MATLAB Central Newsreader - Faster symbolic computations

Faster symbolic computations

Thiago — Tue, 03 Feb 2009 05:57:01 -0500

Folks,

I have a couple of general questions. I'm performing symbolic computations in MATLAB, and I noticed I can avoid memory errors by manually executing blocks of code at a time.

1) Is there a way to orchestrate the execution of my code blocks so that I don't run into those memory errors, while keeping the operation in "auto mode"?

2) Do you have a few other tips to speed up symbolical computations? Maybe the I should do this: digits( n ) with n = 6 at most.

Thanks!

Re: Faster symbolic computations

Alan Weiss — Tue, 03 Feb 2009 13:38:32 -0500

Hi, I am confused about what you are asking. If the value of "digits"
affects your results, then I suppose you are actually doing numerical
computations, not symbolic. In that case, I recommend you employ
matlabFunction to turn your symbolic expressions into function handles
or M-files that will evaluate quickly. For more information, see
http://www.mathworks.com/access/helpdesk/help/toolbox/symbolic/brt7nuv.html#brt7nyh

Alan Weiss
MATLAB mathematical toolbox documentation

Thiago wrote:
> Folks,
>
> I have a couple of general questions. I'm performing symbolic computations in MATLAB, and I noticed I can avoid memory errors by manually executing blocks of code at a time.
>
> 1) Is there a way to orchestrate the execution of my code blocks so that I don't run into those memory errors, while keeping the operation in "auto mode"?
>
> 2) Do you have a few other tips to speed up symbolical computations? Maybe the I should do this: digits( n ) with n = 6 at most.
>
> Thanks!

Re: Faster symbolic computations

Thiago — Tue, 03 Feb 2009 16:43:02 -0500

Hi Alan,

Thanks for your response. We'll I'm actually using a mix of numerical inputs with symbolic values. When those meet in my code, the output is symbolic. That's exactly what I wish, namely, a symbolic function having a few variables I specify. It would be easier if I knew the ultimate expression beforehand, because I would be able to simply numerically evaluate it. Because of the complexity and generality in my routine, that's not the case. An alternative would be to have absolutely everything as symbolic, and then evaluate only the variables I want in the very end. That sounds like a better idea; I'm going to try it.

I did take a look at the link you suggested, but it seems that matlabFunction is only useful for the case of numerical evaluation of all variables. In my case, I want to keep a select few as symbolic.

The reason why I suggested changing the value of digits is that I imagined my code would be processed faster if the huge fractional expressions, which crop up in symbolic mode, can be replaced by an approximate "symbolic floating-point" representation having less significant digits. That's all.

Regards,

Thiago

Re: Faster symbolic computations

Thiago — Tue, 03 Feb 2009 16:53:01 -0500

By the way, I'm sorry for the typo. I meant to say Well, not We'll. You'll probably understand it anyway.

-Thiago