Originally posted by: youngdae

Hello,

I would like to solve LP using callable library which has just bound constraints. For example, when I write down the problem using CPXwriteprob() before calling CPXlpopt(), it generates the following output.

Minimize

 obj: 0 x1 + 0 x2

Bounds

 0 <= x1 <= 1

          x2 Free

End

However, when I call CPXlpopt(), it failed with an error code 1017. Error code 1017 is CPXERR_NOT_FOR_MIP: The requested operation can not be performed for mixed integer programs. Change the problem type.

Could anyone help with this? Should I add at least one constraint with just 0 coefficients?

Originally posted by: T_O

For some reason, your problem has become a MIP. You have several opportunities:

• Find out why you created a MIP. (Where there integer variables that you removed?) (recommended)
• Change the problem type to lp (CPXchgprobtype).
• Call CPXmipopt instead of CPXlpopt (your problem should be solved at the root node).

Best regards,
Thomas

BTW: An LP with just bound constraints is trivial. You can "see" the solution.

I guess you used a non-NULL ctype argument for function CPXnewcols. The below program works just fine. It creates your model and solves it without issue.

#include <stdio.h>
#include <stdlib.h>
#include <ilcplex/cplexx.h>

static double const obj[] = { 0.0, 0.0 };
static double const lb[] = { 0, -CPX_INFBOUND };
static double const ub[] = { 1, CPX_INFBOUND };
static char const *const name[] = { "x1", "x2" };
#ifdef PRODUCE_MIP
static char const ctype[] = { 'C', 'C' };
#else
static char const *ctype = NULL;
#endif

int
main(void)
{
   CPXENVptr env;
   CPXLPptr lp;
   int status;

   if ( (env = CPXXopenCPLEX(&status)) == NULL || status != 0 ) {
      fprintf (stderr, "CPXXopenCPLEX: %d\n", status);
      abort();
   }
   if ( (lp = CPXXcreateprob(env, &status, "model")) == NULL || status != 0 ) {
      fprintf (stderr, "CPXXcreateprob: %d\n", status);
      abort ();
   }

   status = CPXXnewcols (env, lp, 2, obj, lb, ub, ctype, name);
   if ( status != 0 ) {
      fprintf (stderr, "CPXXnewcols: %d\n", status);
      abort ();
   }
   status = CPXXwriteprob (env, lp, "model.lp", NULL);
   if ( status != 0 ) {
      fprintf (stderr, "CPXXwriteprob: %d\n", status);
      abort ();
   }
   status = CPXXlpopt (env, lp);
   if ( status != 0 ) {
      fprintf (stderr, "CPXXlpopt: %d\n", status);
      abort ();
   }

   CPXXfreeprob (env, &lp);
   CPXXcloseCPLEX (&env);
   printf ("Success\n");
   return 0;
}

Note that I pass a NULL pointer as ctype to CPXnewcols. When I pass a non-NULL pointer as ctype (define PRODUCE_MIP) then CPLEX will always consider the problem as a MIP, even if all entries in ctype are 'C'.

This is expected behavior and is specified in the reference documentation for CPXnewcols.