[GiNaC-list] Ordering of expressions

[Luigi Capozza]

Dear people,

I have the following issue with the canonicalization of
expressions. If I run many times the same program without changing
anything, I get the output for the same expression ordered in
different ways. As far as I understand, that seems to depend on the
hash value that is assigned to each symbol, which happens to be
different every time I run the program.

Here is an example (example.cxx):

------------------------------------------
#include <iostream>

#include "ginac/ginac.h"

using namespace std;
using namespace GiNaC;


int main()
{
  symbol x("x"),y("y"),z("z");
  
  ex Ex1 = 2*(x + y);   
  ex Ex2 = z*(x + y);   

  cout<<x.gethash()<<endl
      <<y.gethash()<<endl
      <<z.gethash()<<endl;

  cout<<Ex1<<endl
      <<Ex2<<endl;

  return 0;
}
------------------------------------------

And here is the output I get (I am running on Ubuntu 8.04.1 with the
GCC 4.2.4 compiler):

--------------------------------------------------
=>g++ -lginac example.cxx -o example
=>example
2178355982
851138097
3818887508
2*y+2*x
(y+x)*z
=>example
3784401678
2457183793
1129965908
2*y+2*x
(y+x)*z
=>example
2632340238
1305122353
4272871764
2*y+2*x
(y+x)*z
=>example
3711329038
2384111153
1056893268
2*y+2*x
z*(y+x)
=>
...
--------------------------------------------------

With more complicated expressions also the run time can vary from
execution to execution up to a factor of 2 or more.

Is this normal or am I doing something wrong? I saw that the hash
value is calculated in a fairly sophisticated manner but is it
eventually possible to set it by hand?

Thanks in advance.
Regards,
Luigi