The basic problem with your code is that you haven't enclosed your string variables in quotes. You also haven't dimensioned your 'i' variable and your code could be made more efficient. Try:
Code:
Sub group()
Dim i As Long
For i = 1 To 6
Select Case Range("a" & i).Value
Case "apple": Range("C" & i).Value = 1
Case "orange": Range("C" & i).Value = 2
Case "banana": Range("C" & i).Value = 3
End Select
Next i
End Sub
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