Loops are driving me loopy! How to exit a loop as soon as condition fails

I'm having trouble with conditional loops. I want to test a number of boolean functions and exit the loop immediately after the first fail. (see dummy code below. Even when condition fails on line 50, the code keeps blindly checking all remaining tests in the loop)

Is there any way to exit loops on condition failure? Short of changing all the functions to

Please Login or Register  to view this content.

(That would be as bad as what I had before which was a mass of nested IF ELSEIF. I was trying to change to a DO LOOP to improve readability but I keep hitting this problem)

Please Login or Register  to view this content.

You need to test after each assignment:

Please Login or Register  to view this content.

Or

Please Login or Register  to view this content.

Thanks for the suggestions shg. I guess I am looking for a native method to exit a loop immediately on condition fail *.

I had been googling before I started this thread and found this ( http://stackoverflow.com/questions/1...oop-in-vbs-vba ) - however the answers don't work or they fall back on test (and Exit Do) after every single function call within the loop (as I excluded in my OP)


* As opposed to having to test the variable after each function call (and then Exit Do) or abusing On Error. If they were the only options, I'd go back to the nested IF ELSE IFs I was using before!

I guess I am looking for a native method to exit a loop immediately on condition fail

There's no such construct as "on false exit do"

Originally Posted by shg

There's no such construct as "on false exit do"

I'll put it another way. Why doesn't DO UNTIL and DO WHILE behave as their names imply? When the condition breaks, why does VBA persist in processing the remainder of the current loop???

I have limited experience with different programming languages. VBA, like the few other, older programming languages that I'm familiar with, only evaluate the "Do while" condition at the Loop/Do statement. This means that the entire loop is executed each time that the loop logic decides to execute the loop. (Wikipedia describes generic Do While loops here: https://en.wikipedia.org/wiki/Do_while_loop ). I think this is the reason that the Exit Do statement exists in those programming languages that have it -- is to allow the programmer to test the while condition (or other condition) and exit the loop at a point other than the Do/Loop statements.

I would be interested to know if there are programming languages that behave the way you suggest this should. I can see how the behavior you describe would be detrimental to the Do While loops that I usually use.

I know that this explanation really doesn't explain "why" programming languages work this way. We may need to dig quite deep into the "science" of developing high level languages to see what the rationale is for this. I can understand the logic in implementing the Do While .. Loop either way. I suspect that it is easier to implement an "Exit Do" type statement and let programmers choose if they want to test the while condition in the middle of the loop, than to have a statement that says "change loop condition variable but don't test it yet" kind of statement.

Please Login or Register  to view this content.

Why doesn't the For loop exit as soon as 11 is assigned to i?

@shg - Well actually that example doesn't worry me. It's merely a simple FOR...NEXT loop - I don't expect that loop to evaluate a fail condition during the loop.

Whereas on a DO...WHILE or DO...UNTIL it implies (to me anyway!) that the condition is to be evaluated during the loop, not at the start/end of each iteration.

It's no big deal if VBA cant do this as I can always return to nested Ifs (i.e. IF NOT...THEN ELSEIF NOT...THEN ELSEIF NOT etc)

I just find it frustrating that some threads out there indicate that there IS a way that this can be done in VBA/VBS. It don't work for me.

Thanks Mr Shorty. Food for thought.

Originally Posted by MrShorty

VBA, like the few other, older programming languages that I'm familiar with, only evaluate the "Do while" condition at the Loop/Do statement. This means that the entire loop is executed each time that the loop logic decides to execute the loop.

That's what I thought too. However I found threads out there that indicated that there is a way of writing the loop in VBA to make it exit immediately on fail. I couldn't get this to work which is why I started this thread.

Why not just use the VBA function CBOOL to evaluate your arguments i.e. this will return False...

Please Login or Register  to view this content.

...and this will return True:

Please Login or Register  to view this content.

HTH

Robert

Originally Posted by InvisibleMan

@shg: is/was that a rhetorical question?]

Yup.

It was always my understanding that the loop control values were held internally, and that you cannot change them within the loop]

You can change them, as your code demonstrates, but it's horrible practice.

@ Trebor - Because the problem isn't about determining the condition result. The problem is that VBA only tests the condition change at the start/end of each loop.

It would appear that WHILE or UNTIL loops in VBA (despite what other forum threads out there are saying ) cant test the condition fail while the current loops iteration is running.

The only options so far are: 1) to repeatedly insert code into the loop to check the condition and then Exit Do or 2) set up an On Error Go To based on the condition fail

I think I will go back to nested IF...ELSEIF - It's is a superior method to either of the above two workarounds

The OP has an open loop, i.e. neither the Do nor the Loop statements involve While or Until.

One can construct loops where the "continue looping" condition is tested at the Do or Loop statement.

despite what other forum threads out there are saying )

It might provide some context or some discussion fodder if you could provide one or more of these forum threads that are claiming this kind of Do While..Loop behavior. My gut feel at this point is that either the readers of these posts are misunderstanding what is being written, or the authors of these posts are misunderstanding the exact behavior of Do While...Loops.

I think I will go back to nested IF...ELSEIF - It's is a superior method to either of the above two workarounds

It might be interesting to explain exactly what this section of code is supposed to be doing. I have always envisioned significant differences between the "branching" control structure of a block If..then..elseif..endif and the "repeating" control structure of a loop. In a block if, I use one or more boolean variables/expressions to choose which sequence of statements to execute. A loop structure repeats a block of statements multiple times, where the boolean variable/expression is used to determine how many times the loop is executed. The two programming structures can be nested and otherwise combined in many different ways, but I don't usually think of trying to substitute a "loop" structure for a "branched" structure like this. It makes me wonder exactly what you are doing, and if trying to use a loop is even a better approach for what you are trying to do.

If you are interested in furthering the discussion, it might be valuable to describe where you are getting your information about this behavior of Do While..Loops and what you are trying to do that would substitute a Do While..Loop for a Block If.

More Unicorns?

deleted post

Originally Posted by MrShorty

It might be interesting to explain exactly what this section of code is supposed to be doing. I have always envisioned significant differences between the "branching" control structure of a block If..then..elseif..endif and the "repeating" control structure of a loop. In a block if, I use one or more boolean variables/expressions to choose which sequence of statements to execute. A loop structure repeats a block of statements multiple times, where the boolean variable/expression is used to determine how many times the loop is executed. The two programming structures can be nested and otherwise combined in many different ways, but I don't usually think of trying to substitute a "loop" structure for a "branched" structure like this. It makes me wonder exactly what you are doing, and if trying to use a loop is even a better approach for what you are trying to do.

Thanks Mr Shorty. I've given up expecting a VBA loop that will do this but I'll post a real world example at bottom of this post FYI.

This function is an opening check on a complex project. It checks each of the start-up/install pre-reqs to ensure that all of these checks return TRUE. If any check returns FALSE, the function won't bother checking the remaining checks and return the result FALSE (i.e. fail)

Now I know that this function isn't loop material and that nested Ifs are doing their job. But it occurred to me that if I could alter this code to a loop it would (slightly) improve code readability. (As soon as one of these check functions returned FALSE, the loop would automatically exit and the functions result would return FALSE)
(BTW the final part of the loop would have contained an Exit Do - it would never have been a true loop)


Code Readability was the only reason I was interested in seeing if Nested Ifs could be substituted with a Loop.

Please Login or Register  to view this content.

It's a perfect case for the check procedures raising an error handled by the calling routine.

Originally Posted by shg

It's a perfect case for the check procedures raising an error handled by the calling routine.

[blinks] Yes. You are correct. Thanks shg. (I had been doing the error handling in each sub-function )

You're welcome.