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### workforce2_vb.vb

' Copyright 2021, Gurobi Optimization, LLC
'
' Assign workers to shifts; each worker may or may not be available on a
' particular day. If the problem cannot be solved, use IIS iteratively to
' find all conflicting constraints.

Imports System
Imports System.Collections.Generic
Imports Gurobi

Class workforce2_vb
Shared Sub Main()
Try

' Sample data
' Sets of days and workers
Dim Shifts As String() = New String() {"Mon1", "Tue2", "Wed3", "Thu4", _
"Fri5", "Sat6", "Sun7", "Mon8", _
"Tue9", "Wed10", "Thu11", _
"Fri12", "Sat13", "Sun14"}
Dim Workers As String() = New String() {"Amy", "Bob", "Cathy", "Dan", _
"Ed", "Fred", "Gu"}

Dim nShifts As Integer = Shifts.Length
Dim nWorkers As Integer = Workers.Length

' Number of workers required for each shift
Dim shiftRequirements As Double() = New Double() {3, 2, 4, 4, 5, 6, _
5, 2, 2, 3, 4, 6, _
7, 5}

' Amount each worker is paid to work one shift
Dim pay As Double() = New Double() {10, 12, 10, 8, 8, 9, 11}

' Worker availability: 0 if the worker is unavailable for a shift
Dim availability As Double(,) = New Double(,) { _
{0, 1, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1, 1, 1}, _
{1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1, 0, 1, 0}, _
{0, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1}, _
{0, 1, 1, 0, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1}, _
{1, 1, 1, 1, 1, 0, 1, 1, 1, 0, 1, 0, 1, 1}, _
{1, 1, 1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 1}, _
{1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1}}

' Model
Dim env As New GRBEnv()
Dim model As New GRBModel(env)

model.ModelName = "assignment"

' Assignment variables: x(w)(s) == 1 if worker w is assigned
' to shift s. Since an assignment model always produces integer
' solutions, we use continuous variables and solve as an LP.
Dim x As GRBVar(,) = New GRBVar(nWorkers - 1, nShifts - 1) {}
For w As Integer = 0 To nWorkers - 1
For s As Integer = 0 To nShifts - 1
x(w, s) = model.AddVar(0, availability(w, s), pay(w), _
GRB.CONTINUOUS, _
Workers(w) & "." & Shifts(s))
Next
Next

' The objective is to minimize the total pay costs
model.ModelSense = GRB.MINIMIZE

' Constraint: assign exactly shiftRequirements(s) workers
' to each shift s
For s As Integer = 0 To nShifts - 1
Dim lhs As GRBLinExpr = 0
For w As Integer = 0 To nWorkers - 1
Next
Next

' Optimize
model.Optimize()
Dim status As Integer = model.Status
If status = GRB.Status.UNBOUNDED Then
Console.WriteLine("The model cannot be solved " & _
"because it is unbounded")
Exit Sub
End If
If status = GRB.Status.OPTIMAL Then
Console.WriteLine("The optimal objective is " & model.ObjVal)
Exit Sub
End If
If (status <> GRB.Status.INF_OR_UNBD) AndAlso _
(status <> GRB.Status.INFEASIBLE) Then
Console.WriteLine("Optimization was stopped with status " & status)
Exit Sub
End If

' Do IIS
Console.WriteLine("The model is infeasible; computing IIS")

' Loop until we reduce to a model that can be solved
While True
model.ComputeIIS()
Console.WriteLine(vbLf & "The following constraint cannot be satisfied:")
For Each c As GRBConstr In model.GetConstrs()
If c.IISConstr = 1 Then
Console.WriteLine(c.ConstrName)
' Remove a single constraint from the model
model.Remove(c)
Exit For
End If
Next

Console.WriteLine()
model.Optimize()
status = model.Status

If status = GRB.Status.UNBOUNDED Then
Console.WriteLine("The model cannot be solved " & _
"because it is unbounded")
Exit Sub
End If
If status = GRB.Status.OPTIMAL Then
Exit While
End If
If (status <> GRB.Status.INF_OR_UNBD) AndAlso _
(status <> GRB.Status.INFEASIBLE) Then
Console.WriteLine("Optimization was stopped with status " & _
status)
Exit Sub
End If
End While

Console.WriteLine(vbLf & "The following constraints were removed " & _
"to get a feasible LP:")
For Each s As String In removed
Console.Write(s & " ")
Next

Console.WriteLine()

' Dispose of model and env
model.Dispose()
env.Dispose()

Catch e As GRBException
Console.WriteLine("Error code: " & e.ErrorCode & ". " & e.Message)
End Try
End Sub
End Class


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