[GiNaC-list] Re: Disabling flattening of sums and products?

[Alexei Sheplyakov]

Hello!

On Wed, Nov 14, 2007 at 02:38:44PM +0100, Martin Sandve Alnæs wrote:
> Is it possible to disable the automatic flattening of expressions like
> f = (x+y) + (x+y)
> into
> g = 2*y+2*x
> ?

No.
 
> In other words, I want to be able to retain the additions in
> expression f as binary operators.

And GiNaC wants to 1) canonicalize the tree (expression), 2) make
the tree as flat as possible, so large expressions can be stored
reasonably well.

> The reason for this is that I want
> to examine some expression to detect subtrees that occur multiple
> times,

ex::to_polynomial() does somethings similar, so you might want to look
how different classes implement this method (see the ginac/normal.cpp file).

N.B.: the concept is a bit ill-defined: it depends not on the mathematical
properties of the expression in question, but on its internal representation.
E.g. is x*y a subtree of x^2*y?

> and this automatic flattening destroys some opportunities for
> this.

Not flattening the tree destroys some opportunities for this. Example:

a = c*x^2*y
b = (c*x)*(x*y)

> Example:
> 
> a = x*y
> b = (x*y) * (x*y)

#include <ginac/ginac.h>
#include <iostream>
using namespace std;
using namespace GiNaC;

int main(int argc, char** argv) {
	symbol x("x"), y("y"), t("t");
	ex a = x*y;
	ex b = (x*y)*(x*y);
	cout << "b = " << b << endl;
	// b = y^2*x^2
	symbol foo;
	b = b.subs(a == foo, subs_options::algebraic);
	cout << "b = " << b << " with: " << foo << " = " << a << endl;
	// b = symbol4^2 with: symbol4 = y*x
	return 0;
}

Best regards,
	Alexei

-- 
All science is either physics or stamp collecting.

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