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Optimizing a simple code

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AJ1
AJ1 on 20 Dec 2014
Commented: Roger Stafford on 22 Dec 2014
Hi People
I trying to optimize the following code and get ride of the for loop. What the code dose, is simple. It estimates the next column in a matrix based on the previous column values times a matrix.
clc
clear
close all
dt=0.01;
t=0:dt:10;
sls=zeros(2,numel(t));
M_c=[0 1;0 0];
M_c=M_c+[1 0;0 1]*1/dt;
V_s=[0;1];
for i=1:numel(t)-1
sls(:,i+1)=(M_c*sls(:,i)+V_s)*dt;
end
Any suggestion for a built-in function that would do the for loop part?
  3 Comments
John D'Errico
John D'Errico on 20 Dec 2014
After responding, I see it looks like Geoff and I think alike. Ergo, my condolences are due to Geoff. :)
AJ1
AJ1 on 21 Dec 2014
Yeah the whole idea is to gain speed ups!!!

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Accepted Answer

Roger Stafford
Roger Stafford on 20 Dec 2014
For your particular problem there is a way to avoid the for-loop using the colon operator, but I doubt if it is what you had in mind as a "built-in" function. Just do this:
n = numel(t);
s2 = 0:dt:(n-1)*dt;
sls = [s2.*(-dt:dt:(n-2)*dt);s2];
I tend to agree with John and Geoff that the advantage of avoiding for-loops has been greatly exaggerated among many users of matlab. It is often the very best way to accomplish a given task, and as John states, even if an advantage is gained, it is often not worth the extra programming effort to achieve.
  1 Comment
AJ1
AJ1 on 21 Dec 2014
I accepted this answer but actually this dose not do what the loop did!

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More Answers (1)

John D'Errico
John D'Errico on 20 Dec 2014
People spend too much programmer time worrying about optimizing code that does not need it. In the end, they often get something that looks impressive, but uses as much or more time to run. Or you get something that is fine in theory, but in terms of numerical computer code does something potentially nasty.
A loop is fine there unless you are running the same code millions of times, and as long as you preallocate your sls matrix to the proper size, and you have done the latter.
On my machine, that entire code fragment took 0.0136 seconds to complete. I've spent something like a thousand times that just answering this question. If we added in the time spent by you posting it, and others reading it...
If you have something that works, USE IT. Only if it becomes a problem should you then look for bottlenecks to optimize. Remember that computer time is cheap. Programmer time is not. And, well, my time is free, worth every cent you paid for it. :)
set('soapBoxMode','down')
  6 Comments
Image Analyst
Image Analyst on 22 Dec 2014
Edited: Image Analyst on 22 Dec 2014
AJ1's on my black list too. Even if he does apologize for saying "Ooos I wish people without solutions just stopped answering questions! Bro you simply don't know [profanity deleted] about programming!" to John, I won't answer. And of course there was at least one other prior comment with another name calling which has since been deleted.
Roger Stafford
Roger Stafford on 22 Dec 2014
This was the original insult:
Dear John! When asking for help it is important to pose the most simple problem possible, it helps every one!. It is just unfortunate when an smart ass comes with advices rather than a solution!!!
That one needs an apology too.

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