Error: Unable to classify a variable in the body of the parfor - loop

Hello everyone,

I have the following code:

orden = perms(1:n)
for i = 1:factorial(n)
    for j = 1:n
        pos(i,orden(i,j)) = j;
    end
end

As it is, the code works fine but I want to change the first for loop to "Parfor", I get the following error:

"Error: Unable to classify the variable 'pos' in the body of the parfor-loop. For more information, see Parallel for Loops in MATLAB, "Solve Variable Classification Issues in parfor-Loops"."

I think it must be because "orden(i,j)" is used as an index of "pos" and "i" and "j" are indexes of "pos". Any help would be appreciated.

Answer 1

[Edric Ellis]

This doesn't work as written because the indexing form into pos doesn't meet the requirements for a "sliced" variable. The simplest way to fix this is to create a temporary vector in each iteration of the parfor loop, fill it using a for loop, and then assign a whole "slice" of pos. Like this:

n = 3;
orden = perms(1:n);
parfor i = 1:factorial(n)
    % Make a temporary vector
    tmp = zeros(1, n);
    for j = 1:n
        % Fill the temporary vector
        tmp(orden(i,j)) = j;
    end
    % Assign a whole "slice" of pos
    pos(i, :) = tmp;
end
disp(pos)

     3     2     1
     2     3     1
     3     1     2
     2     1     3
     1     3     2
     1     2     3