Question about write_lp

[Jernej]

Hello,

I think there might be a bug in how write_lp handles the objective function. Consider the following simple ILP formulation for signed domination 

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def signed(G):

    I = MixedIntegerLinearProgram(maximization=false)

    x = I.new_variable(binary=True)

    for u in G:

        I.add_constraint(I.sum(2*x[v]-1 for v in (G[u]+[u])) >=1 )

    I.set_objective(I.sum(2*x[u]-1 for u in G))

    return I

===

The respective formulation for the Petersen graph gives the following objective function,

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G = graphs.PetersenGraph()

I = signed(G)

I.show()

Minimization:

2.0 x_0 + 2.0 x_1 + 2.0 x_2 + 2.0 x_3 + 2.0 x_4 + 2.0 x_5 + 2.0 x_6 + 2.0 x_7 + 2.0 x_8 + 2.0 x_9 - 10.0

.................

===

which is fine. Now if we write the LP problem to a file the subtracted constant is missed:

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I.write_lp('foo')

$ cat foo.lp

Minimize
obj: 2 x0 + 2 x1 + 2 x2 + 2 x3 + 2 x4 + 2 x5 + 2 x6 + 2 x7 + 2 x8 + 2 x9

.........

===

Hence by solving this LP in Sage or with a *.lp solver, one obtains different results. Am I missing something or is this a bug in write_lp?

Best,

Jernej