Re: Changing symbol string names on-the-fly

11 Jul 2003

      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@cheng.cam.ac.uk

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----- Original Message -----
From: "Christian Bauer" <Christian.Bauer@Uni-Mainz.DE>
To: <ginac-list@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
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