Re: [Help-glpk] Re: Intermediate solutions

[Carlos Javier Sosa Gonzalez]

Hello,

I apologize for my previous email. I have found the error. It is in my
C code, at line:

 fprintf(file, "%s %d\n", lpx_get_col_name(lp, col),
         (int)lpx_get_mip_col(lp, col));

The (int) is the problem. lpx_get_mip_col returns a double value. if this
value is printed for some values, it returns a low number (x.xxxE-16). The
automatic round gives a wrong 1 round when the right value is 0. 

With the following modification in this line, the intermediate solution
stored in the file is the right.

 fprintf(file, "%s %d\n", lpx_get_col_name(lp, col),
         (lpx_get_mip_col(lp,col) > 0.5)?1:0);


Best Regards,
Javier


On Sun, 29 Jun 2003, Carlos Javier Sosa Gonzalez wrote:

> 
> Hi, me again.
> 
> I have done your recomendations, but I have several intermediate solutions
> that the problem said LPX_I_FEAS, and it violates several constrains.
> On the other hand the final solution report gives a feasible solution
> that satifies all constrains.
> 
> The code added is:
> 
> int inter_sol = 0;
> 
> void write_intermediate_solution(MIPTREE *tree)
> {
>   LPX   *lp;
>   FILE  *file;
>   int    col;
>   int    ncols;
> 
>   lp = (LPX *)tree->info;
> 
>   if((lp->i_stat == LPX_I_FEAS)&&(tree->found)) {
>     file = fopen("intermediate.out", "a");
>     
>     fprintf(file,"\nStart Solution %d %d\n", inter_sol,
>                lpx_get_mip_obj(lp));
>     ncols = lpx_get_num_cols(lp);
>     for(col = 1; col <= ncols; col++){
>       fprintf(file, "%s %d\n",
>             lpx_get_col_name(lp, col),
>             (int)lpx_get_mip_col(lp, col));
>     }
>     fprintf(file,"END\n");
>     
>     fclose(file);
>     inter_sol++;
>   }
> }
> 
> ...
> 
>          case MIP_V_BINGO:
>             /* better integer feasible solution has been found */
>             /* copy components of this solution to the original problem
>                object */
>             {  int k;
>                mip->i_stat = LPX_I_FEAS;
>                for (k = 1; k <= tree->orig_m + tree->orig_n; k++)
>                   mip->mipx[k] = tree->best[k];
>             }
>           write_intermediate_solution(tree);
>             break;
> 
> 
> Thanks in advance,
> Javier Sosa
> 
> 
> On Tue, 24 Jun 2003, [windows-1251] "Andrew Makhorin[windows-1251] "  wrote:
> 
> > >* Are these non-optimal solutions of the problem? or can they violate any
> > >problem constrain or bound? are they feasible solutions?
> > 
> > Each line printed after the message "Integer optimization begins..."
> > correspond to best integer feasible solution known at the current point.
> > "Integer feasible" means the solution satisfies to all constraints and
> > all integer variables have integral values.
> > 
> > >* If they are solutions of the problem, how can I access to these
> > >MIP non-optimal solutions?
> > 
> > You cannot do that on api level. However you can add your code directly
> > in the b&b driver as explained below.
> > 
> > Get into the module glplpx6c.c which is a b&b driver (lpx_integer) and
> > find the following fragment (lines 466-475 in glpk 4.0):
> > 
> >          case MIP_V_BINGO:
> >             /* better integer feasible solution has been found */
> >             /* copy components of this solution to the original problem
> >                object */
> >             {  int k;
> >                mip->i_stat = LPX_I_FEAS;
> >                for (k = 1; k <= tree->orig_m + tree->orig_n; k++)
> >                   mip->mipx[k] = tree->best[k];
> >             }
> >             break;
> > 
> > This fragment is executed whenever the b&b solver discovers a new integer
> > feasible solution which is better than the currently known one.
> > 
> > You should add your code immediately before the break statement. At that
> > point 'mip' is a pointer to your problem object passed to the routine
> > lpx_integer. Using that pointer you can access components of the current
> > mip solution (which is integer feasible) with the following routines:
> > 
> > lpx_get_num_rows - obtain number of auxiliary variables (rows);
> > 
> > lpx_get_num_cols - obtain number of structural variables (columns);
> > 
> > lpx_get_mip_obj - obtain value of the objective function;
> > 
> > lpx_get_mip_row - obtain value of i-th auxiliary variable (row);
> > 
> > lpx_get_mip_col - obtain value of j-th structural variable (column).
> > 
> > Note that you must not change the problem object within the b&b driver.
> > 
> 
> --
>  Carlos Javier Sosa González  E.T.S.I. de Telecomunicación
>   address@hidden         Campus Universitario de Tafira, Pab. A, Desp. 301 
>   Tel.: +34 928 457 324       Univ. de Las Palmas de Gran Canaria
>   Fax : +34 928 451 243       35017 Las Palmas de Gran Canaria
>   ICQ : 1756350985            Canary Islands. SPAIN
> 
> 
> 
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> Help-glpk mailing list
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> 

--
 Carlos Javier Sosa González  E.T.S.I. de Telecomunicación
  address@hidden         Campus Universitario de Tafira, Pab. A, Desp. 301 
  Tel.: +34 928 457 324       Univ. de Las Palmas de Gran Canaria
  Fax : +34 928 451 243       35017 Las Palmas de Gran Canaria
  ICQ : 1756350985            Canary Islands. SPAIN