LINQ & Collections

' --- Define a simple class ---
Public Class Student
    Public Property Name  As String
    Public Property Grade As Double
    Public Property Class As String
End Class

' --- Create and populate ---
Dim students As New List(Of Student) From {
    New Student With {.Name="Ali",   .Grade=88, .Class="4A"},
    New Student With {.Name="Alice", .Grade=92, .Class="4B"},
    New Student With {.Name="David", .Grade=55, .Class="4B"}
}

' --- Common methods ---
students.Add(New Student With {.Name="Farah", .Grade=95, .Class="5A"})
students.Remove(students(0))            ' remove by reference
students.RemoveAt(0)                    ' remove by index
students.RemoveAll(Function(s) s.Grade < 60)  ' remove matching
Dim count = students.Count              ' number of items
Dim found = students.Find(Function(s) s.Name = "Alice")  ' first match
Dim exists = students.Any(Function(s) s.Grade > 90)       ' LINQ: any match?

' --- Sort in place ---
students.Sort(Function(a, b) a.Grade.CompareTo(b.Grade))   ' ascending by grade

' --- Iterate ---
For Each s In students
    Console.WriteLine($"{s.Name}: {s.Grade}")
Next

' --- Create: Integer key → Student value ---
Dim index As New Dictionary(Of Integer, Student)()

' --- Populate from a List ---
For Each s In students
    index(s.StudentID) = s   ' add or overwrite
Next

' --- Or with LINQ ToDictionary ---
Dim byId = students.ToDictionary(Function(s) s.StudentID)

' --- Safe lookup (never throws) ---
Dim found As Student = Nothing
If index.TryGetValue(3, found) Then
    Console.WriteLine(found.Name)
Else
    Console.WriteLine("Student 3 not found")
End If

' --- Check existence ---
If index.ContainsKey(5) Then index.Remove(5)

' --- Iterate keys, values, or pairs ---
For Each pair In index
    Console.WriteLine($"ID {pair.Key}: {pair.Value.Name}")
Next

' --- Count-by-category pattern ---
Dim classCounts As New Dictionary(Of String, Integer)()
For Each s In students
    If classCounts.ContainsKey(s.Class) Then
        classCounts(s.Class) += 1
    Else
        classCounts(s.Class) = 1
    End If
Next
' Equivalent with LINQ GroupBy (see section 32.4)

' ═══ QUERY SYNTAX ═══
Dim topStudents = From s In students
                  Where s.Grade >= 80
                  Order By s.Grade Descending
                  Select s.Name, s.Grade, s.Class

' ═══ METHOD SYNTAX (identical result) ═══
Dim topStudents2 = students _
    .Where(Function(s) s.Grade >= 80) _
    .OrderByDescending(Function(s) s.Grade) _
    .Select(Function(s) New With {s.Name, s.Grade, s.Class})

' --- Where: filter ---
Dim passers  = students.Where(Function(s) s.Grade >= 60).ToList()
Dim class4A  = students.Where(Function(s) s.Class = "4A").ToList()

' --- Select: project / transform ---
Dim names  = students.Select(Function(s) s.Name).ToList()          ' List(Of String)
Dim scaled = students.Select(Function(s) s.Grade * 1.1).ToList()   ' scaled grades

' --- Ordering ---
Dim byGrade = students.OrderByDescending(Function(s) s.Grade) _
                       .ThenBy(Function(s) s.Name).ToList()

' --- Aggregates ---
Dim avg  = students.Average(Function(s) s.Grade)          ' Double
Dim max  = students.Max(Function(s) s.Grade)              ' highest grade
Dim sum  = students.Sum(Function(s) s.Grade)              ' total
Dim cnt  = students.Count(Function(s) s.Grade >= 80)     ' conditional count

' --- Existence checks ---
Dim anyFail = students.Any(Function(s) s.Grade < 50)
Dim allPass = students.All(Function(s) s.Grade >= 60)

' --- First / Single ---
Dim top    = students.OrderByDescending(Function(s) s.Grade).First()
Dim ali    = students.FirstOrDefault(Function(s) s.Name = "Ali")
If ali IsNot Nothing Then Console.WriteLine(ali.Grade)

' --- Lazy vs eager ---
Dim query  = students.Where(Function(s) s.Grade > 70)  ' LAZY — not executed yet
Dim result = query.ToList()                                 ' EAGER — runs now

' --- Group by class ---
Dim byClass = students.GroupBy(Function(s) s.Class)
For Each grp In byClass
    Console.WriteLine($"Class {grp.Key}: {grp.Count()} students, avg={grp.Average(Function(s) s.Grade):F1}")
Next

' --- Project each group into an anonymous type ---
Dim summary = students _
    .GroupBy(Function(s) s.Class) _
    .Select(Function(g) New With {
        .ClassName  = g.Key,
        .Count      = g.Count(),
        .Average    = Math.Round(g.Average(Function(s) s.Grade), 1),
        .Highest    = g.Max(Function(s) s.Grade),
        .Lowest     = g.Min(Function(s) s.Grade),
        .PassRate   = g.Count(Function(s) s.Grade >= 60) * 100 \ g.Count()
    }) _
    .OrderBy(Function(g) g.ClassName) _
    .ToList()

' Bind to DataGridView
dgvSummary.DataSource = summary  ' anonymous types work as DataSource!

' --- HAVING equivalent (filter groups) ---
Dim largeClasses = students _
    .GroupBy(Function(s) s.Class) _
    .Where(Function(g) g.Count() >= 5)  ' only classes with 5+ students
    .Select(Function(g) g.Key)
    .ToList()

Imports System.Data   ' for DataTableExtensions

' --- Filter a DataTable with LINQ ---
Dim highGraders = _dt.AsEnumerable() _
    .Where(Function(r) r.Field(Of Double)("Grade") >= 80) _
    .OrderByDescending(Function(r) r.Field(Of Double)("Grade"))

' --- Convert result back to DataTable for DataGridView ---
Dim filtered = highGraders.CopyToDataTable()
dgv.DataSource = filtered

' --- Distinct values (e.g. populate a ComboBox) ---
Dim classes = _dt.AsEnumerable() _
    .Select(Function(r) r.Field(Of String)("Class")) _
    .Distinct() _
    .OrderBy(Function(c) c) _
    .ToList()
cboClass.DataSource = classes

' --- Aggregate directly on DataTable ---
Dim avgGrade = _dt.AsEnumerable() _
    .Average(Function(r) r.Field(Of Double)("Grade"))

' --- Group DataTable rows by Class ---
Dim grouped = _dt.AsEnumerable() _
    .GroupBy(Function(r) r.Field(Of String)("Class")) _
    .Select(Function(g) New With {
        .Class   = g.Key,
        .Count   = g.Count(),
        .Average = g.Average(Function(r) r.Field(Of Double)("Grade"))
    }).ToList()
dgvSummary.DataSource = grouped