Working with Do...Loop statement

 Basically Do...Loop statement consist 2 types:

• While Condition
  Either Check First or Check Last
  
• Until Condition
  Either Check First or Check Last

Before proceed we must understand basic operator as below:

Basic Operator
1	<	Less than
2	<=	Less than or equal to
3	>	Greater than
4	>=	Greater than or equal to
5	=	Equal to
6	<>	Not equal to
7	""	Empty

Base on table below we are going to use Do..Loop statement to get total monthly sales.

In case we use Range("A3") to Range("A14"). For Do...Loop While Check whether Not Equal to empty (<> "") but for Do...Loop Until  Check whether Equal to empty (=""). Under certain circumstances the result may be different can cause bug in your code. Please be careful.

VBA Code:

1) While Condition Check first.

Option Explicit
Sub DoWhile_CheckFirst()
    
    Dim i As Integer
    i = 3
    With ActiveSheet
        Do While .Range("A" & i) <> ""
            .Range("D" & i) = .Range("B" & i) + .Range("C" & i)
            i = i + 1
        Loop
    End With

End Sub

2) While Condition Check last.

Option Explicit
Sub DoWhile_CheckLast()
    
    Dim i As Integer
    i = 3
    With ActiveSheet
        Do
            .Range("D" & i) = .Range("B" & i) + .Range("C" & i)
            i = i + 1
        Loop While .Range("A" & i) <> ""
    End With

End Sub

3) Until Condition Check first.

Option Explicit
Sub DoUntil_CheckFirst()
    
    Dim i As Integer
    i = 3
    With ActiveSheet
        Do Until .Range("A" & i) = ""
            .Range("D" & i) = .Range("B" & i) + .Range("C" & i)
            i = i + 1
        Loop
    End With

End Sub

4) While Condition Check last.

Option Explicit
Sub DoUntil_CheckLast()
    
    Dim i As Integer
    i = 3
    With ActiveSheet
        Do
            .Range("D" & i) = .Range("B" & i) + .Range("C" & i)
            i = i + 1
        Loop Until .Range("A" & i) = ""
    End With

End Sub

Note:
Do Loop without counter will create infinite loop and your computer will hang. The counter for above example is i = i + 1.

Microsoft Reference - Do..Loop statement
Microsoft Reference - Using Do..Loop statement

Practice makes perfect. Thank You.