MATLAB: Creating a temporary variable

memory sparse

This is a quick question. Compare

A = sparse(…); B = A – C;

and

B = sparse(…) – C;

Memory-wise spoken, is there a difference? Will the second one use less memory because I'm not creating a variable?

Best Answer

There will be very little memory difference. For a transient value, MATLAB creates everything about the storage except the symbol table entry with the name and a pointer to the descriptor.

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MATLAB: Matrix multiplication result – where does it go

Interesting question. The answer will probably depend on MATLAB version and JIT settings. TMW likely keeps stuffing more & more optimization like this into their JIT code with each version release. (E.g., the in-place operations that can sometimes take place for certain function call syntaxes.) Probably the only way to get any insight into it would be to do some timing and/or memory tests using very large arrays.

(That reminds me ... I need to finish testing & upload the new version of MTIMESX which allows in-place operations)

MATLAB: Are the times took for consequent tocs not consistent

I would personally use multiple tic tocs instead of tic then multiple tocks. tic tocs are meant to be used as a pair, that's the primay function. Although your usage is supported, it's probably be so by logistics in an obscure way, (e.g. 1st toc release resources and 2nd toc need to dig the last used tic ID from somewhere.) If one uses multiple tocs for less critical timings (several milliseconds) it will be fine. In your case you are measurement is back to front. In addition, the documentation says it is much more accuracte to loop over multiple calculations between the tic and the toc if the calculations take too short of a time.

If you modify your code to include the tic IDs the picture changes, supporting my 'logistics' theory. Now the 1st toc uses slightly more time than the second, than the 3rd, and similar from then on. 2nd still uses more than the 3rd probably as it still have to check through why am I being called a 2nd time and set stage for the 3rd and so on.

for i = 1:k
    ID = tic;
    tocs(i,1) = toc(ID); 
    tocs(i,2) = toc(ID); % the second toc!
    tocs(i,3) = toc(ID); 
    tocs(i,4) = toc(ID);
    tocs(i,5) = toc(ID);
    tocs(i,6) = toc(ID);
    pause(0.001); % this also affects the results.
end

If you run paired tic tocs:

for i = 1:k
    pause(0.001); 
    for j=1:6
    tic;
    tocs(i,j) = toc; 
    end
end
elapsed = tocs;

Then, only the first toc take longer than the rest, which is forgiveable.

In anycase, you would not use tic toc for measuring very fast timings like this. If you need to do so, use:

tic
for i=1:1000
%some fast code%
end
time = toc;
time = time/1000;

Best Answer

Related Solutions

MATLAB: Matrix multiplication result – where does it go

MATLAB: Are the times took for consequent tocs not consistent

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