Re: [HELP-GLPK]about the presolver

[Andrew Makhorin]

> from the glpk.pdf i know that Presolving is an optional feature of the
> routine glp_simplex, and by default it is disabled. but when i
> use glpsol.exe to solve LP problem presolving is enabled by default
> ,right?(because when i solve some problem it gave me some messages like
> "if you need actual output for non-optimal solution, use --nopresol")

> when i  use glp_simplex through the API only, if the problem have no
> optimal solution(a computed solution is infeasible or non-optimal,), it
> also can give me a result just like using "--nopresol" in the glpsol.exe 
> ,right?

Yes.

If the lp presolver is enabled and the problem has no optimal solution,
glp_simplex returns GLP_ENOPFS or GLP_ENODFS, and the solution remains
undefined, because the lp presolver is unable to recover non-optimal
solution to the original problem. However, if the lp presolver is
disabled (by default), and glp_simplex returns 0, the basic solution
components stored in the problem object always correspond to the last
point reached, even if the solution is non-optimal. (In the latter case
you can discover the solution status with glp_get_status or
glp_get_prim_stat/glp_get_dual_stat.)