Implementing parallel while loops in matlab.

Hi all,

I am writing code that has two while loops in a random sampling exercise. I have two questions.

I would like the while loops to be performed simultaneously as opposed to the nested loops below, which does not yield the desired results. How can one modify the code to perform a parallel while loop/ simultaneously sample each of the while loops? And secondly, how can I pass the DG_uptake and DG2_uptake variables into one variable in the while loop as opposed to obtaining each one of them separately? Thank you.

while (modules_DG>=1)                                       % number of DG
    DG_PoC=randsample(MCS_nodes,1,true)                     % randomly select the position of DG1      
    while modules_DG2>=1                                     % number of DG2
        DG2_PoC=randsample(MCS_nodes_DG2,1,true)              % random selection of DG2 position
        if rem(DG2_PoC,3)==0
            DG2_node_pos=fix(DG2_PoC/3);
            DG2_phase_pos=3;
        else
            DG2_phase_pos=rem(DG2_PoC,3);
            DG2_node_pos=fix(DG2_PoC/3)+1;
        end
        DG2_uptake(DG2_node_pos,DG2_phase_pos) = DG2_uptake(DG2_node_pos,DG2_phase_pos)+ 1

        if DG2_uptake(DG2_node_pos,DG2_phase_pos)> DG2_modules(DG2_node_pos,DG2_phase_pos)
            DG2_uptake(DG2_node_pos,DG2_phase_pos) = DG2_uptake(DG2_node_pos,DG2_phase_pos)- 1
            remove_PoC = MCS_nodes_DG2== (DG2_node_pos - 1)*3 + DG2_phase_pos; %determine the full PoC
            MCS_nodes_DG2(remove_PoC) = [];%remove node from allocation array
        else
            modules_DG2=modules_DG2-1;
        end
    end
    if rem(DG_PoC,3)==0
        DG_node_pos=fix(DG_PoC/3);
        DG_phase_pos=3;
    else
        DG_phase_pos=rem(DG_PoC,3);
        DG_node_pos=fix(DG_PoC/3)+1;
    end
    DG_uptake(DG_node_pos,DG_phase_pos) = DG_uptake(DG_node_pos,DG_phase_pos)+ 1;
    if DG_uptake(DG_node_pos,DG_phase_pos)> DG_modules(DG_node_pos,DG_phase_pos)
        DG_uptake(DG_node_pos,DG_phase_pos) = DG_uptake(DG_node_pos,DG_phase_pos)- 1
        remove_PoC = MCS_nodes== (DG_node_pos - 1)*3 + DG_phase_pos; %determine the full PoC
        MCS_nodes(remove_PoC) = [];%remove node from allocation array
    else
        modules_DG = modules_DG - 1
    end
end

[MarKf]

The variable modules at the end should probably be the one designated as modules_DG. The nested DG2 while-loop does not seem to depend on any variable defined in the main DG one and viceversa. This seems the result of some quick and dirty distracted code duplication/addition/editing, since it seems there are no differences between the 2 loops.

You could then, yes, easily separate (while, end; while, end) or integrate (while,DG(1,:),DG(2,:),end or while,for dgi=1:2,end,end) them. You'd have to put together any variables which have the 2 versions DG and DG2, also outside the loop that we see hjere, if they are matrices or cells that's easy, just add a dimension (e.g DG_node_pos(1,:)= fix(DG_PoC(1,:)/3);) otherwise you could use a struct (e.g DG_variable.(['dg' dgi])).

[Lewis Waswa]

The two loops perform the same function, hence the duplication. The thing is modules_DG and modules DG2 are not of the same length and are different. I would like to know if these two loops can be re-written to be implemented simulatneously. You are also right, I edited the code significantly but I hope one gets the idea of random sampling of two variables by implementing a while loop. @Marco Fuscà, your first suggestion gives me the same results as the above nested code, and therefore not very helpful.

[MarKf]

if the length of modules_DG and modules_DG2, which are used to define the while loops, are different, one would need to know how they differ and which is the logic to create them to ascertain whether you could integrate them or not, sice they are defined outside of the loop and we have no way of knowing how and if this code runs.

It's likely you can only do it separately (if by simultaneously you mean together in the same while loop that is, otherwise you can parfor, that's real simultaneity) that is while, DG, end; while, DG2, end then. Just copy and paste the nested DG2 while loop ouside, otherwise you are repeating that nested one a lot needlessly.

[Lewis Waswa]

Thanks for your input @Marco Fuscà. I appreciate

Answer 1

[sneha]

• Running while loops simultaneously

MATLAB does not execute while loops in parallel by default — they execute sequentially. If you want the two loops to progress together, you can restructure them into a single loop that updates both DG and DG2 conditions in each iteration, or use parallel computing.

Approach A: Merge into a single loop

Instead of:

    while modules_DG >= 1 
        % DG loop 
        while modules_DG2 >= 1 
            % DG2 loop 
        end 
    end 

You can write:

    while (modules_DG >= 1) || (modules_DG2 >= 1) 
        if modules_DG >= 1 
            % Process DG allocation 
        end 
         
        if modules_DG2 >= 1 
            % Process DG2 allocation 
        end 
    end 

This way, both DG and DG2 allocations are handled in one loop iteration without nesting.

Approach B: Use Parallel Computing Toolbox

If you want true parallel execution, you can use parfor or spmd from the Parallel Computing Toolbox.

Example with parfor:

     parfor i = 1:2 
        if i == 1 
            % DG allocation code 
        else 
            % DG2 allocation code 
        end 
    end 

Note: parfor works with for loops, not while loops, so you may need to restructure your logic to a fixed-iteration loop.

• Combining DG_uptake and DG2_uptake

If DG_uptake and DG2_uptake have the same dimensions, you can store them in one 3D array or a cell array.

Option A: 3D Array

     DG_all(:,:,1) = DG_uptake;   % Layer 1 for DG 
     DG_all(:,:,2) = DG2_uptake;  % Layer 2 for DG2 

Access:

    DG_all(node_pos, phase_pos, 1)  % DG 
    DG_all(node_pos, phase_pos, 2)  % DG2 

Option B: Cell Array

    DG_all = {DG_uptake, DG2_uptake}; 

Access:

   DG_all{1}(node_pos, phase_pos)  % DG 
   DG_all{2}(node_pos, phase_pos)  % DG2 

For more information you can reffer to MathWorks documentation:

https://www.mathworks.com/help/matlab/ref/while.html

https://www.mathworks.com/help/matlab/math/array-indexing.html

https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-cell-array.html

https://www.mathworks.com/help/matlab/ref/parfor.html

Similar answers:

https://www.mathworks.com/matlabcentral/answers/274902-is-there-anything-like-parfor-for-while-loops