[PPL-devel] Another question

[Ezio Bartocci]

Dear PPL developers,

       I forward the message that I sent to Roberto. Since he is
traveling, maybe some of you can help us...

Thanks a lot

Best Regards

Ezio

On Thu, May 3, 2012 at 4:42 PM, Ezio Bartocci <ezio.bartocci at gmail.com> wrote:
> Dear Roberto,
>
>
>     I have another question. This time in English so that also Radu
> and Gregory can read. I have the following points, describing a
> polytope:
>
> 1                  0.005             0.007883
> 0.9967750    0.004769   0.007988
> 1                  0.000250     0.007883
> 0.9967750         1.900e-05     0.007988
> 1                 0.005000      0.004225
> 0.9967750   0.004769    0.004331
> 1                 0.000250      0.004225
> 0.9967750   1.900e-05   0.004331
> 1                 0.0050000     0.007883
> 0.4194590         0.0047690     0.007988
> 1                 0.0002500     0.007883
> 0.4194590         1.9000e-05    0.007988
> 1                 0.0050000     0.004225
> 0.4194590   0.0047690   0.004331
> 1                 0.0002500     0.004225
> 0.4194590  1.90000e-05  0.004331
>
>
> From this points stored in a matrix P with rows_P = 16 and cols_P = 3.
>
> I call this function that I developed to generate a C_Polyhedron (or
> Polytope), that uses
>
> add_generator
>
> as you suggested and assign  to P the result that is private member of
> the class ppl_wrap
>
>
> ppl_wrap::ppl_wrap (double * Points, int rows_P, int
> cols_P):rows_P(rows_P), cols_P(cols_P)
> {
>
>        int debug = 1;
>        int i, j, k;
>
>    // Convert the standard double representation to a much
>    // accurate representation
>    vector<mpf_class> *P1_row = double2mpzclass(Points, &rows_P, &cols_P);
>    vector<vector<mpf_class> > *P1 = vec2mat(P1_row, &rows_P, &cols_P);
>    vector<mpf_class> single_point;
>
>    list<Variable> VarList;
>        list<Variable>::iterator it;
>    Linear_Expression lin_exp[rows_P];
>    if (debug)
>        cout << "Cols num: " << cols_P << "\n";
>
>    for(i=0; i< cols_P; i++)                            // create a list of n_col_C "Variable" objects
>                VarList.push_back(Variable(i));
>
>    C_Polyhedron ph (cols_P, EMPTY);
>
>    if (debug)
>        cout << "Rows num: " << rows_P << "\n";
>    for (j = 0; j < rows_P; j++)
>    {
>        if (debug)
>            cout << " j = " << j << "\n";
>        single_point.clear();
>
>        /*Create linear expression for expressing the point*/
>                if (debug)
>            cout << " k = ";
>        for (k = 0; k < cols_P; k++){                   // pick the needed row of matrix C
>                        if (debug)
>                    cout << "(" << k << ") " << (*P1)[k][j] << " ";
>                        single_point.push_back((*P1)[k][j]);
>
>                }
>        if (debug)
>            cout << ";\n";
>
>
>        int m;
>        for ( it=VarList.begin() ; it != VarList.end(); it++ )  //
> creates linear expression "lin_exp_temp" by using the iterator of the
> Variable-List
>        {
>                m = it->id();
>                if (debug)
>                    cout << " m = " << m <<"\n";
>                lin_exp[j]+=single_point[m]*(*it);
>        }
>
>        ph.add_generator(point(lin_exp[j]));
>    }
>    P = ph;
> }
>
>
>
> After obtaining a polytope I need to have the following representation
> to store the polytope back in matlab C * x <= d.
>
> Now as I wrote in a previous email I saw that with:
>
> Constraint_System ConSys_P;
> ConSys_P = P.constraints();
>
> we can actually obtain the representation: C * x >= d
>
> but if I print the constraints:
>
> ConSys_P.print();
>
> -A + C >= 0, -C >= -1, A >= 0, B >= 0, -B >= -1
>
> This does not seem to match with the points and it seems an over
> approximation. I would expected constraints more tight constraints.
>
> Why is like that ?
>
>
> Thanks,
>
> Best
>
>
> Ezio
>
>
>
>
>
>
>
>
> --
> Dr. Ezio Bartocci, Assistant Professor
> Dependable Systems Group
> Department of Computer Engineering
> Faculty of informatics
> TU Wien, Vienna University of Technology, Austria
> Treitlstraße   3, 1040 Vienna, Austria
> E-Mail:     ezio.bartocci at tuwien.ac.at
> Phone:     +43 (1) 58801 - 18210
> Webiste: http://www.eziobartocci.com



-- 
Dr. Ezio Bartocci, Assistant Professor
Dependable Systems Group
Department of Computer Engineering
Faculty of informatics
TU Wien, Vienna University of Technology, Austria
Treitlstraße   3, 1040 Vienna, Austria
E-Mail:     ezio.bartocci at tuwien.ac.at
Phone:     +43 (1) 58801 - 18210
Webiste: http://www.eziobartocci.com