<div dir="ltr">Thank you Gerald.<div><br></div><div>What I did was to give the initial column to the pricer, price it, and then add it to the MP in the very beginning. The rest is done as before. The problem is solved by this additional step.</div><div><br></div><div>Best,</div><div>Saleh<br></div><div class="gmail_extra"><br><div class="gmail_quote">On Sat, Oct 3, 2015 at 9:37 PM, Gerald Gamrath <span dir="ltr"><<a href="mailto:gamrath@zib.de" target="_blank">gamrath@zib.de</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
Dear Saleh,<br>
<br>
this sounds like you have a bug in your pricer and sometimes miss to
generate needed variables. Given that adding initial columns will
lead to different LP optima with potentially different dual
multipliers, it may just be luck that sometimes, you end up with the
optimal solution and sometimes not.<br>
<br>
Sorry for not having a constructive answer to your question...<br>
<br>
Best,<br>
Gerald<div><div><br>
<br>
<div>Am 27.09.2015 um 14:48 schrieb M.
Farham:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Dear Gerald,
<div><br>
</div>
<div>Thank you for your kind response.</div>
<div>However, I got another problem. I am dealing with the VRP
and I am trying to add an initial column before solving the
problem. In some cases, the solver fails to find the optimal
solution when the initial column is added to the MP. I also
noticed that sometimes, the dual bound is higher than the
primal bound at the final node for this minimization problem.<br>
</div>
<div><br>
</div>
<div>Marking the initial variable as deletable and setting
'lp/cleanupcolsroot' and 'pricing/delvarsroot' parameters to
TRUE did not help. But, if I reduce the 'lp/colagelimit'
value, the problem is solved to optimality and no bound
conflicts are observed (I have to set that parameter to 0 for
some instances to get the correct solution).</div>
<div><br>
</div>
<div>I have already checked the initial column for feasibility
and I wonder why this happens.</div>
<div><br>
</div>
<div>Bests,</div>
<div>Saleh</div>
<div><br>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Tue, Sep 22, 2015 at 12:52 AM,
Gerald Gamrath <span dir="ltr"><<a href="mailto:gamrath@zib.de" target="_blank">gamrath@zib.de</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000"> Dear Saleh,<br>
<br>
it is ok to add some of the variables already during
problem creation and then add others in pricing.<br>
<br>
If you give the optimal solution as initial columns, this
solution will be found directly in the beginning, but its
optimality still has to be proven. And normally, there are
other variables that help to improve the LP solution when
priced. If this would not be the case, you would already
be done without any branching. But often, you need some
branching to prove optimality, which implies that your
root dual bound is smaller (in case of minimization) than
the optimum, so there are some variables not contained in
the optimal solution which can improve the LP solution. In
order to close the gap, you need to solve multiple node
LPs and potentially generate variables there.<br>
<br>
In branch-and-bound (without price), you see a similar
thing: If you provide the optimal solution at the
beginning of the solving process, this speeds up the
process, but you still need a significant amount of
branching for most of the instances.<br>
<br>
Best,<br>
Gerald
<div>
<div><br>
<br>
<div>Am 21.09.2015 um 14:02 schrieb M. Farham:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>Hi all,<br>
<br>
</div>
<div>Same question here.<br>
But I am not sure if I am giving the initial
columns correctly. What I do is to create a
variable and add its coefficients to the
objective function and corresponding constraints
before calling the pricer. Is this ok, or should
I price this variable and add it in the pricing
stage?<br>
<br>
</div>
<div>Bests,<br>
<br>
</div>
<div>Saleh<br>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Tue, Mar 10, 2015 at
12:36 AM, Abdelkader Ouali <span dir="ltr"><<a href="mailto:oualiaek@hotmail.fr" target="_blank">oualiaek@hotmail.fr</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div>
<div dir="ltr"><font style="font-size:12pt" face="Calibri,sans-serif" size="3" color="#000000">Hi all,</font>
<div><br>
</div>
<div>
<div>I have implemented branch-and-price
using Scip,</div>
<div>and launch it to solve short
examples before attacking big ones.</div>
<div>I noticed that when I give an
optimal solution as initial columns </div>
<div>for the master or also during the
solving, the pricing process generates
unused columns, which are in final
all zero (because they not participate
in optimal solution).</div>
<div>So, I wonder why the pricing
generates columns while an already
optimal solution was given, is it due
to duals given by Scip or ... ?</div>
</div>
<div>
<div><font style="font-size:12pt" face="Calibri,sans-serif" size="3" color="#000000"><br>
</font></div>
<div><font style="font-size:12pt" face="Calibri,sans-serif" size="3" color="#000000">Thanks for any help</font></div>
<div><font style="font-size:12pt" face="Calibri,sans-serif" size="3" color="#000000">With regards</font></div>
<div><font style="font-size:12pt" face="Calibri,sans-serif" size="3" color="#000000">---</font></div>
<div><font style="font-size:12pt" face="Calibri,sans-serif" size="3" color="#000000">A. Ouali</font></div>
</div>
</div>
</div>
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<br>
-- <br>
<div>
<div dir="ltr">M Saleh F<br>
</div>
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</blockquote></div><br><br clear="all"><div><br></div>-- <br><div><div dir="ltr">M Saleh F<br></div></div>
</div></div>