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<p>Hi,</p>
<p>I confirm the bug.</p>
<p>It seems that when mipstart is informed (and a good bound is
available at the start) cut generators "cut too much" in this
particular problem and it becomes infeasible.</p>
<p>This fixation of the "one" variable should not interfere... <br>
</p>
<p>Cbc0045I Trying just fixing integer variables.<br>
Cbc0045I MIPStart provided solution with cost 181978<br>
Cgl0002I 301 variables fixed<br>
Cgl0003I 63 fixed, 0 tightened bounds, 275 strengthened rows, 0
substitutions<br>
Cgl0003I 0 fixed, 0 tightened bounds, 124 strengthened rows, 0
substitutions<br>
Cgl0003I 0 fixed, 0 tightened bounds, 146 strengthened rows, 0
substitutions<br>
Cgl0000I Cut generators found to be infeasible! (or unbounded)<br>
Pre-processing says infeasible or unbounded<br>
Total time (CPU seconds): 0.28 (Wallclock seconds):
0.28<br>
<br>
</p>
<br>
<div class="moz-cite-prefix">Em 12/01/2017 13:22, Rudi Araújo
escreveu:<br>
</div>
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<div class="WordSection1">
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">John,<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">Regarding the other thread (assertion failure),
your fix seems to work, thanks.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">I have unearthed this week-old thread because
we came across a different situation where CBC reports
infeasibility, even though the problem is indeed feasible.
See files here:<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><a class="moz-txt-link-freetext" href="https://drive.google.com/open?id=0B-5kPpSmX5imVllVaUdxVWY3TGc">https://drive.google.com/open?id=0B-5kPpSmX5imVllVaUdxVWY3TGc</a><o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">Breaking it down:<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoListParagraph"
style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><!--[if !supportLists]--><span
style="font-size:11.0pt;font-family:Symbol;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><span style="mso-list:Ignore">·<span
style="font:7.0pt "Times New Roman"">
</span></span></span><!--[endif]--><span
style="font-size:11.0pt;font-family:Consolas;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">cbc -import test.lp -mipstart ms.before.txt<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:35.4pt"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">Reports infeasibility.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoListParagraph"
style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><!--[if !supportLists]--><span
style="font-size:11.0pt;font-family:Symbol;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><span style="mso-list:Ignore">·<span
style="font:7.0pt "Times New Roman"">
</span></span></span><!--[endif]--><span
style="font-size:11.0pt;font-family:Consolas;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">cbc -import test.lp -mipstart ms.txt<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:35.4pt"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">Solves problem to optimality, but rejects MIP
start.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">The files ms.txt and ms.before.txt have exactly
the same solution.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">We traced the issue to this constraint:<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:Consolas;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"> __one = 1<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">If we remove it, the .before MIP start is
accepted and the model is solved to optimality.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">In this particular case, we can do without the
</span><span
style="font-size:11.0pt;font-family:Consolas;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">__one</span><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"> var, but in the general case we add it because
it is sometimes convenient for us to pass a constant term in
the objective function.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">Can you take a look at it?<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB">Rúdi Araújo.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-GB"><o:p> </o:p></span></p>
<div>
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"
lang="EN-US">From:</span></b><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"
lang="EN-US"> John Forrest
[<a class="moz-txt-link-freetext" href="mailto:john.forrest@fastercoin.com">mailto:john.forrest@fastercoin.com</a>] <br>
<b>Sent:</b> quinta-feira, 5 de janeiro de 2017 15:29<br>
<b>To:</b> Rudi Araújo <a class="moz-txt-link-rfc2396E" href="mailto:rudi.araujo@siscog.pt"><rudi.araujo@siscog.pt></a><br>
<b>Cc:</b> 'Tiago Maduro Dias' <a class="moz-txt-link-rfc2396E" href="mailto:tiago@siscog.pt"><tiago@siscog.pt></a>;
'Luís Borges de Oliveira' <a class="moz-txt-link-rfc2396E" href="mailto:lbo@siscog.pt"><lbo@siscog.pt></a><br>
<b>Subject:</b> Re: CBC rejecting its own solution as
MIP start<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Rudi,<br>
<br>
On 04/01/17 17:39, Rudi Araújo wrote:<br>
> <o:p></o:p></p>
<p class="MsoNormal">> John,<o:p></o:p></p>
<p class="MsoNormal">> <o:p></o:p></p>
<p class="MsoNormal">> <o:p></o:p></p>
<p class="MsoNormal">> <o:p></o:p></p>
<p class="MsoNormal">> Thank you the fast reply.<o:p></o:p></p>
<p class="MsoNormal">> <o:p></o:p></p>
<p class="MsoNormal">> <o:p></o:p></p>
<p class="MsoNormal">> <o:p></o:p></p>
<p class="MsoNormal">> The .before hint did the trick for the
LP I sent you. However, we<o:p></o:p></p>
<p class="MsoNormal">> came across a different situation
where the mipstart is accepted but<o:p></o:p></p>
<p class="MsoNormal">> CBC reports infeasibility and exits
without finding an integer<o:p></o:p></p>
<p class="MsoNormal">> solution, even though it exists:<o:p></o:p></p>
<p class="MsoNormal">> <o:p></o:p></p>
<p class="MsoNormal">><br>
That was more interesting - a bug which was unlikely to occur
in normal use, but could. As the code had a cutoff value, it
added that as a constraint in preprocessing and there was a
bug there.<br>
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> By the way, what is the reason for
building the initial solution only<o:p></o:p></p>
<p class="MsoNormal">> after pre-processing by default? Would
it make sense to do it the<o:p></o:p></p>
<p class="MsoNormal">> other way around - by default before,
with an option to do it only<o:p></o:p></p>
<p class="MsoNormal">> after?<o:p></o:p></p>
<p class="MsoNormal">> <o:p></o:p></p>
<p class="MsoNormal">> <o:p></o:p></p>
<p class="MsoNormal">I did not write that bit of code and don't
really want to do too much to it. Some information which
could get used in heuristics can not easily be passed across
when creating solution before - so on the whole it is more
efficient to try and do work on preprocessed problem.<br>
> <o:p></o:p></p>
<p class="MsoNormal">> One last thing: trunk appears to be
broken. We tried compiling it and<o:p></o:p></p>
<p class="MsoNormal">> tripped over this error:<o:p></o:p></p>
<p class="MsoNormal">> <o:p></o:p></p>
<p class="MsoNormal">><br>
My error (you picked it up before I was told off this morning
after standard Coin builds were done). I am doing some
development in trunk and thought I might as well commit some
extra changes instead of taking them out temporarily. However
I forgot that I would also need to commit changes from
CoinUtils.<br>
<o:p></o:p></p>
<p class="MsoNormal">John Forrest<o:p></o:p></p>
</div>
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<pre class="moz-signature" cols="72">--
==================================================
Haroldo Gambini Santos
D.Sc, Computer Science
Universidade Federal de Ouro Preto
<a class="moz-txt-link-freetext" href="http://www.decom.ufop.br/haroldo/">http://www.decom.ufop.br/haroldo/</a></pre>
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