Reducing the number of for loops

How can I reduce the number of for loops in the code below:

 l = find(lamda >= fmin & lamda <= fmax); 
    lam = lamda(l);
    n = length(lam);
    C = zeros(n,n,n,n);
    for i = 1:n
        for ii = 1:n
            for iii = 1:n
                for iv = 1:n
                A = [data(l(i),1) data(l(i),2) data(l(i),3);data(l(ii),1) data(l(ii),2) data(l(ii),3); data(l(iii),1) data(l(iii),2) data(l(iii),3); data(l(iv),1) data(l(iv),2) data(l(iv),3)];
                  C(i,ii,iii,iv) = cond(A);
                end
            end
        end
        C(i,i,i,i) = mean2(C);
        i
    end

Answer 1

[Walter Roberson]

    A = data(l([i ii iii iv]), 1:3);

Other than that minor change, I think the best you will be able to do is hide a couple of loops using arrayfun() or pagefun(), and doing so will not necessarily improve performance. If you have the Parallel Processing Toolkit you might be able to work with a gpu array, possibly. And certainly you could run different combinations of the values in parallel.

[Muhannad]

Is it possible to reduce the number of for loops used in this code? as I think this will dramatically increase the speed.

Also can you give me an example for arrayfun() or pagefun() related to my code?

[Walter Roberson]

arrayfun() is internally implemented with loops, and requires function calls, so it can be slower than plain loops.

l = find(lamda >= fmin & lamda <= fmax); 
lam = lamda(l);
n = length(lam);
C = zeros(n,n,n,n);
for i = 1:n
  A1 = data(l(i),:);
  for ii = 1:n
    A2 = [A1; data(l(ii),:)];
    for iii = 1:n
      A3 = [A2; data(l(iii),:)];
      C(i,ii,iii,:) = arrayfun(@(iv) cond([A3;data(l(iv),:)]), 1:n);
    end
  end
  C(i,i,i,i) = mean2(C);
end

Your mean2(C) looks wrong. C has not been fully initialized yet because it has not had anything for large i put in yet. Are you counting on the fact that those values will be 0, and yet you are including the number of those 0s in the mean2 calculation? And do you really want the value to depend upon the order you fill C?