CGAL users discussion list

[Daniel Russel <>]

Try replacing
typedef CGAL::Exact_predicates_exact_constructions_kernel Static_kernel;
with
typedef CGAL::Cartesian<CGAL::Gmpq> Static_kernel;

That should shift things to use the Gmpq number type rather than the  
Lazy_exact_nt. The latter doesn't ever print the exact representation  
(as it might be quite complicated). If you can't reproduce the problem  
with Gmpq. there may be an issue with Laxy_exact_nt and if you can  
reproduce it, it should be clearer what is going on.

Sorry about the slow response, I have been at a meeting.


On Apr 25, 2008, at 6:15 AM, Mathieu Brédif wrote:

> If it can help, I get an other problem that seems related in :
> -----------------
> template<class K, class Time>
> unsigned int get_multiplicity(const typename K::Function& f, const K&
> k, const Time& t) {
>    if(f.degree() == -1) return (unsigned int) -1; // null function
>    typename K::Function h=f;
>    unsigned int deg=0;
>    typename K::Sign_at sign= k.sign_at_object();
>    typename K::Differentiate diff=k.differentiate_object();
>    while (sign(h, t)==CGAL::ZERO) { <---------------- throws exception
>      ++deg;
>      h=diff(h);
>    }
>    return deg;
> };
> ------------------------
>
> the exception is :
>
> --------------------------------------------------
> CGAL error: assertion violation!
> Expr: p_(v) == 0
> File: /usr/include/CGAL/Polynomial/Sturm_root_stack.h
> Line: 167
> -----------------------------------------
> the corresponding code is:
>
>    if (p_.degree() == 1) {
>      NT v= -p_[0]/p_[1];
>      CGAL_assertion(p_(v) == 0);
>      ....
>    }
> so it does not seem related to the particular root t but rather to the
> polynom p_ and the exactness of p_[0]-p_[1]*p_[0]/p_[1]==CGAL::ZERO.
>
> The values ouput by "std::cout <<" that cause the problem are :
> f=-0.894427191-0.105572809*t
> t=0
>
> The roots that cause the problem in the previous mail were both  
> output as :
> -0.12132034356+0.192388155425*t+0.100505063388*t^2 in [0,1] =  
> 0.5...0.5
>
> Is there a way to backup exactly the value of a Function to be able to
> reproduce my problem with a small piece of code ?
>
> Thanks,
> Mathieu
>
> On Fri, Apr 25, 2008 at 2:17 PM, Mathieu Brédif
> <>
>  wrote:
> > Hi,
> > I have some trouble tracking done the cause of this  
> > precondition_exception :
> >
> > ----------------------
> > Expr: s0 != CGAL::ZERO
> > File: /usr/include/CGAL/Polynomial/internal/Rational/ 
> > Sign_Sturm_sequence.h
> > Line: 57
> > Explanation:
> > terminate called after throwing an instance of  
> > 'CGAL::Precondition_exception'
> >  what():  CGAL ERROR: precondition violation!
> > Expr: s0 != CGAL::ZERO
> > File: /usr/include/CGAL/Polynomial/internal/Rational/ 
> > Sign_Sturm_sequence.h
> > Line: 57
> > -----------------
> >
> > This exception is triggered when i am comparing 2 roots for equality.
> > The traits i use are :
> >
> >  typedef CGAL::Exact_predicates_exact_constructions_kernel  
> > Static_kernel;
> >  typedef CGAL::POLYNOMIAL::Polynomial<Static_kernel::FT> Function;
> >  typedef CGAL::POLYNOMIAL::Sturm_root_stack_traits<Function>  
> > Root_stack_traits;
> >  typedef CGAL::POLYNOMIAL::Sturm_root_stack<Root_stack_traits>  
> > Root_stack;
> >  typedef CGAL::POLYNOMIAL::Kernel<Function, Root_stack>  
> > Function_kernel;
> >
> > Am i doing something wrong ?
> >
> > thanks
> > Mathieu Bredif
>
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