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main routine, the arrays cstat and rstat set the status of the initial basis.
CPXcopybase().
The application begins with declarations of arrays to store the solution of the problem. Then, before it calls any other ILOG CPLEX routine, the application invokes the Callable Library routine CPXopenCPLEX() to initialize the ILOG CPLEX environment. Once the environment has been initialized, the application calls other ILOG CPLEX Callable Library routines, such as CPXsetintparam() with the argument CPX_PARAM_SCRIND to direct output to the screen and most importantly, CPXcreateprob() to create the problem object. The routine populatebycolumn() builds the problem object, and as we noted earlier, CPXcopybase() copies the advanced starting basis.
Before the application ends, it calls CPXfreeprob() to free space allocated to the problem object and CPXcloseCPLEX() to free the environment.
The complete program, lpex6.c, appears here or online in the standard distribution
#include <ilcplex/cplex.h> /* Bring in the declarations for the string functions */ #include <string.h> /* Include declaration for function at end of program */ #ifndef CPX_PROTOTYPE_MIN static int populatebycolumn (CPXENVptr env, CPXLPptr lp); #else static int populatebycolumn (); #endif /* The problem we are optimizing will have 2 rows, 3 columns and 6 nonzeros. */ #define NUMROWS 2 #define NUMCOLS 3 #define NUMNZ 6 #ifndef CPX_PROTOTYPE_MIN int main (void) #else int main () #endif { char probname[16]; /* Problem name is max 16 characters */ int cstat[NUMCOLS]; int rstat[NUMROWS]; /* Declare and allocate space for the variables and arrays where we will store the optimization results including the status, objective value, variable values, dual values, row slacks and variable reduced costs. */ int solstat; double objval; double x[NUMCOLS]; double pi[NUMROWS]; double slack[NUMROWS]; double dj[NUMCOLS]; CPXENVptr env = NULL; CPXLPptr lp = NULL; int status; int i, j; int cur_numrows, cur_numcols; /* Initialize the CPLEX environment */ env = CPXopenCPLEX (&status); /* If an error occurs, the status value indicates the reason for failure. A call to CPXgeterrorstring will produce the text of the error message. Note that CPXopenCPLEX produces no output, so the only way to see the cause of the error is to use CPXgeterrorstring. For other CPLEX routines, the errors will be seen if the CPX_PARAM_SCRIND indicator is set to CPX_ON. */ if ( env == NULL ) { char errmsg[1024]; fprintf (stderr, "Could not open CPLEX environment.\n"); CPXgeterrorstring (env, status, errmsg); fprintf (stderr, "%s", errmsg); goto TERMINATE; } /* Turn on output to the screen */ status = CPXsetintparam (env, CPX_PARAM_SCRIND, CPX_ON); if ( status ) { fprintf (stderr, "Failure to turn on screen indicator, error %d.\n", status); goto TERMINATE; } /* Create the problem. */ strcpy (probname, "example"); lp = CPXcreateprob (env, &status, probname); /* A returned pointer of NULL may mean that not enough memory was available or there was some other problem. In the case of failure, an error message will have been written to the error channel from inside CPLEX. In this example, the setting of the parameter CPX_PARAM_SCRIND causes the error message to appear on stdout. */ if ( lp == NULL ) { fprintf (stderr, "Failed to create LP.\n"); goto TERMINATE; } /* Now populate the problem with the data. */ status = populatebycolumn (env, lp); if ( status ) { fprintf (stderr, "Failed to populate problem data.\n"); goto TERMINATE; } /* We assume we know the optimal basis. Variables 1 and 2 are basic, while variable 0 is at its upper bound */ cstat[0] = CPX_AT_UPPER; cstat[1] = CPX_BASIC; cstat[2] = CPX_BASIC; /* The row statuses are all nonbasic for this problem */ rstat[0] = CPX_AT_LOWER; rstat[1] = CPX_AT_LOWER; /* Now copy the basis */ status = CPXcopybase (env, lp, cstat, rstat); if ( status ) { fprintf (stderr, "Failed to copy the basis.\n"); goto TERMINATE; } /* Optimize the problem and obtain solution. */ status = CPXlpopt (env, lp); if ( status ) { fprintf (stderr, "Failed to optimize LP.\n"); goto TERMINATE; } status = CPXsolution (env, lp, &solstat, &objval, x, pi, slack, dj); if ( status ) { fprintf (stderr, "Failed to obtain solution.\n"); goto TERMINATE; } /* Write the output to the screen. */ printf ("\nSolution status = %d\n", solstat); printf ("Solution value = %f\n", objval); printf ("Iteration count = %d\n\n", CPXgetitcnt (env, lp)); /* The size of the problem should be obtained by asking CPLEX what the actual size is, rather than using sizes from when the problem was built. cur_numrows and cur_numcols store the current number of rows and columns, respectively. */ cur_numrows = CPXgetnumrows (env, lp); cur_numcols = CPXgetnumcols (env, lp); for (i = 0; i < cur_numrows; i++) { printf ("Row %d: Slack = %10f Pi = %10f\n", i, slack[i], pi[i]); } for (j = 0; j < cur_numcols; j++) { printf ("Column %d: Value = %10f Reduced cost = %10f\n", j, x[j], dj[j]); } /* Finally, write a copy of the problem to a file. */ status = CPXwriteprob (env, lp, "lpex6.sav", NULL); if ( status ) { fprintf (stderr, "Failed to write LP to disk.\n"); goto TERMINATE; } TERMINATE: /* Free up the problem as allocated by CPXcreateprob, if necessary */ if ( lp != NULL ) { status = CPXfreeprob (env, &lp); if ( status ) { fprintf (stderr, "CPXfreeprob failed, error code %d.\n", status); } } /* Free up the CPLEX environment, if necessary */ if ( env != NULL ) { status = CPXcloseCPLEX (&env); /* Note that CPXcloseCPLEX produces no output, so the only way to see the cause of the error is to use CPXgeterrorstring. For other CPLEX routines, the errors will be seen if the CPX_PARAM_SCRIND indicator is set to CPX_ON. */ if ( status ) { char errmsg[1024]; fprintf (stderr, "Could not close CPLEX environment.\n"); CPXgeterrorstring (env, status, errmsg); fprintf (stderr, "%s", errmsg); } } return (status); } /* END main */ /* This function builds by column the linear program: Maximize obj: x1 + 2 x2 + 3 x3 Subject To c1: - x1 + x2 + x3 <= 20 c2: x1 - 3 x2 + x3 <= 30 Bounds 0 <= x1 <= 40 End */ #ifndef CPX_PROTOTYPE_MIN static int populatebycolumn (CPXENVptr env, CPXLPptr lp) #else static int populatebycolumn (env, lp) CPXENVptr env; CPXLPptr lp; #endif { int status = 0; double obj[NUMCOLS]; double lb[NUMCOLS]; double ub[NUMCOLS]; char *colname[NUMCOLS]; int matbeg[NUMCOLS]; int matind[NUMNZ]; double matval[NUMNZ]; double rhs[NUMROWS]; char sense[NUMROWS]; char *rowname[NUMROWS]; /* To build the problem by column, create the rows, and then add the columns. */ CPXchgobjsen (env, lp, CPX_MAX); /* Problem is maximization */ /* Now create the new rows. First, populate the arrays. */ rowname[0] = "c1"; sense[0] = `L'; rhs[0] = 20.0; rowname[1] = "c2"; sense[1] = `L'; rhs[1] = 30.0; status = CPXnewrows (env, lp, NUMROWS, rhs, sense, NULL, rowname); if ( status ) goto TERMINATE; /* Now add the new columns. First, populate the arrays. */ obj[0] = 1.0; obj[1] = 2.0; obj[2] = 3.0; matbeg[0] = 0; matbeg[1] = 2; matbeg[2] = 4; matind[0] = 0; matind[2] = 0; matind[4] = 0; matval[0] = -1.0; matval[2] = 1.0; matval[4] = 1.0; matind[1] = 1; matind[3] = 1; matind[5] = 1; matval[1] = 1.0; matval[3] = -3.0; matval[5] = 1.0; lb[0] = 0.0; lb[1] = 0.0; lb[2] = 0.0; ub[0] = 40.0; ub[1] = CPX_INFBOUND; ub[2] = CPX_INFBOUND; colname[0] = "x1"; colname[1] = "x2"; colname[2] = "x3"; status = CPXaddcols (env, lp, NUMCOLS, NUMNZ, obj, matbeg, matind, matval, lb, ub, colname); if ( status ) goto TERMINATE; TERMINATE: return (status); } /* END populatebycolumn */ |
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