Struct variable save in parfor loop
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seung ho yeom
am 1 Feb. 2019
Kommentiert: seung ho yeom
am 3 Feb. 2019
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?
3 Kommentare
Stephen23
am 1 Feb. 2019
Bearbeitet: Stephen23
am 1 Feb. 2019
"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?:
Do you understand the overhead involved in running parallel code?
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Stephen23
am 1 Feb. 2019
Bearbeitet: Stephen23
am 1 Feb. 2019
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
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