Runtime error = '1004' - vba

Question

I'm trying to write to a program that will indicate an alphabet grade to each student in accord with their marks. Basically, the indicated grade will be shown in column C. From the data table, John will receive a grade "A" as his mark is higher than or equal to 85. Plus, the cell of his grade will be filled green and center aligned. However, for students whose mark is lower than 35, the students will receive a grade "F" in column C. Moreover, the whole row(Column A,B,C) of that student will be filled red. When I try to run the program, I get a
run time error = '1004'.
Does anyone know how to solve the problem?
**Also is my method of counting the row and column with data right?
Sub Green()
Dim Mark As Integer
Dim c As Integer
Dim r As Integer
Dim i As Integer
Dim j As Integer
r = Cells(Rows.Count, 1).End(xlUp).row
c = (Selection.End(xlToRight).row) / r
For i = 0 To r
For j = 0 To c
Mark = Range(i, "B").Value
If Mark >= 85 And Mark <= 100 Then
Range(i, "C").Value = "A"
Range(i, "C").Interior.Color = RGB(0, 255, 0)
Range(i, "C").HorizontalAlignment = xlCenter
ElseIf Mark >= 0 And Mark <= 35 Then
Range(i, "C").Value = "F"
Range(i, j).Interior.Color = RGB(255, 0, 0)
Range(i, "C").HorizontalAlignment = xlCenter
Else
End If
Next j
Next i
MsgBox ("Rows = " & r)
MsgBox ("Columns =" & c)
End Sub

Answer 1

My recommendation is that you handle conditional formatting to the application's built-in features. If you disagree, feel free to modify this code.
Function LastRow(ByVal ws As Worksheet, Optional ByVal col As Variant = 1) As Long
With ws
LastRow = .Cells(.Rows.Count, col).End(xlUp).row
End With
End Function
Sub assignLetterGrade()
Dim ws As Worksheet, i As Long, ltr As String
Set ws = ThisWorkbook.Worksheets(1)
For i = 1 To LastRow(ws, "B")
Select Case ws.Cells(i, "B")
Case 85 To 100
ltr = "A"
Case 75 To 84
ltr = "B"
Case 65 To 74
ltr = "C"
Case 55 To 64
ltr = "D"
Case 0 To 54
ltr = "F"
Case Else
MsgBox "Unknown Grade! Aborting!"
Exit Sub
End Select
ws.Cells(i, "C").Value = ltr
Next
End Sub
For conditional Formatting, you could try these steps:
Select the entire column of which you plan to format conditionally
(in my example, it's column C.
Let's create a new rule.
Now enter your Rules, one at a time. In this example, I am highlighting all cells containing "A" in green.
The question remains. Why not format within VBA?
The first thing you should understand about VBA is that it's very inefficient. While minimal code can take fractions of a second, once your loops get larger and larger the more you will notice that it takes longer to complete its task.
Also, you run into issues where your code is not pristine and may have minor errors. Minor errors can turn into major errors when you start building your project (example: using ActiveSheet comes to mind. It might work for your small project, but next thing you know you are applying formats to the wrong sheet, referencing the wrong cell's value, so on and so forth).
If you can avoid VBA, then do it. Take advantage of Microsoft's excellent conditional formatting UI. It's very efficient and much easier to manage.
Also, again this entire VBA project could have been done without using VBA at all. VBA is slow. You could have just as easily used formulas in column C to provide you with the letter grade.
A less-advanced version of a formula is to use nested If Statements.
=IF(B1>=85,"A", IF(B1>=75,"B", IF(B1>=65,"C", IF(B1>=55,"D","F"))))