Opened 4 years ago

Last modified 21 months ago

#19523 new defect

Raise an error when constraints are added to the wrong MILP

Reported by: jdemeyer Owned by:
Priority: major Milestone: sage-7.3
Component: linear programming Keywords:
Cc: Merged in:
Authors: Reviewers:
Report Upstream: N/A Work issues:
Branch: Commit:
Dependencies: Stopgaps:

Description (last modified by jdemeyer)

sage: p = MixedIntegerLinearProgram(solver="glpk")
sage: q = MixedIntegerLinearProgram(solver="glpk")
sage: q.add_constraint(p[0] <= 1)
---------------------------------------------------------------------------
GLPKError                                 Traceback (most recent call last)
...
GLPKError: glp_set_mat_row: i = 1; len = 1; invalid row length 
Error detected in file glpapi01.c at line 760

Before #19525, this would instead crash Sage. A crash still happens with COIN (#20360).

This ticket is to actually fix the error completely or give a better error message.

And low-level error message with the PPL and InteractiveLP (#20296 ) backends.

The CVX backend silently adds a new variable that is accessible only to the backend:.

sage: sage: default_mip_solver("cvxopt")
sage: sage: p = MixedIntegerLinearProgram()
sage: sage: q = MixedIntegerLinearProgram()
sage: sage: q.add_constraint(p[0] <= 1)
sage: q.number_of_variables()
1

Change History (18)

comment:1 Changed 4 years ago by jdemeyer

  • Dependencies set to #19525
  • Description modified (diff)

comment:2 Changed 4 years ago by ncohen

  • Cc ncohen added

comment:3 Changed 4 years ago by jdemeyer

  • Description modified (diff)
  • Priority changed from critical to major

comment:4 Changed 3 years ago by mkoeppe

While MIPVariable objects know which MIP they belong to, their "components" gotten by p[0] etc. are elements of LinearFunctionsParent(base_ring), and do not remember their MIP (nor even their name). So with the current design it does not seem possible to catch the error of adding constraints to the wrong MIP. So unfortunately only lower-level error checking, catching out-of-bounds indices, is possible.

comment:5 Changed 3 years ago by mkoeppe

  • Description modified (diff)

comment:6 Changed 3 years ago by jdemeyer

Given that p[0] is just a linear function, it would make the most sense if q.add_constraint(p[0] <= 1) would simply work, not give an error message.

comment:7 follow-up: Changed 3 years ago by mkoeppe

No, q has no variables at this point.

comment:8 in reply to: ↑ 7 Changed 3 years ago by jdemeyer

Replying to mkoeppe:

No, q has no variables at this point.

That's exactly my point: the variables should be added by q.add_constraint().

comment:9 follow-up: Changed 3 years ago by mkoeppe

I'm not sure that this would be a good interface. It would allow to add variables, but only variables with default settings (continuous, lower bound 0, no upper bound, no name). I think it's better to raise an error, which could help spot programming mistakes -- at least when there's a dimension mismatch between the two problems. This bound checking should be done by the backends -- see #10232.

Last edited 3 years ago by mkoeppe (previous) (diff)

comment:10 in reply to: ↑ 9 Changed 3 years ago by jdemeyer

Replying to mkoeppe:

at least when there's a dimension mismatch between the two problems.

No. It should be always an error, or never an error. If it's only an error "when there's a dimension mismatch between the two problems", that will be more confusing than either extreme.

comment:11 Changed 3 years ago by mkoeppe

Then I strongly prefer to always raise an error in this situation. This requires changes to LinearFunction (a class that is I think only used in the context of MixedIntegerLinearProgram), so that it remembers the MIP that it relates to.

The mapping from MIP variables (and their indexed components) to integer indices (designating backend columns) is determined dynamically, adding a backend column when a MIP variable component is accessed. Because of this there's simply no good way to write correct code that interchanges MIP variables between two MixedIntegerLinearPrograms, or to use LinearFunctions directly somehow (without referring to the correct MIP's variables).

Last edited 3 years ago by mkoeppe (previous) (diff)

comment:12 Changed 3 years ago by mkoeppe

  • Dependencies changed from #19525 to #19525, #20360
  • Description modified (diff)

comment:13 Changed 3 years ago by mkoeppe

  • Summary changed from Adding constraints for the wrong MILP crashes Sage to Raise an error when constraints are added to the wrong MILP

Since the crashes are now on separate tickets, I have changed the title of this ticket.

comment:14 Changed 3 years ago by mkoeppe

See also #15159, which raises a similar question when q started out as a copy of p.

comment:15 Changed 3 years ago by mkoeppe

A solution could be to change LinearFunctionsParent etc. so that it not only has remembers the base_ring but also the MIP.

comment:16 Changed 3 years ago by mkoeppe

  • Milestone changed from sage-6.10 to sage-7.3

comment:17 Changed 21 months ago by jdemeyer

  • Cc ncohen removed
  • Dependencies #19525, #20360 deleted
  • Description modified (diff)

comment:18 Changed 21 months ago by jdemeyer

  • Description modified (diff)
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