Excel VBA For...Next - vba

Question

I'm a VBA newcomer. I wonder why does sum1() show 10 while sum2() show 11? Thanks in advance.
Sub sum1()
Dim x As Integer
x = 0
For x = 5 To 5
x = x + x
MsgBox x
Next
End Sub
Sub sum2()
Dim x As Integer
x = 0
For x = 5 To 5
x = x + x
Next
MsgBox x
End Sub

Answer 1

A vba for-loop increments the index variable at the end of each iteration. That is what the Next keyword does. In sum1() You get:
Sub sum1()
Dim x As Integer
x = 0 ' x is 0
For x = 5 To 5 ' x set to 5
x = x + x 'x gets 5+5=10
Next 'x gets 10+1=11
MsgBox x 'display x=11
End Sub
but in sum2() you get:
Sub sum2()
Dim x As Integer
x = 0 ' x is 0
For x = 5 To 5 ' x is 5
x = x + x 'x gets 5+5=10
MsgBox x 'display x=10
Next 'x gets 10+1 and is now 11
End Sub
I agree with #MitchWheat, it usually not good practice to modify your index variable while inside a loop. A better approach would be this:
Sub sum3()
Dim x as Integer
Dim i as Integer
x= 1
For i = 5 To 5
x = x + x
Next
MsgBox x
End Sub

Answer 2

For loops increment the loop variable at the end of the loop.
In the second code snippet, the for loop increments x from 10 to 11, and then you display it.
Whereas the first code snippets increments x AFTER you display it. This is not something specific to VBA.
To see this, run:
Sub sum1_1()
Dim x As Integer
x = 0
For x = 5 To 5
x = x + x
MsgBox x
Next
MsgBox x
End Sub
As an aside: It's not best practice to modify loop counters from inside a loop. It can lead to code that is hard to understand.