Lesson 17: Creating User-Defined Functions in VB2022

Private Function CalculateBMI(height As Single, weight As Single) As Double
    Return weight / (height * height)
End Function

Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
    Dim h As Single = Val(txtHeight.Text)
    Dim w As Single = Val(txtWeight.Text)
    
    Dim bmi As Double = CalculateBMI(h, w)
    lblResult.Text = "Your BMI is: " & Math.Round(bmi, 2)
End Sub

Private Function CalculateFutureValue(pv As Double, rate As Double, periods As Integer) As Double
    Return pv * Math.Pow(1 + rate / 100, periods)
End Function

Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
    Dim presentVal As Double = Val(txtPresent.Text)
    Dim interestRate As Double = Val(txtRate.Text)
    Dim years As Integer = Val(txtYears.Text)
    
    Dim futureVal As Double = CalculateFutureValue(presentVal, interestRate, years)
    lblResult.Text = "Future Value: $" & futureVal.ToString("#,##0.00")
End Sub

Private Function Square(ByVal num As Integer) As Integer
    num = num * num  ' Changes local copy
    Return num
End Function

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim value As Integer = 5
    Dim result As Integer = Square(value)
    
    MessageBox.Show("Result: " & result)  ' Shows 25
    MessageBox.Show("Original Value: " & value)  ' Still 5
End Sub

Private Function Square(ByRef num As Integer) As Integer
    num = num * num  ' Changes original variable
    Return num
End Function

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim value As Integer = 5
    Dim result As Integer = Square(value)
    
    MessageBox.Show("Result: " & result)  ' Shows 25
    MessageBox.Show("Original Value: " & value)  ' Now 25
End Sub

Private Function IsPrime(num As Integer) As Boolean
    If num < 2 Then Return False
    If num = 2 Then Return True
    If num Mod 2 = 0 Then Return False
    
    Dim limit As Integer = Math.Sqrt(num)
    For i As Integer = 3 To limit Step 2
        If num Mod i = 0 Then Return False
    Next
    Return True
End Function

Private Sub btnCheck_Click(sender As Object, e As EventArgs) Handles btnCheck.Click
    Dim number As Integer
    If Integer.TryParse(txtNumber.Text, number) Then
        If IsPrime(number) Then
            lblResult.Text = number & " is a prime number"
        Else
            lblResult.Text = number & " is NOT a prime number"
        End If
    Else
        MessageBox.Show("Please enter a valid integer")
    End If
End Sub

Private Function CelsiusToFahrenheit(c As Double) As Double
    Return (c * 9 / 5) + 32
End Function

Private Function FahrenheitToCelsius(f As Double) As Double
    Return (f - 32) * 5 / 9
End Function

Private Sub btnConvertCtoF_Click(sender As Object, e As EventArgs) Handles btnConvertCtoF.Click
    Dim celsius As Double
    If Double.TryParse(txtCelsius.Text, celsius) Then
        txtFahrenheit.Text = CelsiusToFahrenheit(celsius).ToString("0.0")
    Else
        MessageBox.Show("Please enter a valid number")
    End If
End Sub

Private Function CalculateGrade(score As Integer) As String
    Select Case score
        Case Is >= 90 : Return "A"
        Case Is >= 80 : Return "B"
        Case Is >= 70 : Return "C"
        Case Is >= 60 : Return "D"
        Case Else : Return "F"
    End Select
End Function

Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
    Dim score As Integer
    If Integer.TryParse(txtScore.Text, score) Then
        If score >= 0 And score <= 100 Then
            lblGrade.Text = "Grade: " & CalculateGrade(score)
        Else
            MessageBox.Show("Score must be between 0 and 100")
        End If
    Else
        MessageBox.Show("Please enter a valid number")
    End If
End Sub