On Thu, 7 Oct 2004, Roberto Bagnara wrote:
Enrico Oliosi wrote:
I analysed this program:
BEGINPRG Read(i); j:=0; IF (i >= 0) THEN { j:= i+2; [p1]={j=0; i>=0; j-i=2} } ELSE { j:= i; [p2]={j=0; -i>0; j-i=0} } FI; [1=0] ENDPRG
when I done the convex-hull using:
[p1].poly_hull_assign_and_minimize([p2]);
the result has been the empty polyhedron ([1=0]).
Dear Enrico,
what you obtain is not surprising at all: since both p1 and p2 are empty polyhedra, their poly-hull is the empty polyhedron.
(if I use the function [p1].poly_hull_assign([p2]) the poly_hull returns [p2]);
This is still OK: the method poly_hull_assign() does not minimize the result and, since p1 is empty it simply gives you back p2.
I think that the problem is elsewhere: the constraint `j=0' should not, I believe, be in p1 nor in p2.
OK but in Cousot&Halbwachs76 if I have a linear test C and P the enter polyhedron P-true is defined as:
P-true = P intersection C ( P-false = P intersecrion not(C) ).
If P is [j=0] and C is [i>=0] the intersection (P.intersection_assign_and_minimize(C);) returns the Polyhedron P' = [j=0,i>=0]; the same holds for P_false ([j=0,i<0]).
What do I wrong? Probably I don't understand the guard rules very well....
Thanks,
Enrico