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<div class="moz-cite-prefix">Unable to reproduce bug.<br>
<br>
There was a bug in CglProbing which I fixed.<br>
<br>
I get attached log<br>
<br>
John Forrest<br>
<br>
On 12/01/17 15:43, Haroldo Gambini Santos wrote:<br>
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cite="mid:09c06a42-c2e1-de6b-4ba0-e6a0efdcdaad@gmail.com"
type="cite">
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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>
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<blockquote cite="mid:02ed01d26ce7$aa5bbdf0$ff1339d0$@siscog.pt"
type="cite">
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<p class="MsoNormal"><span lang="EN-GB">John,</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoNormal"><span lang="EN-GB">Regarding the other
thread (assertion failure), your fix seems to work,
thanks.</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoNormal"><span 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:</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoNormal"><span lang="EN-GB"><a
moz-do-not-send="true" class="moz-txt-link-freetext"
href="https://drive.google.com/open?id=0B-5kPpSmX5imVllVaUdxVWY3TGc"><a class="moz-txt-link-freetext" href="https://drive.google.com/open?id=0B-5kPpSmX5imVllVaUdxVWY3TGc">https://drive.google.com/open?id=0B-5kPpSmX5imVllVaUdxVWY3TGc</a></a></span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoNormal"><span lang="EN-GB">Breaking it down:</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoListParagraph"><span lang="EN-GB"><span>·<span>
</span></span></span><span lang="EN-GB">cbc -import
test.lp -mipstart ms.before.txt</span></p>
<p class="MsoNormal"><span lang="EN-GB">Reports infeasibility.</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoListParagraph"><span lang="EN-GB"><span>·<span>
</span></span></span><span lang="EN-GB">cbc -import
test.lp -mipstart ms.txt</span></p>
<p class="MsoNormal"><span lang="EN-GB">Solves problem to
optimality, but rejects MIP start.</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoNormal"><span lang="EN-GB">The files ms.txt and
ms.before.txt have exactly the same solution.</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoNormal"><span lang="EN-GB">We traced the issue to
this constraint:</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoNormal"><span lang="EN-GB"> __one = 1</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoNormal"><span lang="EN-GB">If we remove it, the
.before MIP start is accepted and the model is solved to
optimality.</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoNormal"><span lang="EN-GB">In this particular
case, we can do without the </span><span lang="EN-GB">__one</span><span
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.</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoNormal"><span lang="EN-GB">Can you take a look at
it?</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<p class="MsoNormal"><span lang="EN-GB">Thanks,</span></p>
<p class="MsoNormal"><span lang="EN-GB">Rúdi Araújo.</span></p>
<p class="MsoNormal"><span lang="EN-GB"> </span></p>
<div>
<div>
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span
lang="EN-US"> John Forrest [<a moz-do-not-send="true"
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 moz-do-not-send="true"
class="moz-txt-link-rfc2396E"
href="mailto:rudi.araujo@siscog.pt"><rudi.araujo@siscog.pt></a><br>
<b>Cc:</b> 'Tiago Maduro Dias' <a
moz-do-not-send="true" class="moz-txt-link-rfc2396E"
href="mailto:tiago@siscog.pt"><a class="moz-txt-link-rfc2396E" href="mailto:tiago@siscog.pt"><tiago@siscog.pt></a></a>;
'Luís Borges de Oliveira' <a moz-do-not-send="true"
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</span></p>
</div>
</div>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Rudi,<br>
<br>
On 04/01/17 17:39, Rudi Araújo wrote:<br>
> </p>
<p class="MsoNormal">> John,</p>
<p class="MsoNormal">> </p>
<p class="MsoNormal">> </p>
<p class="MsoNormal">> </p>
<p class="MsoNormal">> Thank you the fast reply.</p>
<p class="MsoNormal">> </p>
<p class="MsoNormal">> </p>
<p class="MsoNormal">> </p>
<p class="MsoNormal">> The .before hint did the trick for
the LP I sent you. However, we</p>
<p class="MsoNormal">> came across a different situation
where the mipstart is accepted but</p>
<p class="MsoNormal">> CBC reports infeasibility and exits
without finding an integer</p>
<p class="MsoNormal">> solution, even though it exists:</p>
<p class="MsoNormal">> </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>
</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">> By the way, what is the reason for
building the initial solution only</p>
<p class="MsoNormal">> after pre-processing by default?
Would it make sense to do it the</p>
<p class="MsoNormal">> other way around - by default
before, with an option to do it only</p>
<p class="MsoNormal">> after?</p>
<p class="MsoNormal">> </p>
<p class="MsoNormal">> </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>
> </p>
<p class="MsoNormal">> One last thing: trunk appears to be
broken. We tried compiling it and</p>
<p class="MsoNormal">> tripped over this error:</p>
<p class="MsoNormal">> </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>
</p>
<p class="MsoNormal">John Forrest</p>
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<pre class="moz-signature" cols="72">--
==================================================
Haroldo Gambini Santos
D.Sc, Computer Science
Universidade Federal de Ouro Preto
<a moz-do-not-send="true" class="moz-txt-link-freetext" href="http://www.decom.ufop.br/haroldo/">http://www.decom.ufop.br/haroldo/</a></pre>
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