Struct variable save in parfor loop

I made simple code to test for saving struct variable in parfor loop.

parfor k=1:3
    my_field = sprintf('v%d', k);
    variable.(my_field) = k*2;
end

But, i did not get the results I wanted, and there was only a variable with an empty value in the workspace.

If I use for instead of parfor, I get the result I want perfectly. However, the function(not the simple like above code) actually used takes a long time, so I want to use parfor.

How do I save a struct variable such as variable.v1, variable.v2 in a struct variable using parfor?

[KSSV]

YOu should first try to vectorise and see the possibilites of avoiding loop..won't you think you can achieve your code without loop?

[Stephen23]

"If I use for instead of parfor, I get the result I want perfectly. "

I very much doubt that. Lets try it:

variable=struct();
for k=1:3
    my_field = sprintf('v%d', k);
    variable=struct();  % why do you overwrite this on every iteration?
    variable.(my_field) = k*2;
end

On every loop iteration you completely overwrite variable, discarding anything that was stored in the variable on other loop iterations. Is that really what you mean by "I get the results I want perfectly" ?:

>> fieldnames(variable)
ans = 
    'v3' 

"However, the function(not the simple like above code) actually used takes a long time, so I want to use parfor."

I very much doubt that parfor is a magic silver bullet for your runtime. Have you profiled your code to identify which lines take the most time? Have you followed the guidelines for writing efficient MATLAB code?:

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

Do you understand the overhead involved in running parallel code?

[seung ho yeom]

oops, sorry i change the code.

Answer 1

[Stephen23]

I doubt that parfor wil work happily with dynamic fieldnames, because their order of definition is ambiguous (e.g .consider naming them using the mod of your loop counter: parfor cannot predict in which order the fields should be overwritten, which for a normal for loop is unambiguous).

The best solution is to avoid the awkward, ambiguous, anti-pattern dynamic fieldnames, and just use simpler indexing with a non-scalar structure:

N = 3;
S = struct('val',cell(1,N));
parfor k = 1:N
    S(k).val = k*2;
end

Unlike your "perfect" code, this really does keep all of the data that you calculate:

>> S.val
ans =
     2
ans =
     4
ans =
     6

https://www.mathworks.com/help/matlab/matlab_prog/access-multiple-elements-of-a-nonscalar-struct-array.html

[seung ho yeom]

Now, i use a non scalar strucure as you taught me, it works great! thank you!

Next, i think it is necessary to devise a method of making using vectorization method.