CGAL users discussion list

["Mathieu Brédif" <>]

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
>