"I do not know why the maximum value is 0 when it should have been -5.
Not just this example, but with any other negative value, the output is
always zero. How can I fix this?"
/*
* Maximize z = 0*x - 1*y
* subject to :
* x<=1
* x>=1
* y<=5
* y>=5
*/
#include <iostream>
#include <stdlib.h>
#include <glpk.h>
using namespace std;
int main(void)
{
glp_prob *lp;
int ia[1+1000], ja[1+1000];
double ar[1+1000], z, x, y;
lp = glp_create_prob();
glp_set_prob_name(lp, "sample");
glp_set_obj_dir(lp, GLP_MAX);
glp_add_rows(lp, 4);
glp_set_row_name(lp, 1, "p");
glp_set_row_bnds(lp, 1, GLP_UP, 0.0, 1.0);
glp_set_row_name(lp, 2, "q");
glp_set_row_bnds(lp, 2, GLP_LO, 0.0, 1.0);
glp_set_row_name(lp, 3, "r");
glp_set_row_bnds(lp, 3, GLP_UP, 0.0, 5.0);
glp_set_row_name(lp, 4, "s");
glp_set_row_bnds(lp, 4, GLP_LO, 0.0, 5.0);
glp_add_cols(lp, 2);
glp_set_col_name(lp, 1, "x");
glp_set_col_bnds(lp, 1, GLP_LO, 0.0, 0.0); //why GLP_LO here ?
glp_set_obj_coef(lp, 1, 0.0);
glp_set_col_name(lp, 2, "x");
glp_set_col_bnds(lp, 2, GLP_LO, 0.0, 0.0); //why GLP_LO here ?
glp_set_obj_coef(lp, 2, -1.0);
ia[1] = 1, ja[1] = 1, ar[1] = 1.0;
ia[2] = 1, ja[2] = 2, ar[2] = 0.0;
ia[3] = 2, ja[3] = 1, ar[3] = 1.0;
ia[4] = 2, ja[4] = 2, ar[4] = 0.0;
ia[5] = 3, ja[5] = 1, ar[5] = 0.0;
ia[6] = 3, ja[6] = 2, ar[6] = 1.0;
ia[7] = 4, ja[7] = 1, ar[7] = 0.0;
ia[8] = 4, ja[8] = 2, ar[8] = 1.0;
glp_load_matrix(lp, 8, ia, ja, ar);
glp_simplex(lp, NULL);
z = glp_get_obj_val(lp);
x = glp_get_col_prim(lp, 1);
y = glp_get_col_prim(lp, 2);
//printf("\nz = %g; x1 = %g; x2 = %g; x3 = %g\n",z, x1, x2, x3);
cout <<"z = " << z << " x = " << x << " y = " << y;
cout << endl;
glp_delete_prob(lp);
return 0;
}
--
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