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Re: [Help-glpk] Cannot get integer case to work

From: Andrew Makhorin
Subject: Re: [Help-glpk] Cannot get integer case to work
Date: Tue, 25 Aug 2009 17:49:18 +0400

> That's funny, I specifically issued the statement
> control_params.presolve = GLP_ON;
> before calling glp_intopt().  My understanding from the documentation
> is that this would tell it to do the simplex pre-solve on its own. 
> Furthermore, the trace it showed on the screen indicated that it had in
> fact found the LP solution already, before tackling the integer
> programming problem.

Sorry, I did not notice that. Yes, if the mip presolver is enabled,
there is no need to call glp_simplex.

However, to obtain mip solution components you need to use api
routine glp_mip_col_val rather than glp_get_col_prim (the latter
retrieves basic lp solution, not mip solution).

> Please see partial screen output below, from the original attempt. 
> Does the "Crashing..." part mean anything bad?

That message means constructing an initial basis. In glpk 4.39 (which
is the most recent version) it was replaced by a more friendly message.

> As a follow-up attempt I worked in a glp_simplex() call prior to the
> glp_intopt() call, just to see if that made a difference.  In that
> scenario it gave the LP solution as the integer solution, with
> fractional values for x and y.  I still haven't found a path that seems
> to work.

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