April 2015 - GiNaC-list

compile_ex with more than two symbols in the expression
by William Silva 01 May '15

01 May '15

Dear all, Is there a way to use "compile_ex" with more than 2 free variables? I am generating several expressions using .diff() and would like to compile them into C functions and attach function pointers. Using the .subs() method is too slow for my code. Unfortunately my expressions contain more than just two symbolic variables. Ideally I would love to have a FUNCP_nP typedef that can accept a list or array of n symbolic variables and use a compile_ex function that accepts an lst of the symbols. I noticed that the FUNCP_CUBA typedef sort of does this, but I am not sure how to access the compiled function. Any help or pointers would be greatly appreciated! Best, Will S.

2 2

Re: [GiNaC-list] [GiNaC-devel] Symbolic Integration of exponential functions
by Matthias Reis 27 Apr '15

27 Apr '15

Dear Matthias, thanks a lot for your work. However, I have implemented a numeric workaround that does not need symbolic integration a few weeks ago. Since this approach works and I am running out of time with my PhD, I will unfortunately not be able to look further into ginac. At some point in the future maybe. Best regards and thanks again, Matthias Am Mon, 27 Apr 2015 16:27:43 +0200 schrieb Matthias Dellweg <dellweg(a)tp1.uni-duesseldorf.de>: > Am Mon, 27 Apr 2015 11:37:39 +0100 > schrieb "Vladimir V. Kisil" <kisilv(a)maths.leeds.ac.uk>: > > > Dear All, > > > > What is easy to do is to add to function class a primitive > > feature analogously to their knowledge of derivatives. Then, > > integral::eval_integ() may call such methods to evaluate simple > > integrals of plain functions. > > > > If this looks reasonable to be included in GiNaC, then I may write > > such a patch. > > > > Best wishes, > > Vladimir > > >>>>> On Sat, 14 Mar 2015 17:35:10 +0100, Matthias Reis > > >>>>> <matthias.reis(a)biologie.hu-berlin.de> said: > > > > MR> Dear all, as far as I understand, ginac cannot integrate > > MR> anything other than polynomials. How hard would it be (for > > MR> me maybe) to implement exponentials? What needs to be done? > > MR> This feature would be great for me. I just need to > > MR> integrate simple e^{x * constant} terms. > > > > MR> Best regards, Matthias > > Dear Matthias, > i have an implementation ready that integrates expressions like > step(x-a)*x^5*exp(b*x). > You can find it at https://bitbucket.org/mdellweg/ginac.git . > It just can not be done in the way described above, since it needs to > know stuff outside of exp(...). > I would love to hear comments on this. > Kindest regards, Matthias

1 0

Symbolic Integration of exponential functions
by Matthias Reis 27 Apr '15

27 Apr '15

Dear all, as far as I understand, ginac cannot integrate anything other than polynomials. How hard would it be (for me maybe) to implement exponentials? What needs to be done? This feature would be great for me. I just need to integrate simple e^{x * constant} terms. Best regards, Matthias

2 1

Re: [GiNaC-list] Piecewise function
by Vincent Huber 22 Apr '15

22 Apr '15

Dear Dr. Kisil Thank you for your answer I'll carefully read. Pr. C. Prud'homme has hacked our GiNaC fork to let people use this kind of expression. https://github.com/feelpp/feelpp/commit/547c08b66e5b6019ab258477ab19a329cdf… Now, the one that parse the string "(x<0.5)*1000+(x>0.5)*10:x:y" will find in the source file: f[0] = 10.0*(a[0]>5.0000000000000000e-01)+1000.0*(a[0]<5.0000000000000000e-01); Thank again for your quick answer ! All the best, VH 2015-04-22 18:50 GMT+02:00 Vladimir V. Kisil <kisilv(a)maths.leeds.ac.uk>: > Dear Vincent, > > You are right that GiNaC expression do not admit boolean > terms directly. However with your example you can proceed as follow (I > do understand that your actual target is much more complicated): > > 1. Use the step function from GiNaC: > > f=2*x*step(x)+3*x*step(-x). > > [but for x=0 you need take care as step(0)=.5] > > Of course, if you have a hundred points where the function changes the > expression, this becomes much more complicated > > 2. Define your own function of three variables S(a,b,x) such that > S(a,b,x)=1 for a<x<b and zero otherwise. GiNaC tutorial > contains a demonstration how make a user-defined function. Then, any > piece-wise function F will be a sum of terms f_i(x)*S(a_i,_b_i,x) > where f_i is the expression for F on [a_i,b_i] (assuming disjoint > partition. > > 3. Finally, you may derive your own class from GiNaC::basic to hold > your piece-wise function (again, GiNaC tutorial gives an example). As > I see it shall have two lists: one with points of the partition, > another with the respective expression. Then you add your custom > methods to operate this class. > > > The approaches are given in the order of increasing complexity, but > the efficiently shall grow in the same direction. > > Best wishes, > Vladimir > -- > Vladimir V. Kisil http://www.maths.leeds.ac.uk/~kisilv/ > Book: Geometry of Mobius Transformations http://goo.gl/EaG2Vu > Software: Geometry of cycles http://moebinv.sourceforge.net/ > >>>>> On Wed, 22 Apr 2015 16:47:13 +0200, Vincent Huber < > vincent.huber(a)cemosis.fr> said: > > VH> Hello Dr. Kisil, > > VH> The question from C. Trophime is not about evaluating the > VH> function but to define it. Feel++ <http://www.feelpp.org> is a > VH> Finite Element library that main goal is to solve the > VH> variational problem a(u,v)=f(v) that arises from a lot of > VH> physical problems. > > VH> We use GiNaC to dynamically define the right hand side of our > VH> problem, or our boundaries conditions and whatever can be > VH> expressed with it. > > VH> The question - I guess - can be explained like that : I would > VH> like to define > > VH> - > > VH> f(x) = 2*x if x>0 - > > VH> f(x) = 3*x if x<=0. > > VH> Thus, basically, I would like to write > > > VH> ex MyEx = (x<=0)?3*x:2*x > > VH> To my best knowledge, boolean expression are not handled in > VH> GiNaC, isn’t it ? > > VH> all the best, > > VH> Vincent H. -- Docteur Ingénieur de recherche CeMoSiS > VH> <http://www.cemosis.fr> - vincent.huber(a)cemosis.fr Tel: +33 (0)3 > VH> 68 8*5 02 06* IRMA - 7, rue René Descartes 67 000 Strasbourg > > VH> ---------------------------------------------------- > VH> Alternatives: > > VH> ---------------------------------------------------- > VH> _______________________________________________ GiNaC-list > VH> mailing list GiNaC-list(a)ginac.de > VH> https://www.cebix.net/mailman/listinfo/ginac-list > -- Docteur Ingénieur de recherche CeMoSiS <http://www.cemosis.fr> - vincent.huber(a)cemosis.fr Tel: +33 (0)3 68 8*5 02 06* IRMA - 7, rue René Descartes 67 000 Strasbourg

1 0

Piecewise function
by Vincent Huber 22 Apr '15

22 Apr '15

Hello Dr. Kisil, The question from C. Trophime is not about evaluating the function but to define it. Feel++ <http://www.feelpp.org> is a Finite Element library that main goal is to solve the variational problem a(u,v)=f(v) that arises from a lot of physical problems. We use GiNaC to dynamically define the right hand side of our problem, or our boundaries conditions and whatever can be expressed with it. The question - I guess - can be explained like that : I would like to define - f(x) = 2*x if x>0 - f(x) = 3*x if x<=0. Thus, basically, I would like to write ex MyEx = (x<=0)?3*x:2*x To my best knowledge, boolean expression are not handled in GiNaC, isn’t it ? all the best, Vincent H. -- Docteur Ingénieur de recherche CeMoSiS <http://www.cemosis.fr> - vincent.huber(a)cemosis.fr Tel: +33 (0)3 68 8*5 02 06* IRMA - 7, rue René Descartes 67 000 Strasbourg

2 1