Bug when objective changed with different constant

[odow]

Reported on https://discourse.julialang.org/t/bug-in-detecting-changed-constant-in-objective-function/91235

julia> using Gurobi

julia> const MOI = Gurobi.MOI
MathOptInterface

julia> model = Gurobi.Optimizer();

julia> x = MOI.add_variable(model)
MathOptInterface.VariableIndex(1)

julia> MOI.set(model, MOI.ObjectiveSense(), MOI.MAX_SENSE)

julia> attr = MOI.ObjectiveFunction{MOI.ScalarAffineFunction{Float64}}()
MathOptInterface.ObjectiveFunction{MathOptInterface.ScalarAffineFunction{Float64}}()

julia> MOI.set(model, attr, 1.0 * x - 1.0)

julia> MOI.set(model, attr, 1.0 * x + 1.0)

julia> MOI.get(model, attr)
MathOptInterface.ScalarAffineFunction{Float64}(MathOptInterface.ScalarAffineTerm{Float64}[MathOptInterface.ScalarAffineTerm{Float64}(1.0, MathOptInterface.VariableIndex(1))], -1.0)

Final constant should be +1.0, not -1.0.

[odow]

Issue is upstream:

julia> using Gurobi

julia> p = Ref{Cdouble}(NaN)
Base.RefValue{Float64}(NaN)

julia> envP, modelP = Ref{Ptr{Cvoid}}(), Ref{Ptr{Cvoid}}()
(Base.RefValue{Ptr{Nothing}}(Ptr{Nothing} @0x0000000000000006), Base.RefValue{Ptr{Nothing}}(Ptr{Nothing} @0x000000000000000c))

julia> GRBemptyenv(envP)
0

julia> GRBstartenv(envP[])
0

julia> GRBnewmodel(envP[], modelP, "", 0, C_NULL, C_NULL, C_NULL, C_NULL, C_NULL)
0

julia> model = modelP[]
Ptr{Nothing} @0x00007faf975b4e80

julia> GRBsetintattr(model, "ModelSense", -1)
0

julia> GRBsetdblattr(model, "ObjCon", -1.0)
0

julia> GRBupdatemodel(model)
0

julia> GRBgetdblattr(model, "ObjCon", p)
0

julia> p[]
-1.0

julia> GRBsetdblattr(model, "ObjCon", 1.0)
0

julia> GRBupdatemodel(model)
0

julia> GRBgetdblattr(model, "ObjCon", p)
0

julia> p[]
-1.0

julia> # Last result should be +1.0!

[odow]

Closing because it is fixed in Gurobi v10:

julia> using Gurobi

julia> const MOI = Gurobi.MOI
MathOptInterface

julia> model = Gurobi.Optimizer();

julia> x = MOI.add_variable(model)
MathOptInterface.VariableIndex(1)

julia> MOI.set(model, MOI.ObjectiveSense(), MOI.MAX_SENSE)

julia> attr = MOI.ObjectiveFunction{MOI.ScalarAffineFunction{Float64}}()
MathOptInterface.ObjectiveFunction{MathOptInterface.ScalarAffineFunction{Float64}}()

julia> MOI.set(model, attr, 1.0 * x - 1.0)
(f.constant, ret) = (-1.0, 0)

julia> MOI.set(model, attr, 1.0 * x + 1.0)
(f.constant, ret) = (1.0, 0)

julia> MOI.get(model, attr)
MathOptInterface.ScalarAffineFunction{Float64}(MathOptInterface.ScalarAffineTerm{Float64}[MathOptInterface.ScalarAffineTerm{Float64}(1.0, MathOptInterface.VariableIndex(1))], 1.0)

julia> Gurobi._GUROBI_VERSION
v"10.0.0"