September 2005 - GiNaC-list

Geometric Algebra
by Alan Bromborsky 28 Sep '05

28 Sep '05

I am trying to use the clifford algebra classes to do geometric algebra. To do this I must do a grade decomposition of multivectors. Is there a way to use clifford to list to do this other than the case of a simple vector (not a multivector). In the call: lst clifford_to_lst(const ex & e, const ex & c, bool algebraic = true); what should "c" be to get a complete multivector expansion in terms of the basis blades of the algebra. Letting "c = clifford_unit()" only expands simple vectors. After doing a grade decompostion I need to calculate the inner and outer products of homogenous (pure) grade multivector.

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Maps and sets of symbols?
by John Pye 14 Sep '05

14 Sep '05

Hi again, I've got an application I'm working on which needs the ability to handle STL maps and sets containing symbols. In order to be able to use these classes, I have to define an ordering (a less-than operator) for symbols. At the moment I use the 'get_name' method and then compare symbol names as strings, but this isn't really ideal, since it takes a a lot of processing, plus GiNaC is quite happy to tolerate more than one symbol with the same name, so my map/set should be as well. I notice that GiNaC's symbol class has a 'serial number' as one of its properties. Is there any way I can access that value via a public method? Or is there another method suggested for creating sets and maps of symbols? //// Ordering for symbols based on name (there should be a better way)/ *class* symbol_lt{ *public*: bool *operator*()(const GiNaC::symbol &a, const GiNaC::symbol &b) const; }; Cheers JP

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Getting a pointer to an element in a list?
by John Pye 10 Sep '05

10 Sep '05

Hi all A hopefully pretty simple question about lists in GiNaC. I have a little fragment of code like this: class SymbolicSystem{ private: GiNaC::lst F; public: const ex * getExpressionPtr(const unsigned &i) const{ *return* &(F[i]); } } Is there something about the implementation of the operator[] in GiNaC::list that stops me from being able to do this? GCC gives me a warning, like this: warning: taking address of temporary This might be an elementary C++ mistake, but just in case, I thought I'd ask. Cheers JP

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Symbolic functions with multiple outputs
by John Pye 08 Sep '05

08 Sep '05

Hi again Just reading the GiNaC 1.3.1 manual, in the section about adding new symbolic functions, section 6.2 Is it possible to create symbolic functions that have multiple, matrix, or list outputs? Is there a way to assign the outputs of a multiple-valued symbolic function to elements of a list? lst(a,b,c) = three_valued_function(d,e); For example, a case where one might want to do this sort of thing would be evaluation of fluid properties, like so lst(p,h) = pressure_and_enthalpy_of_water(T, rho); When finally evaluating this numerically, eg once T and rho are known, having this as one symbolic function allows some intrinsic optimisation of calls to the thermodynamic property calculation routines. Thanks for your help, and sorry for the stream of silly questions... Cheers JP -- John Pye School of Mechanical and Manufacturing Engineering The University of New South Wales Sydney NSW 2052 Australia t +61 2 9385 5127 f +61 2 9663 1222 mailto:john.pye_AT_student_DOT_unsw.edu.au http://pye.dyndns.org/

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Referencing array of symbols?
by John Pye 07 Sep '05

07 Sep '05

Hi there I'm struggling to understand how to correctly use the 'idx' functionality in GiNaC. If anyway would be able to offer some suggestions or relevant examples, I'd be very appreciative. My application is for something like a simple one-dimensional pipe flow calculation. For example, I would like to evaluate the pressures in the flow alow a pipe. I will set up some expressions relating the values of my pressures at different nodes in my pipe, then I will substitute values for those pressures and then evaluate residuals from my expressions. So I am hoping to write something like sym p[100] = sym[]("p"); sym mdot[100] = sym[]("mdot"); sym rho("rho"); lst f; for(int i=1; i<100; ++i){ f.append( p[i] - p[i-1] - 0.5 * rho * pow( mdot[i-1] / (rho * A) , 2) ); } for(int i=1; i<100; ++i){ f.subs(p[i] == 1.5454223494); } cerr << f << endl; In this way, I will have an array of symbols and a lst of expressions. I can then use those to generate equations which can be exported to s numerical solver. Perhaps the best way to do this is to create standard C-style array of symbols? I am hoping there's a more elegant way. Also, I'd like to know your thoughts on how efficient this sort of approach might be as the number of equations / symbols grows. Am I right in thinking I shuold be using the 'idx' class? Cheers JP -- John Pye School of Mechanical and Manufacturing Engineering The University of New South Wales Sydney NSW 2052 Australia t +61 2 9385 5127 f +61 2 9663 1222 mailto:john.pye_AT_student_DOT_unsw.edu.au http://pye.dyndns.org/

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GiNaC inside ROOT?
by John Pye 02 Sep '05

02 Sep '05

Hi all Since ginsh is built on top of CINT, it seemed reasonable to hope that GiNaC might able to be run from insite ROOT -- has anybody been able to do this? ROOT is here: http://root.cern.ch/ With the following simple test program, > #include <iostream> > #include <ginac/ginac.h> > > using namespace std; > using namespace GiNaC; > > int main(){ > symbol x("x"); > > ex y = pow(x+1,3); > > ex dydx = y.diff(x); > > cerr << dydx; > } I get an error message from ROOT > Processing roottest.C... > Limitation: Reference member not supported. Please use pointer > FILE:/usr/include/ginac/print.h LINE:101 So maybe I can't use GiNaC inside ROOT? This is with GiNaC 1.3.2. Would appreciate any suggestions. How does gTybalt get around this? Cheers JP

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