VBA oddity, modifying For loop

[Gates Is Antichrist]

It's refreshing to know that to make VBA even more lightning fast it doesn't check the For limiting variable. /sarcasm mode off

Does the following surprise anyone? Surprises me. (And yes, I know what an Exit For is. This code is designed solely to illustrate the point.)

Sub JimmyLoopVar()
    Dim i As Long, K As Long
    K = 6
    For i = 0 To K
        If (i > 3) Then K = 2
        Debug.Print i; ; K
    Next
End Sub

So the loop runs 7 times regardless of K! (At least on XL03 )

 

[Jonmo1]

K = 6
For i = 0 To K

The first loop is the only time it calculates K in the For/Next...

at the time it first reads for i = 0 to K,
it actually reads for i = 0 to 6.

after the first loop when it reads next, it doesn't REread for i = 0 to K...it goes straight to the first line after the for line..

You will see this demonstrated by Stepping Through the macro using F8.
it will only highlight the line "for i = 0 to K" on the first time through. Each loop after the first will go directly to the line "If (i > 3) Then K = 2"

 

[Gates Is Antichrist]

That's my point! Wow, VBA is more optimized than C

 

[Boller]

Would you expect this to do something other than it does? :-

Dim rng As Range, cell As Range
Set rng = [A1:A5]
For Each cell In rng
    Set rng = rng.Offset(, 1)
    MsgBox rng.Address
Next

 

[Gates Is Antichrist]

Where I said "it doesn't check" it should have said "but once," so jonmo1's expansive explanation fits what I literally said. But man! The way this goes, VBA is more optimized than C

 

[Gates Is Antichrist]

That is a clever and twisted example Boller Now THAT's something I'd never touch - messing for For Each's province!

 

[Boller]

So I suppose you are no longer surprised.

 

[Gates Is Antichrist]

Well For Each is another matter. It necessitates a "list" or "collection," loosely speaking. Meanwhile integer for loops are very simple - only about 6 assembler statements. Now, it looks more like 5

BTW I'm not getting subscription notifications today...

 

[Scott Huish]

If you had expected an endless loop, you are changing the wrong variable:

Sub JimmyLoopVar()
    Dim i As Long, K As Long
    K = 6
    For i = 0 To K
        If (i > 3) Then i = 2
        Debug.Print i; ; K
    Next
End Sub

 

[Gates Is Antichrist]

No, I expected the loop to bail out. (You didn't?!) Here's the thing: in my recollection I' ve never see a compiled or interpreted language that did NOT check the limiting For variable every time. (Someone please check BASIC and confirm or deny this.) The key here is the word "variable."

Everyone's probably aware of the side effect of modifying the looping control variable, in almost any language. And there's always been controversy over whether the looping variable gets incremented "one extra time" or not when it's made it to the limiting variable. Now, in VBA there's no side effect on the limiting variable.

BASIC is what probably should be the best guide. Could someone check that?