[Cbc] "Integer infeasible..." issue?

[James.Gunning at csiro.au]

Hi John,
        Terrific! OK, done. That's a subtle one I must say....
Cheers & thanks
James.

James Gunning
CSIRO
Clayton, Vic., Australia
________________________________
From: cbc-bounces at list.coin-or.org [cbc-bounces at list.coin-or.org] on behalf of John Forrest [john.forrest at fastercoin.com]
Sent: Friday, 8 May 2015 9:15 PM
To: cbc at list.coin-or.org
Subject: Re: [Cbc] "Integer infeasible..." issue?

James,

It is not a bug in Cbc, but an error (feature?) in Coin's lp format reader.  The reader does not like the last two constraints - if I replace

p0_2 + p1_2 = x2
p0_3 + p1_3 = x3

by

p0_2 + p1_2 - x2 = 0
p0_3 + p1_3 - x3 = 0

everything is fine

John Forrest


On 08/05/15 09:07, James Gunning wrote:
Hello All,
        This is probably a dumb question: why does this simple .lp file

Min
obj: -10.9139 p1_3 +15.3211 x1 -3.30662 x3
s.t.
x0 - p0_2 >= 0
x2 - p0_2 >= 0
p0_2 - x0 - x2 >= -1
x0 - p0_3 >= 0
x3 - p0_3 >= 0
p0_3 - x0 - x3 >= -1
x1 - p1_2 >= 0
x2 - p1_2 >= 0
p1_2 - x1 - x2 >= -1
x1 - p1_3 >= 0
x3 - p1_3 >= 0
p1_3 - x1 - x3 >= -1
x0 + x1 = 1
x2 + x3 = 1
p0_2 + p1_2 = x2
p0_3 + p1_3 = x3
Binaries
x0 x1 x2 x3 p0_2 p0_3 p1_2 p1_3
End



Give me "Integer infeasible..." under cbc?

lp_solve gives me an integer solution

p1_3                            0
x1                              0
x3                              1
x0                              1
p0_2                            0
x2                              0
p0_3                            1
p1_2                            0



I'm sure I've done something stupid, but it may be a subtle roundoff thing?
This is cbc 2.8.12.

Best wishes all & kudos to John for keeping things going so well...
       James.






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