Hellooooooooooooo !!!
It seems that Integer Programs solved with CPLEX sometimes have the wrong
> bounds on binary variables. For instance,
>
Well, as you say CPLEX defined a "binary" type. And does not associate
bounds with such variables, because it knows it is binary. Anyway :
sage: p=MixedIntegerLinearProgram(solver="cplex");
sage: b=p.new_variable(binary=True)
sage: p.add_constraint(b[0] == 2)
sage: p.solve()
...
MIPSolverException: 'CPLEX: The primal has no feasible solution'
Soooooo well. No problem.
{0: 2.0, 1: 0.0}
>
> Note that x[0] has the solution 2, which shouldn't happen for a binary
> variable.
>
>
When I ran the example of code you gave (removing the comment before
binary=True, I get the solution 0:1, 1:1, which is good. If you really get
a variable equal to 2 on this example, something is dead wrong indeed.
> In the output, the upper bound of the variable is not printed correctly, but
> I'm not sure if that's a problem, since an upper bound should not need to be
> explicitly set for a binary variable.
>
>
Hmmmm... The trick is that Sage stores no information on the LP. Everything
is stored in the backend. So if CPLEX does not store as "variable bounds" 0
and 1, then Sage does not know it and displays the bounds that are stored
in CPLEX's data structure. Though it remembers that the variable is binary
of course.
> In other instances when using binary variables with CPLEX, they seem to
> behave as they should.
>
> This issue is more problematic than it appears, since if CPLEX is installed
> it becomes the default IP/LP solver, and other things can randomly fail. For
> instance,
>
> sage.combinat.integer_vector.gale_ryser_theorem([2]*5,[5]*2)
>
> should give a 5x2 matrix of all 1's but instead gives
>
> [2 0]
> [2 0]
> [1 1]
> [0 2]
> [0 2]
>
> I am using Sage 5.12 and IBM ILOG CPLEX 12.5.1.0. I took a look in
> cplex_backend.pyx, but nothing immediately jumped out as a problem.
>
>
Well...Really, when I run this what I get is :
sage: sage.combinat.integer_vector.gale_ryser_theorem([2]*5,[5]*2)
[1 1]
[1 1]
[1 1]
[1 1]
[1 1]
> Thanks for any insight that anyone has!
>
>
Well, I would be glad to help but I would need to be able to reproduce one
of the bugs for a start :-P
Nathann
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