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What's the cost of overwriting a variable versus clearing it every time it changes?

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Alonso Trejo-Mora
Alonso Trejo-Mora 2017-7-24
编辑： per isakson 2017-7-25
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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Stephen23
Stephen23 2017-7-25
编辑：Stephen23 2017-7-25
Why not simply read the documentation, which states clearly: "Avoid clearing more code than necessary. Do not use clear all programmatically. For more information, see clear."
source: https://www.mathworks.com/help/matlab/matlab_prog/techniques-for-improving-performance.html

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per isakson
per isakson 2017-7-24
编辑：per isakson 2017-7-25
Comments
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)

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