I'm having trouble in Visual Basic with a grade calculator for class

Question

 Public Class convertGrades
'Here is where I declare my variables
Dim numbergrade As Integer
Dim lettergrade As Char

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    'This is an If/Else statement that allows the user to
    'convert their number grade into a letter grade
    txtNumberGrade.Text = numbergrade
    lblLetterGrade.Text = lettergrade

    If (numbergrade >= 90 Or numbergrade <= 100) Then
        lettergrade = "A"
    ElseIf (numbergrade >= 80 Or numbergrade <= 79) Then
        lettergrade = "B"
    ElseIf (numbergrade >= 70 Or numbergrade <= 89) Then
        lettergrade = "C"
    ElseIf (numbergrade >= 60 Or numbergrade <= 69) Then
        lettergrade = "D"
    ElseIf (numbergrade >= 50 Or numbergrade <= 59) Then
        lettergrade = "F"
    End If
    txtNumberGrade.Focus()
End Sub

This is the exact code I have, I am using Visual Basic 2012 and am having trouble converting these numbers into letter grades. VB says I have no errors but when I run it, the program does not convert the numbers into letters. Any suggestions?

Answer 1

You swapped the high number for the B and C grades. Also, you probably want AndAlso for these checks, rather than Or . While I'm here, as a matter of good program design, you should abstract this out to a separate method. The final result:

Public Function LetterGradeFromScore(ByVal score As Integer) As Char
    If numbergrade >= 90 Then
         Return "A"c
    ElseIf numbergrade >= 80 AndAlso numbergrade <= 89 Then
        Return "B"c
    ElseIf numbergrade >= 70 AndAlso numbergrade <= 79 Then
        Return "C"c
    ElseIf numbergrade >= 60 AndAlso numbergrade <= 69 Then
        Return "D"c
    Else
        Return "F"c
    End If
End Function

'The only code that lives in this method is code that directly updates or responds to the UI
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click   
    txtNumberGrade.Text = numbergrade
    lblLetterGrade.Text = LetterGradeFromScore(numbergrade)
    txtNumberGrade.Focus()
End Sub

Another thing: VB.Net has a really nice Select Case syntax you can use for this:

Public Function LetterGradeFromScore(ByVal score As Integer) As Char
    Select Case score
        Case >= 90
            Return "A"c
        Case 80 to 89
            Return "B"c
        Case 70 to 79
            Return "C"c
        Case 60 to 69
            Return "D"c
        Case Else
           Return "F"c
    End Select
End Function

Finally, you could avoid a problem like swapping the number above, and as well as improve the code, by converting this to a kind of table lookup:

Public Function LetterGradeFromScore(ByVal score As Integer) As Char 

    'This could be a Shared Member somewhere, or even stored in a DB and populated on load
    Dim gradeMap As New SortedList(Of Integer, Char)(5)
    gradeMap.Add(90, "A"c)
    gradeMap.Add(80, "B"c)
    gradeMap.Add(70, "C"c)
    gradeMap.Add(60, "D"c)
    gradeMap.Add( 0, "F"c)

    Return gradeMap.Last(Function(m) m.Key < score).Value
End Function