A way to optimize this piece of code..

I have a switch..case inside a nested 'for-loop'. This makes the program very slow, as the for-loops themselves run for a number of iterations. Can someone suggest me a way to optimize this piece of code?

for iter = 1 : 10
  for i = 1 : 10
            for j = 1 : 5
                if (H(j,i) == 1)
                    tempr = [tempr, newbitvalues(j,i)];
                end
            end
            nZeros = nnz(~tempr);
            nOnes = nnz(tempr);
            switch recdmsg(i)
                case 1
                    if nZeros > nOnes
                        recdmsg(i) = ~recdmsg(i);
                    end
                case 0
                    if nOnes > nZeros
                        recdmsg(i) = ~recdmsg(i);
                    end
            end
        tempr = [];    
   end
end

[Cedric]

It would be easier if you explained what you are trying to achieve and provided test data/arrays. I assume that the outer loop is for multiplying the run time by 10 for measuring the performance, but in the rest of the code many operations make little sense (to me).

[Vijay]

Well, H is 5 by 10 logical matrix. 'newbitvalues' is again a 5 by 10 logical matrix. 'recdmsg' is a 1 by 10 logical vector.

Answer 1

[OCDER]

First, let's look at the original code and see how to improve that. Then I'll show the vectorized code.

 %Making up some variables
 % recdmsg0 = rand(1, 10) > 0.5;
 newbitvalues = 10*randn(10, 10);
 newbitvalues(newbitvalues < 0) = 0;
 H = newbitvalues > 5;
 recdmsg0 = [1 1 1 0 0 0 1 1 1 1]; 

 recdmsg = recdmsg0;
 for iter = 1 : 10
      for i = 1 : 10
          %Avoid this loop - growing a matrix is very slow since it's copying and deleting of matrices.
          %for j = 1 : 5
          %    if (H(j,i) == 1)
          %        tempr = [tempr, newbitvalues(j,i)];
          %    end
          %end
          tempr = newbitvalues(H(:, i), i); %Use this instead, assuming H is a 2D logical array 

          nOnes  = nnz(tempr);
          nZeros = numel(tempr) - nOnes; % to prevent using nnz again for simple math. nnz(~tempr);     

          %switch recdmsg(i) %Looks like recdmsg is a logical array of 1, 0, and no other values. 
          %  case 1
          %    if nZeros > nOnes
          %        recdmsg(i) = ~recdmsg(i);  %This is the same operation as the elseif operation
          %    end
          %  case 0 
          %    if nOnes > nZeros
          %        recdmsg(i) = ~recdmsg(i);
          %    end
          %end
          if (recdmsg(i)  && nZeros > nOnes) || (~recdmsg(i) && nZeros < nOnes)  
                recdmsg(i) = ~recdmsg(i);
          end

          %tempr = []; No need for deletion of variable if you're going to override it completely    
      end
 end

However, the loop inside "for i = 1:10 ... end" seems to just operate on all column, which can be avoided by:

 recdmsg = recdmsg0;
 for iter = 1 : 10
      nZeros = sum((newbitvalues.*H + ~H) == 0, 1);
      nOnes  = sum(H, 1) - nZeros;
      condit =  (recdmsg & nZeros > nOnes) | (~recdmsg & nZeros < nOnes);
      recdmsg(condit) = ~recdmsg(condit);
 end

So why did I do "(newbitvalues.*H + ~H) == 0" ? It's just a workaround.

• H seems to be a logical array storing the index in newbitvalues for extracting certain elements of newbitvalues.
• However, accessing newbitvalues(H) does not work, as you lose the column location, and need to do extra steps to preserve that info.
• newbitvalues.*H will return a matrix similar to newbitvalues, but with H == 0 area set to 0 too.
• adding ~H ensures that 0 values in this matrix are ~0 so they don't get counted as zeros

[Vijay]

Sorry, for the late reply. But when I use this command

tempr = newbitvalues(LDPC.H(:, i), i);

MATLAB throws me an error

'Subscript indices must either be real positive integers or logicals.'

[Vijay]

While the vectorized version is undoubtedly more efficient and faster, it combines three steps of the algorithm into one single line of code. That will raise a lot of questions from my advisor. Thanks for the help anyway.

[Walter Roberson]

Try

tempr = newbitvalues(logical(LDPC.H(:, i)), i);

[OCDER]

Hm, interesting that your advisor would question an "undoubtedly more efficient and faster" and simpler code...