Add has_start_value function (#3157)

Author: jd-lara

docs/src/reference/variables.md

@@ -38,6 +38,7 @@ variable_by_name
 ## Start values
 
 ```@docs
+has_start_value
 set_start_value
 start_value
 ```

src/variables.jl

@@ -1000,6 +1000,15 @@ function start_value(v::VariableRef)::Union{Nothing,Float64}
     return MOI.get(owner_model(v), MOI.VariablePrimalStart(), v)
 end
 
+"""
+    has_start_value(variable::AbstractVariableRef)
+
+Return `true` if the variable has a start value set otherwise return `false`.
+
+See also [`set_start_value`](@ref).
+"""
+has_start_value(v::AbstractVariableRef)::Bool = start_value(v) !== nothing
+
 """
     set_start_value(variable::VariableRef, value::Union{Real,Nothing})
 

test/JuMPExtension.jl

@@ -247,7 +247,8 @@ function JuMP.unfix(vref::MyVariableRef)
     )
 end
 function JuMP.start_value(vref::MyVariableRef)::Union{Nothing,Float64}
-    return variable_info(vref).start
+    info = variable_info(vref)
+    return info.has_start ? info.start : nothing
 end
 function JuMP.set_start_value(vref::MyVariableRef, start)
     info = variable_info(vref)

test/variable.jl

@@ -220,8 +220,9 @@ function test_variable_starts_set_get(ModelType, ::Any)
     JuMP.set_start_value.(x, x0)
     @test JuMP.start_value.(x) == x0
     @test JuMP.start_value.([x[1], x[2], x[3]]) == x0
-
+    @test JuMP.has_start_value(x[1]) == true
     @variable(model, y[1:3, 1:2])
+    @test JuMP.has_start_value(y[1, 1]) == false
     @test_throws DimensionMismatch JuMP.set_start_value.(y, collect(1:6))
 end