) = -iq * dy;
        coord(7, 2) = -iq * dy;
        coord(2, 2) = (coord(1, 2) + coord(3, 2)) * 0.5;
        coord(6, 2) = coord(2, 2);
          for i=1:8
              g_coordx(nm, i) = coord(i, 1);
              g_coordy(nm, i) = coord(i, 2);
          end
    end
end

[Stephen23]

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

https://www.mathworks.com/help/matlab/matlab_prog/techniques-for-improving-performance.html

Answer 1

[KSSV]

Read abput preallocating/ initializing the variables.

nx1=1216;
nye=160;
dx=1.973684;
dy=1.875;
nm=0;
t1 = tic ;
% See this
g_coordx = zeros(nx1*nye,8) ;
g_coordy = zeros(nx1*nye,8) ;
%
for ip=1:nx1
    fprintf('% d of %d\n',ip,nx1) ; 
    for iq=1:nye
        nm=nm+1;
        coord(1, 1) = (ip - 1) * dx;
        coord(2, 1) = (ip - 1) * dx;
        coord(3, 1) = (ip - 1) * dx;
        coord(5, 1) = ip * dx;
        coord(6, 1) = ip * dx;
        coord(7, 1) = ip * dx;
        coord(4, 1) = (coord(3, 1) + coord(5, 1)) * 0.5;
        coord(8, 1) = coord(4, 1);
        coord(3, 2) = -(iq - 1) * dy;
        coord(4, 2) = -(iq - 1) * dy;
        coord(5, 2) = -(iq - 1) * dy;
        coord(1, 2) = -iq * dy;
        coord(8, 2) = -iq * dy;
        coord(7, 2) = -iq * dy;
        coord(2, 2) = (coord(1, 2) + coord(3, 2)) * 0.5;
        coord(6, 2) = coord(2, 2);
        for i=1:8
            g_coordx(nm, i) = coord(i, 1);
            g_coordy(nm, i) = coord(i, 2);
        end
    end
end
t1 = toc(t1) ;

Answer 2

[Swastik Sarkar]

• Assuming you have not preallocated the variables g_coordx & g_coordy, you can consider preallocating them to improve time and performance of the code. Read about it here
• The first part of the loops only depend on variable ip, so you can write it out of the loop

So the final code is:

nx1=1216;
nye=160;
dx=1.973684;
dy=1.875;
nm=0; 
g_coordx = zeros(nx1*nye, 8);
g_coordy = zeros(nx1*nye, 8);

coord = zeros(8, 2);
for ip = 1:nx1
    coord(1, 1) = (ip - 1) * dx;
    coord(2, 1) = (ip - 1) * dx;
    coord(3, 1) = (ip - 1) * dx;
    coord(5, 1) = ip * dx;
    coord(6, 1) = ip * dx;
    coord(7, 1) = ip * dx;
    coord(4, 1) = (coord(3, 1) + coord(5, 1)) * 0.5;
    coord(8, 1) = coord(4, 1);
    
    for iq = 1:nye
        nm = (ip - 1) * nye + iq; % In case you need nm later when you modify it
        coord(3, 2) = -(iq - 1) * dy;
        coord(4, 2) = -(iq - 1) * dy;
        coord(5, 2) = -(iq - 1) * dy;
        coord(1, 2) = -iq * dy;
        coord(8, 2) = -iq * dy;
        coord(7, 2) = -iq * dy;
        coord(2, 2) = (coord(1, 2) + coord(3, 2)) * 0.5;
        coord(6, 2) = coord(2, 2);
        
        g_coordx(nm, :) = coord(:, 1)';
        g_coordy(nm, :) = coord(:, 2)';
    end
end

You can also go through Techniques to improve performance