MIP: Several fixes regarding constant terms in the objective function

[mkoeppe]

Contrary to documentation, one cannot set a constantly zero objective function by passing 0 to set_objective.

    sage: p = MixedIntegerLinearProgram(maximization=True)
    sage: x = p.new_variable(nonnegative=True)
    sage: p.set_objective(0)
    AttributeError: 'sage.rings.integer.Integer' object has no attribute 'dict'

This ticket fixes that and actually allows any field number (which ends up in the obj_constant_term).

Moreover, the backend slot obj_constant_term is accessed directly by MixedIntegerLinearProgram.show; it should instead be exposed by a backend method, which this ticket adds as well.

Finally, the show method added an extraneous blank line when a nonzero obj_constant_term was present. Fixed and doctest added.

CC: @dimpase @videlec @jdemeyer

Component: numerical

Author: Matthias Koeppe

Branch/Commit: ad40508

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/20337

[mkoeppe]

Branch: u/mkoeppe/mip_backends__obj_constant_term_should_be_exposed_by_a_backend_method

[mkoeppe]

New commits:

62a4da0	objective_constant_term: New MIP backend method
edb9d7f	MixedIntegerLinearProgram.show: Remove extraneous blank line when objective constant term is nonzero; add doctest
ad40508	MixedIntegerLinearProgram.set_objective: Accept constants as objective

[mkoeppe]

Commit: ad40508

[dimpase]

comment:3

do all backends support this? how do we know?

[mkoeppe]

comment:4

Objective constant terms are already part of the backend interface.

#20323 addresses the issue of making the testing of backends more rigorous.

[dimpase]

comment:5

ok.

[dimpase]

Reviewer: Dima Pasechnik

[vbraun]

comment:6

Author name

[dimpase]

Author: Matthias Koeppe

[vbraun]

Changed branch from u/mkoeppe/mip_backends__obj_constant_term_should_be_exposed_by_a_backend_method to ad40508