Creating Functions

' Simplest possible function
Private Function Square(n As Double) As Double
    Return n * n
End Function

lblResult.Text = Square(7).ToString()           ' "49"
lblResult.Text = Square(Square(2)).ToString()    ' "16" -- can nest calls

' Function with input validation and multiple Return paths
Private Function Divide(a As Double, b As Double) As String
    If b = 0 Then Return "Cannot divide by zero"
    Return (a / b).ToString("G")
End Function

lblDiv.Text = Divide(10, 3)    ' "3.33333333333333"
lblDiv.Text = Divide(5,  0)    ' "Cannot divide by zero"

' Boolean predicate -- name starts with "Is" or "Has" by convention
Private Function IsEven(n As Integer) As Boolean
    Return n Mod 2 = 0
End Function

If IsEven(42) Then lblStatus.Text = "Even!"

' Function called in a loop -- building a formatted list
Private Function FormatScore(name As String, score As Integer) As String
    Dim grade = If(score >= 90, "A+", If(score >= 80, "A", If(score >= 70, "B", If(score >= 60, "C", "F"))))
    Return $"{name,-20} {score,3}  {grade}"
End Function

lstScores.Items.Add(FormatScore("Alice Lim",   92))
lstScores.Items.Add(FormatScore("Bob Tan",     74))
lstScores.Items.Add(FormatScore("Carol Wong",  58))

' Integer return
Private Function Factorial(n As Integer) As Long
    If n <= 1 Then Return 1
    Dim result As Long = 1
    For i As Integer = 2 To n
        result *= i
    Next
    Return result
End Function

lblFact.Text = Factorial(10).ToString()   ' "3628800"

' Double return -- interest calculation
Private Function CompoundInterest(principal As Decimal, rate As Double,
                                   years As Integer) As Decimal
    Return CDec(principal * (1 + rate) ^ years)
End Function

lblFV.Text = CompoundInterest(10000, 0.05, 10).ToString("C2")   ' "$16,288.95"

' Boolean return -- email validator
Private Function IsValidEmail(email As String) As Boolean
    If String.IsNullOrWhiteSpace(email) Then Return False
    Dim atPos = email.IndexOf("@"c)
    If atPos < 1 Then Return False
    Dim dotPos = email.LastIndexOf("."c)
    Return dotPos > atPos + 1 AndAlso dotPos < email.Length - 1
End Function

btnSubmit.Enabled = IsValidEmail(txtEmail.Text)

' String return -- full name formatter
Private Function FormatFullName(first As String, last As String,
                                   Optional title As String = "") As String
    Dim base = (first.Trim() & " " & last.Trim()).Trim()
    Return If(title <> "", title & " " & base, base)
End Function

lblName.Text = FormatFullName("Alice", "Lim")            ' "Alice Lim"
lblName.Text = FormatFullName("Alice", "Lim", "Dr.")    ' "Dr. Alice Lim"

' Nullable return -- safe TryParse wrapper
Private Function TryParseInt(s As String) As Integer?
    Dim n As Integer
    Return If(Integer.TryParse(s, n), n, CType(Nothing, Integer?))
End Function

Dim age = TryParseInt(txtAge.Text)
If age.HasValue Then
    lblAge.Text = $"Age: {age.Value}"
Else
    lblAge.Text = "Not a number"
End If

' Factorial (recursive) -- base case: n = 0 or 1
Private Function FactR(n As Integer) As Long
    If n <= 1 Then Return 1        ' base case
    Return n * FactR(n - 1)          ' recursive case
End Function

' FactR(5) = 5 * FactR(4)
'           = 5 * 4 * FactR(3)
'           = 5 * 4 * 3 * FactR(2)
'           = 5 * 4 * 3 * 2 * FactR(1)
'           = 5 * 4 * 3 * 2 * 1 = 120

' Fibonacci (recursive)
Private Function FibR(n As Integer) As Long
    If n <= 1 Then Return n           ' base cases: Fib(0)=0, Fib(1)=1
    Return FibR(n - 1) + FibR(n - 2)  ' two recursive calls
End Function

' GCD using Euclid's algorithm (recursive)
Private Function GCD(a As Integer, b As Integer) As Integer
    If b = 0 Then Return a            ' base case
    Return GCD(b, a Mod b)            ' recursive: GCD(48,18) -> GCD(18,12) -> GCD(12,6) -> GCD(6,0)
End Function

lblGCD.Text = GCD(48, 18).ToString()   ' "6"

' Power function (recursive): base ^ exponent
Private Function Pow(base_ As Double, exp As Integer) As Double
    If exp = 0 Then Return 1                                ' base case: x^0 = 1
    If exp < 0 Then Return 1 / Pow(base_, -exp)             ' handle negatives
    Return base_ * Pow(base_, exp - 1)
End Function

' Overloaded Add -- different parameter types
Private Function Add(a As Integer, b As Integer) As Integer
    Return a + b
End Function

Private Function Add(a As Double, b As Double) As Double
    Return a + b
End Function

Private Function Add(a As String, b As String) As String
    Return a.Trim() & " " & b.Trim()
End Function

Add(2, 3)             ' 5           (Integer version called)
Add(2.5, 3.7)        ' 6.2         (Double version called)
Add("Alice", "Lim")  ' "Alice Lim" (String version called)

' Practical overload: FormatMoney
Private Function FormatMoney(amount As Decimal) As String
    Return amount.ToString("C2")        ' uses current culture
End Function

Private Function FormatMoney(amount As Decimal, currency As String) As String
    Return $"{currency} {amount:N2}"
End Function

Private Function FormatMoney(amount As Decimal, currency As String, showSymbol As Boolean) As String
    Return If(showSymbol, $"{currency} {amount:N2}", amount.ToString("N2"))
End Function

FormatMoney(1234.5)                        ' "$1,234.50"
FormatMoney(1234.5, "RM")                  ' "RM 1,234.50"
FormatMoney(1234.5, "USD", False)          ' "1,234.50"

' Single-line lambda function: Function(params) expression
Dim square    = Function(x As Double) x * x
Dim greet     = Function(name As String) "Hello, " & name
Dim isAdult   = Function(age As Integer) age >= 18

lblResult.Text = square(5).ToString()       ' "25"
lblResult.Text = greet("Bob")               ' "Hello, Bob"
If isAdult(20) Then lblStatus.Text = "Adult"

' Pass lambda to a method -- Func(Of T, TResult) delegate
Dim numbers = New List(Of Integer) From {1, 2, 3, 4, 5, 6, 7, 8}

' LINQ with lambda: filter and transform
Dim evens  = numbers.Where(Function(n) n Mod 2 = 0)    ' {2,4,6,8}
Dim squares = numbers.Select(Function(n) n * n)         ' {1,4,9,16,25,36,49,64}
Dim bigEvens = numbers.Where(Function(n) n Mod 2 = 0 AndAlso n > 4) ' {6,8}

lstResult.DataSource = evens.ToList()

' Multi-line lambda function
Dim classify = Function(score As Integer) As String
                    If score >= 90 Then Return "A+"
                    If score >= 80 Then Return "A"
                    If score >= 70 Then Return "B"
                    Return "F"
                End Function

lblGrade.Text = classify(85)   ' "A"

' Lambda Sub (no return value)
Dim logItem = Sub(msg As String) lstLog.Items.Add($"[{DateTime.Now:HH:mm:ss}] {msg}")
logItem("App started")
logItem("Connection established")

' Monthly mortgage payment: P * r*(1+r)^n / ((1+r)^n - 1)
Private Function MortgagePayment(principal As Decimal,
                                  annualRate As Double,
                                  years As Integer) As Decimal
    If annualRate = 0 Then Return principal / (years * 12)   ' zero-interest edge case
    Dim r = annualRate / 12
    Dim n = years * 12
    Return CDec(CDbl(principal) * r * (1 + r) ^ n / ((1 + r) ^ n - 1))
End Function

' Compound interest: A = P*(1 + r/n)^(n*t)
Private Function FutureValue(principal As Decimal, annualRate As Double,
                               compoundsPerYear As Integer, years As Integer) As Decimal
    Return CDec(CDbl(principal) * (1 + annualRate / compoundsPerYear) ^ (compoundsPerYear * years))
End Function

' Simple VAT calculator
Private Function AddSST(amount As Decimal, Optional rate As Double = 0.06) As Decimal
    Return Math.Round(amount * CDec(1 + rate), 2)
End Function

' Usage in event handler
Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
    Dim p As Decimal
    Dim r As Double
    Dim y As Integer
    If Not Decimal.TryParse(txtPrincipal.Text, p) Then Return
    If Not Double.TryParse(txtRate.Text, r)     Then Return
    If Not Integer.TryParse(txtYears.Text, y)   Then Return

    lblMonthly.Text = MortgagePayment(p, r / 100, y).ToString("C2")
    lblFV.Text      = FutureValue(p, r / 100, 12, y).ToString("C2")
End Sub