Allow user to have already created the disjunction binary variable before calling the disjunction macro #24

Author: hdavid16

examples/ex5.jl

@@ -3,6 +3,7 @@ using DisjunctiveProgramming
 
 m = Model()
 @variable(m, -10 ≤ x ≤ 10)
+@variable(m, z[1:2], Bin) #create binary variable for disjunction
 
 nl_con1 = @NLconstraint(m, exp(x) >= 1)
 nl_con2 = @NLconstraint(m, exp(x) <= 2)

src/disjunction.jl

@@ -4,10 +4,20 @@ function add_disjunction(m::Model,disj...;reformulation,M=missing,eps=1e-6,name=
     @assert length(disj) > 1 "At least 2 disjuncts must be included."
     #create binary indicator variables for each disjunction
     disj_name = ismissing(name) ? Symbol("disj",gensym()) : name
-    @assert isnothing(variable_by_name(m,string(disj_name))) "Name for disjunction binary cannot be the same as an existing variable."
-    eval(:(@variable($m, $disj_name[i = 1:length($disj)], Bin)))
+    #check if indicator variable with that name already exists
+    if disj_name in keys(object_dictionary(m))
+        #check that the existing name is for a valid binary variable that can be used for the disjunction
+        type_check = m[disj_name] isa Vector{VariableRef} #check it is a variable
+        type_check2 = all([is_binary(disj_var_i) for disj_var_i in m[disj_name]]) #check it is binary
+        dim_check = length(m[disj_name]) >= length(disj) #check the number of indices is at least the number of disjuncts
+        @assert type_check && dim_check && type_check2 "An object of name $name is already attached to this model, but is not valid for use in the disjunction. $name must be a Vector of binary variables with dimension greater than or equal to that of the disjunction."
+    else #create variable if it doesn't exist
+        eval(:(@variable($m, $disj_name[i = 1:length($disj)], Bin)))
+    end
+    
     #enforce exclussive OR
-    eval(:(@constraint($m,sum($disj_name[i] for i = 1:length($disj)) == 1)))
+    disj_var = m[disj_name]
+    eval(:(@constraint($m,sum($disj_var[i] for i = 1:length($disj)) == 1)))
     #apply reformulation
     param = reformulation == :BMR ? M : eps
     reformulate(m, disj, disj_name, reformulation, param)