I'm trying to create an optimization model using Microsoft Solver Foundation with VB.NET in Visual Studio 2010. Basically I have a list of 3 types of employees that I need to hire (bartenders, waiters, and hostesses) each with different wages and performance ratings. I have created a decision variable for each potential employee. It is set to 0 for a no hire decision and 1 for a hire decision.
When I try to calculate the total cost of employees, the total performance score, or the sum of each type of employee, I receive an error that states that my decision variables do not yet have a value.
Is there an easier way to add these constraints (possibly in a loop) without individually listing every decision variable for the entire employee database in each constraint?
Here is the code I'm working with.
Dim myEmployee As Employee
Dim context As SolverContext = SolverContext.GetContext()
Dim model As Model = context.CreateModel()
For Each myEmployee In employeeList
If myEmployee.Type = "Bartender" Then
Dim BartenderHire As New Decision(Domain.IntegerRange(0, 1), myEmployee.ID)
barDecisionList.Add(BartenderHire)
ElseIf myEmployee.Type = "Host(ess)" Then
Dim HostHire As New Decision(Domain.IntegerRange(0, 1), myEmployee.ID)
hostDecisionList.Add(HostHire)
ElseIf myEmployee.Type = "Waiter/Waitress" Then
Dim WaiterHire As New Decision(Domain.IntegerRange(0, 1), myEmployee.ID)
waitDecisionList.Add(WaiterHire)
End If
Next
For i = 0 To barDecisionList.Count - 1
model.AddDecision(barDecisionList.Item(i))
Next
For i = 0 To hostDecisionList.Count - 1
model.AddDecision(hostDecisionList.Item(i))
Next
For i = 0 To waitDecisionList.Count - 1
model.AddDecision(waitDecisionList.Item(i))
Next
'Calculate cost of hired employees.
Dim cost As Double
cost = 0
For i = 0 To model.Decisions.Count - 1
Dim thisEmployee As Employee = employeeList.Item(i)
cost = cost + (model.Decisions(i).ToDouble * thisEmployee.Wage * 6)
Next
'Calculate total score of hired employees.
Dim totalScore As Double
totalScore = 0
For i = 0 To model.Decisions.Count - 1
Dim thisEmployee As Employee = employeeList.Item(i)
totalScore = totalScore + (model.Decisions(i).ToDouble * thisEmployee.Score)
Next
'Calculate total bartenders hired.
Dim barSum As Integer
barSum = 0
For i = 0 To barDecisionList.Count - 1
barSum = barSum + barDecisionList.Item(i)
Next
'Calculate total waiters hired.
Dim waitSum As Integer
waitSum = 0
For i = 0 To waitDecisionList.Count - 1
waitSum = waitSum + waitDecisionList.Item(i)
Next
'Calculate total hosts hired.
Dim hostSum As Integer
hostSum = 0
For i = 0 To hostDecisionList.Count - 1
hostSum = hostSum + hostDecisionList.Item(i)
Next
'Add constraints
model.AddConstraint("Bartenders_Required", barSum = bartendersRequired)
model.AddConstraint("WaitStaff_Required", waitSum = waitersRequired)
model.AddConstraint("Hosts_Required", hostSum = hostsRequired)
model.AddConstraint("Budget", cost <= budget)
model.AddGoal("Total_Score", GoalKind.Maximize, totalScore)
Dim solution As Solution = context.Solve(New SimplexDirective())
Dim report As Report = solution.GetReport
For i = 0 To model.Decisions.Count - 1
solutionList.Add(model.Decisions(i))
Next
