#12091: bug in add_constraint to MixedIntegerLinearProgram
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Reporter: dimpase | Owner: ncohen
Type: defect | Status: new
Priority: critical | Milestone: sage-5.0
Component: linear programming | Keywords:
Work_issues: | Upstream: N/A
Reviewer: | Author:
Merged: | Dependencies:
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Comment(by ncohen):
I spent days already on this problem, so I should at least update the
ticket. This does not work because of.... that :
{{{
sage: p = MixedIntegerLinearProgram()
sage: b = p.new_variable()
sage: b[2] + b[4] >= 5
5 <= x_0 +x_1
sage: 5 <= b[2] + b[4]
True
}}}
It is as easy as that. And of course, calling ``p.add_constraint(True)``
can not do anything useful. That comes from the fact that the way it is
implemented the comparison methods I wrote for the LP variables are
totally ignored when there is an integer on the left of the equation.
I went through hell (aka the coercion system) to make it work, and after
getting back from hell (writing new classes, using categories, parents
objets, elements objects, ...) Sage finally understood that it has to
coerce integers to "LinearFunction" objects before applying the ``<=``
operation.
I was happy at this point. And then I noticed that when I typed ``3*b[i]``
in Sage, it first converted the "3" to a ``LinearFunction`` object and
then tried to multiply linear functions between themselves. And multiply
linear functions is *never* a good idea.
So unless there is a magical way to make it work (without destroying all
the code... I mean, even reaching that former point amounted to something
like 100 additional lines of code totally exclusively dealing with
categories and coercion), I'd stand for big "DO NOT PUT AN INTEGER BY
ITSELF ON THE LEFT SIDE OF A LP INEQUALITY" in the doc.
But I'd prefer to find a magical way, of course..
Nathann
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/12091#comment:2>
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