VBA iteration not returning value - excel

Question

I am writing a very simple sub process to assign a letter grade to numeric grade values. I have a loop and I am trying to set the cell value to the output of my function. This seems like a very simple task but the first two iterations of my loops are not assigning any values. My Loop only goes through 4 rows.
Function get_letter(grade As Double)
Select Case grade
Case 0 To 59: letter = "F"
Case 60 To 69: letter = "D"
Case 70 To 79: letter = "C"
Case 80 To 89: letter = "B"
Case 90 To 100: letter = "A"
End Select
get_letter = letter
End Function
Sub assign_letter_grade()
Dim x As Integer
Dim grade As Range
Dim letter As Range
num_rows = Range("A2", Range("A2").End(xlDown)).Rows.Count
Set grade = Range("J2")
Set letter = Range("K2")
For x = 1 To num_rows
letter.Value = get_letter(grade.Value)
Set grade = grade.Offset(1, 0)
Set letter = letter.Offset(1, 0)
Next
End Sub
Why don't this set the values of my first two rows in the loop?

Answer 1

Try this:
Function get_letter(grade As Double) As String
if grade < 60 Then
get_letter = "F"
Elseif grade < 70 Then
get_letter = "D"
Elseif grade < 80 Then
get_letter = "C"
Elseif grade < 90 Then
get_letter = "B"
Else
get_letter = "A"
End If
End Function

Answer 2

The problem comes from the scores that is NOT included in the range. For VBA a score of 79.25 does not falls in either Case C or B. You could try to see if below would fix the problem:
Function get_letter(grade As Double)
Select Case grade
Case 0 To 59.99: letter = "F"
Case 60 To 69.99: letter = "D"
Case 70 To 79.99: letter = "C"
Case 80 To 89.99: letter = "B"
Case 90 To 100.99: letter = "A" ' assuming student can get a score over 100
End Select
get_letter = letter
End Function

Answer 3

Or using INDEX/MATCH
Function get_letter(grade As Double) As String
get_letter = Evaluate("INDEX({""F"",""D"",""C"",""B"",""A""},MATCH(" & grade & ",{0,60,70,80,90,100}))")
End Function
sample
Sub b()
Debug.Print get_letter(59.99)
Debug.Print get_letter(60)
End Sub

Answer 4

Because you start at row 2:
Set grade = Range("J2")
Set letter = Range("K2")
and then immediately offset by 1 row in your loop:
Set grade = grade.Offset(1, 0)
Set letter = letter.Offset(1, 0)
therefore missing rows 1 to 2 out and starting at Row 3. Use the x variable in your loop to correct the problem:
For x = 1 To num_rows
letter.Value = get_letter(grade.Value)
Set grade = Range("J" & x)
Set letter = Range("K" & x)
Next