MethodError: no method matching -(::VariableRef, ::Nothing) - julia

Question

I am new to julia and I am still trying to learn about it.
I tried to use a user-defined function in my constraint.
Basically, the lookup function returns the value of the same index as the input in the array:
function lookup(scenario,zw_set,sn_set)
for i= 1:length(sn_set)
if sn_set[i] == scenario
return zw_set[i]
end
end
end
Thus I tried to apply this function in my optimization model:
m =Model(with_optimizer(CPLEX.Optimizer))
T = map(Int64, collect(1:1:5))
R = [1; 2]
#variable(m, sn[R,T] >=0,Int)
#variable(m,zw1[T]>=0, Int)
#constraint(m, [t in T], zw1[t]==lookup(sn[1,t],w1,snx1))
and I got this error:
ERROR: LoadError: MethodError: no method matching -(::VariableRef, ::Nothing)
Closest candidates are:
-(::Union{MathOptInterface.ScalarAffineFunction{T}, MathOptInterface.ScalarQuadraticFunction{T}}, ::T) where T at C:\Users\alkin\.julia\packages\MathOptInterface\DmQBj\src\Utilities\functions.jl:1125
-(::AbstractVariableRef) at C:\Users\alkin\.julia\packages\JuMP\Sp4sR\src\operators.jl:58
-(::AbstractVariableRef, ::Union{Number, LinearAlgebra.UniformScaling}) at C:\Users\alkin\.julia\packages\JuMP\Sp4sR\src\operators.jl:62
...
Stacktrace:
[1] sub_mul(::VariableRef, ::Nothing) at C:\Users\alkin\.julia\packages\MutableArithmetics\h76UA\src\MutableArithmetics.jl:31
[2] operate(::typeof(MutableArithmetics.sub_mul), ::VariableRef, ::Nothing) at C:\Users\alkin\.julia\packages\MutableArithmetics\h76UA\src\interface.jl:88
[3] operate_fallback!(::MutableArithmetics.NotMutable, ::Function, ::VariableRef, ::Nothing) at C:\Users\alkin\.julia\packages\MutableArithmetics\h76UA\src\interface.jl:275
[4] operate!(::typeof(MutableArithmetics.sub_mul), ::VariableRef, ::Nothing) at C:\Users\alkin\.julia\packages\MutableArithmetics\h76UA\src\rewrite.jl:67
[5] macro expansion at C:\Users\alkin\.julia\packages\MutableArithmetics\h76UA\src\rewrite.jl:224 [inlined]
[6] macro expansion at C:\Users\alkin\.julia\packages\JuMP\Sp4sR\src\macros.jl:380 [inlined]
[7] (::var"#5653#5654")(::Int64) at C:\Users\alkin\.julia\packages\JuMP\Sp4sR\src\Containers\macro.jl:183
[8] #28 at C:\Users\alkin\.julia\packages\JuMP\Sp4sR\src\Containers\container.jl:85 [inlined]
[9] iterate at .\generator.jl:47 [inlined]
[10] collect(::Base.Generator{JuMP.Containers.VectorizedProductIterator{Tuple{Array{Int64,1}}},JuMP.Containers.var"#28#29"{var"#5653#5654"}}) at .\array.jl:622
[11] map(::Function, ::JuMP.Containers.VectorizedProductIterator{Tuple{Array{Int64,1}}}) at .\abstractarray.jl:2099
[12] container(::Function, ::JuMP.Containers.VectorizedProductIterator{Tuple{Array{Int64,1}}}, ::Type{JuMP.Containers.DenseAxisArray}) at C:\Users\alkin\.julia\packages\JuMP\Sp4sR\src\Containers\container.jl:85
[13] container(::Function, ::JuMP.Containers.VectorizedProductIterator{Tuple{Array{Int64,1}}}) at C:\Users\alkin\.julia\packages\JuMP\Sp4sR\src\Containers\container.jl:65
[14] top-level scope at C:\Users\alkin\.julia\packages\JuMP\Sp4sR\src\macros.jl:45
w1 and snx1 has the following data:
julia> print(w1)
[0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0,0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0,0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0,1.0, 1.0, 0.0, 1.0]
julia> print(snx1)
[1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0, 20.0, 21.0, 22.0, 23.0, 24.0, 25.0, 26.0, 27.0, 28.0, 29.0, 30.0, 31.0, 32.0, 33.0, 34.0, 35.0, 36.0, 37.0, 38.0, 39.0, 40.0, 41.0, 42.0, 43.0, 44.0, 45.0, 46.0, 47.0, 48.0, 49.0, 50.0, 51.0, 52.0, 53.0, 54.0, 55.0, 56.0, 57.0, 58.0, 59.0, 60.0, 61.0, 62.0, 63.0, 64.0]
I realized that it might have to do something with the data types and array stuff?
Please help

Answer 1

Can you elaborate on what you are trying to do in the lookup function?
Your scenario in the lookup function is a JuMP variable, i.e. a VariableRef. And you are trying to do a comparison with a Julia Int.
if scenario == sn_set[i] # this is always `false`
return zw_set[i]
end
This will always not enter the if block. And your lookup function will always return nothing.
When JuMP tries to create the constraint, it does
#constraint(m, [t in T], zw1[t] == nothing)
which causes your error.
See example below that throws an error in the lookup:
using JuMP
function lookup(scenario, zw_set, sn_set)
for i in 1:length(sn_set)
if scenario == sn_set[i]
return zw_set[i]
end
end
error("Cannot find $scenario in $sn_set.")
end
w1 = [0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0,0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0,0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0,1.0, 1.0, 0.0, 1.0]
snx1 = [1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0, 20.0, 21.0, 22.0, 23.0, 24.0, 25.0, 26.0, 27.0, 28.0, 29.0, 30.0, 31.0, 32.0, 33.0, 34.0, 35.0, 36.0, 37.0, 38.0, 39.0, 40.0, 41.0, 42.0, 43.0, 44.0, 45.0, 46.0, 47.0, 48.0, 49.0, 50.0, 51.0, 52.0, 53.0, 54.0, 55.0, 56.0, 57.0, 58.0, 59.0, 60.0, 61.0, 62.0, 63.0, 64.0]
m = Model()
T = map(Int64, collect(1:1:5))
R = [1; 2]
#variable(m, sn[R,T] >=0, Int)
#variable(m, zw1[T] >= 0, Int)
#constraint(m, [t in T], zw1[t] == lookup(sn[1,t], w1, snx1))