What's the cost of overwriting a variable versus clearing it every time it changes?
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Alonso Trejo-Mora
on 24 Jul 2017
Edited: per isakson
on 25 Jul 2017
As someone who wants to optimize the performance of his code, I want to know if it is more efficient to clear a variable each time I know it will be overwritten (e.g. a "temp" in a loop, or if the name is reused later) OR if the act of telling Matlab to instantiate a variable of the same name every time I will overwrite actually hurts more than it hurts.
I can't reason whether giving Matlab the "freedom" of choosing memory for each instantiation is good or if it is better to overwrite data to a designation of memory that already exists.
P.S. This would be using the "clear" command if that makes any difference.
Thanks!
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Accepted Answer
per isakson
on 24 Jul 2017
Edited: per isakson
on 25 Jul 2017
Comments
- don't speculate; make a test
- make it a habit to use the function, profile
- see premature optimization
and the answer is that clear hurts badly (in the simple case below)
N = 1e6;
tic
val = 17;
for jj = 1 : N
val = jj;
end
toc
tic
for jj = 1 : N
clear val
val = jj;
end
toc
returns
Elapsed time is 0.005792 seconds.
Elapsed time is 17.567347 seconds.
using R2016a (in response to a comment)
2 Comments
Walter Roberson
on 24 Jul 2017
The "clear" appears to be less expensive in newer versions relative to whatever release dpb is using, but he is right that it is much more expensive.
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