Changing symbol string names on-the-fly

[Dr. Vassilis S. Vassiliadis]

On the previous discussion, of changing names & substitutions:

The highest speed with the previous hints provided (below)
the best and most impressive is obtained with the
.assign(...)
method.  I have found it extremely efficient both in terms of
replacing symbols with other symbols, as well as for evaluation
of a large function upon numerical assignments through a temporary
auxiliary expression carrying a numerical value.

Vassilis.

-----------------------------------------------------------------
Dr. Vassilis S. Vassiliadis,
Lecturer,

UNIVERSITY OF CAMBRIDGE,
DEPARTMENT OF CHEMICAL ENGINEERING

Pembroke Street,
Cambridge CB2 3RA, UK.

Tel: (reception): +44 1223 334777
Fax: (departmental): +44 1223 334796

WWW: www.cheng.cam.ac.uk/~vsv20
e-mail: vsv20 at cheng.cam.ac.uk

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----- Original Message -----
From: "Christian Bauer" <Christian.Bauer at Uni-Mainz.DE>
To: <ginac-list at thep.physik.uni-mainz.de>
Sent: Wednesday, July 09, 2003 6:40 PM
Subject: Re: Changing symbol string names on-the-fly


> Hi!
>
> On Wed, Jul 09, 2003 at 11:02:07AM +0100, Dr. Vassilis S. Vassiliadis
wrote:
> > Is it possible to change on-the-fly the string names used in a symbol?
>
> There is a symbol::set_name(), but it only works for symbols that are not
> already part of an expression.
>
> > The names used internally utilize the object-partitioning
> > of my code e.g. :
> >         symbol x("system[1].composition.x[2]")
> >
> > The output in raw C++ form is desired to be a simple counting
> > scheme e.g. for the above symbol:
> >         "var[101]"
> > will be substituted.
>
> Maybe we should add a "csrc" name for symbols in the same manner as there
> is a TeX name. But this seems like overkill...
>
> > I have tried to use an auxiliary vector list with new symbols and
> > then substitute the new symbols for the old, using .subs().
>
> subs() would be the recommended solution to your problem.
>
> > However, when the number of substitutions is beyond say a couple of
> > hundred even a 2.5 GHz machine cannot run the problem efficiently.
>
> Specifying subs_options::subs_no_pattern might speed it up. If it's still
> too slow, find out why and send us a patch. :-)
>
> Maybe subs() should use maps instead of lists?
>
> > Without resorting to brute force outputing and replacing of strings,
> > I wonder if there is an elegant and economical way to do the above in
GiNaC.
>
> There is another way that is undocumented but gets the job done: symbols
> can be assigned expressions, including other symbols:
>
>     symbol a("a"), b("b");
>     ex e = a + 2*b;
>     cout << e << endl;
>      // prints "2*b+a"
>     a.assign(symbol("foo"));
>     b.assign(symbol("bar"));
>     cout << e.eval() << endl;
>      // prints "2*bar+foo", the eval() is necessary
>
> Bye,
> Christian
>
> --
>   / Physics is an algorithm
> \/ http://www.uni-mainz.de/~bauec002/