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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<br>
<br>
<div class="moz-cite-prefix">Am 21.09.2015 um 14:02 schrieb M.
Farham:<br>
</div>
<blockquote
cite="mid:CAGhf1zKixU0B8UQ18imB6MJFmQC3_B2VT8GFFLiMb10t1tfDhQ@mail.gmail.com"
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
moz-do-not-send="true" 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"
color="#000000" face="Calibri,sans-serif" size="3">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" color="#000000"
face="Calibri,sans-serif" size="3"><br>
</font></div>
<div><font style="font-size:12pt" color="#000000"
face="Calibri,sans-serif" size="3">Thanks for any
help</font></div>
<div><font style="font-size:12pt" color="#000000"
face="Calibri,sans-serif" size="3">With regards</font></div>
<div><font style="font-size:12pt" color="#000000"
face="Calibri,sans-serif" size="3">---</font></div>
<div><font style="font-size:12pt" color="#000000"
face="Calibri,sans-serif" size="3">A. Ouali</font></div>
</div>
</div>
</div>
<br>
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</blockquote>
</div>
<br>
<br clear="all">
<br>
-- <br>
<div class="gmail_signature">
<div dir="ltr">M Saleh F<br>
</div>
</div>
</div>
<br>
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