topic Re: Notifier - for loop in LabVIEW

Notifier - for loop

Stavrosyt — Mon, 10 Aug 2015 08:49:17 GMT

Hello,

I am making a parallel program so I need to use a notifier to tell one of this processes that one has an event.

To make it easier I build a more simple program.

I have a FOR loop that iterates for 1000 times. I need that each iteration is executed once a Ready (OK) button is pressed, and if it's not needs to wait until is pressed. Also, I need to have a STOP button that if pressed at anytime, stop the program.

I built a notifier WHILE structure to make the waiting until is Ready button is pressed, and an EVENT structure to poll the READY and STOP buttons.

can you tell me what I am doing wrong?

Re: Notifier - for loop

GerdW — Mon, 10 Aug 2015 09:10:02 GMT

Hi Stavros,

can you tell me what I am doing wrong?

A lot is wrong in your image:

- You constantly request for a notifier and delete it in your inner loop.

- You have a wait function in your FOR loop, when you want to wait for a notifier…

- You have a long running FOR loop in your event case, which is a no-go!

- Why not just read the notifier with a timeout? No need to use a while loop at all!

- You haven't used the automatic block diagram cleanup function…

Re: Notifier - for loop

Stavrosyt — Mon, 10 Aug 2015 09:27:57 GMT

Hi Gerd,

Thanks for answering.

here some clarifications:

A lot is wrong in your image:

- You constantly request for a notifier and delete it in your inner loop.

I delete the notifier constantly because I need to use it only once, maybe I am missunderstanding how it is used.

- You have a wait function in your FOR loop, when you want to wait for a notifier…

The wait function was only to allow me to see the changing numbers.

- You have a long running FOR loop in your event case, which is a no-go!

Why you mean by no-go? I only added the "conditional terminal" in case I wanted to stop it.

- Why not just read the notifiers with a timeout? No need to use a while loop at all!

timeout? do you mean a stop timed structure? I used a while because I wanted to be waiting and don't run another FOR iteration until the Ready button is pressed again.

- You haven't used the automatic block diagram cleanup function…

yes, You are right in this. I just wanted to be focus on the functionality first.

can you give me some tips how can I improve this or make it simpler? maybe changing or replacing some structures......

Re: Notifier - for loop

GerdW — Mon, 10 Aug 2015 09:32:01 GMT

Hi Stavros,

timeout? do you mean a stop timed structure?

No, no additional structure.

Just the timeout input of the ReadNotifier function! (Read the help for this function!)

Re: Notifier - for loop

Stavrosyt — Mon, 10 Aug 2015 10:26:18 GMT

OK, so you meant about the timeout variable in "Wait on notification", but by default is -1. I can't use a certain time, because I need to wait undefinetily until notification is received.

why my stop button is not working?

Re: Notifier - for loop

GerdW — Mon, 10 Aug 2015 10:53:32 GMT

Hi Stavros,

why my stop button is not working?

Because you wait for ever on a notification…

Re: Notifier - for loop

PeterFoerster — Mon, 10 Aug 2015 11:07:19 GMT

Hi Stavros,

waiting indefinitely without the possibility to shut down is rarely a good idea. However I think that you're thinking a bit complicated for this: If you just want to wait for the button to be pushed, a simple event structure within the for loop would be your best bet. That event could also handle the stop case.

Antoher Question I'm wondering about is this: you want a program that has the user clock 1000 times befor it's finished? Why? How would you feel if Microsoft gave you a program like that?

Re: Notifier - for loop

Stavrosyt — Mon, 10 Aug 2015 11:23:59 GMT

Hello Peter,

As I told before, this is a simplifier case of a bigger program.

In my real program I do not count until 1000, but I have an array file which is read by the for loop, line by line.

I need to get those parameters, to execute another loop process, for that reason I need the events. Which in this case, are simplified by buttons, but actually are triggered signals.

You are rigth that I am complicating me a lot.

The case you mentioned it's fair enough, but before this process I am running another task, which I wanted to be holded until a Begin button is pressed.

So to sum up I should required a begin button, stop button, and "variable?"(trigger signal) as events to manipulate this for loop.

Managing this trigger signals, I thought using Notifiers was the best option, maybe the buttons can be used in a single event structure?

Re: Notifier - for loop

Stavrosyt — Mon, 10 Aug 2015 11:29:48 GMT

Maybe it's clearer to see it in an image.

This file is wrong also, but helped me to figure it out a better idea.......

The FOR loop goes until the end of the inserted file.(EOF)

Re: Notifier - for loop

GerdW — Mon, 10 Aug 2015 11:36:53 GMT

Hi Stavros,

Maybe it's clearer to see it in an image.

A VI would be so much better…

But still:

It seems your event structure is called before the loop. You try to react on events in your loop, but the event structure can only process the events once the loop has finished (THINK DATAFLOW!). A classical deadlock loop!

- Put your UI event processing in a parallel running loop.

- Use shift registers for references.

- As told you before: IndexArray can be resized! It does not need constant inputs if used correctly!

- Use the cleanup tool!

- Get the notifier reference once before the loop, close it once after the loop!

- Don't use a while loop in a FOR loop in this application, it's not needed here!

Re: Notifier - for loop

Stavrosyt — Mon, 10 Aug 2015 12:01:11 GMT

Hi Gerd,

You are right, I didn't notice the deadlock loop. But I need to stop the FOR loop in each iteration, so how can I do that? inserting a event structure inside the for can be used but it will not retain the FOR iteration.

I tried to use shift registers but the values where not transfer properly outside the loop.

- Get the notifier reference once before the loop, close it once after the loop!

but I need to stop in each iteration!and in addition, wait until a trigger to continue iterating...

How can I arrange the structures in an optimal way?

Re: Notifier - for loop

GerdW — Mon, 10 Aug 2015 12:10:18 GMT

Hi Stavros,

But I need to stop the FOR loop in each iteration, so how can I do that?

As told before…

inserting a event structure inside the for can be used but it will not retain the FOR iteration.

Yes, it will. If done correctly…

I tried to use shift registers but the values where not transfer properly outside the loop.

We don't know what you tried and in which way you tried…

but I need to stop in each iteration!and in addition, wait until a trigger to continue iterating...

So what's the problem here? Get the notifier once before the loop. Wait for notification in the loop. Close the notifier once after the loop!

How can I arrange the structures in an optimal way?

Handle the event structure in a parallel loop!

Re: Notifier - for loop

billko — Mon, 10 Aug 2015 13:30:46 GMT

Your description is very confusing and your code isn't helping to describe what it is you are trying to do. Why don't we backtrack and have you explain your problem as a set of requirements? I think then we'd be better able to help you. 🙂

Re: Notifier - for loop

Stavrosyt — Mon, 10 Aug 2015 13:53:31 GMT

Ok, what about now....

any tips?

Re: Notifier - for loop

GerdW — Mon, 10 Aug 2015 13:59:17 GMT

Hi Stavros,

you still have your deadlock due to executing the event structure before the case structure!

Even worse: the event loop will only execute once. You either "start" or "ready", but not both!

Have your event structure run in a parallel loop!!!

Re: Notifier - for loop

aputman — Mon, 10 Aug 2015 15:17:00 GMT

Is this what you are trying to do?

EDIT: Not sure what is wrong with the snippet and event structure. That's not what my code looks like.

Re: Notifier - for loop

Stavrosyt — Tue, 11 Aug 2015 08:00:38 GMT

Billko,

The requirements:

-Have a button to START the process (there is a initializing process and then wait until start is pressed)
-There is a file with coordinates ( number, x coordinate, Y coordinate, diameter), where using a FOR loop iteration a row is readed and the parameters are "extracted" to be used in another process.
-Because it is needed to wait until the other process is finished, I am replacing that with a READY button to make sure that the next line of the file can be readed (continue with the FOR loop after a "pause" has been done.

At that's it.

Gerd,

I create the event structure before because I need both, a waiting to START and then a READY that is an unpause button for the FOR loop.
You were right, the event was executed once, that was wrong.
it could be nice to have some structure ideas....

Aputman,

Your snippet was very handy. I did not understand why you release the notifier before it entered the event structure....I imagine in that case it will never enter there!!

I have made another modification taking in account the example gave by Aputman.

But I still can't figure out how can I introduce a "Pause" in my FOR.

please, more recommendations or examples are very welcome!!!

Re: Notifier - for loop

Stavrosyt — Tue, 11 Aug 2015 12:40:51 GMT

The easiest and simpler method to use it in my FOR loop is this(and it works perfectly):

where the first iteration is done, and then it waits until Ready is pressed for the next one. And whenever STOP is pressed, then the program stops.

Now I need to add that the Ready button should come in form of a trigger signal (i thought using a notifier from an external parallel event), and the STOP should be used as a general STOP of the program as well....

Furthermore, a START button should be added to this program......I was thinking on states machines (event structure inside an case structure) but maybe there's a simpler way.

any suggestions?

Re: Notifier - for loop

RavensFan — Tue, 11 Aug 2015 12:44:04 GMT

@aputman wrote:

Is this what you are trying to do?

EDIT: Not sure what is wrong with the snippet and event structure. That's not what my code looks like.

LabVIEW's built-in Snippets don't work well with Event Structures or Property Nodes.

Use the Code Capture Tool. It works much, much better.

Re: Notifier - for loop

aputman — Tue, 11 Aug 2015 13:45:10 GMT

@Stavrosyt wrote:

Aputman,

Your snippet was very handy. I did not understand why you release the notifier before it entered the event structure....I imagine in that case it will never enter there!!

I did not release the notifier "before it entered the event structure." The notifier is released after the top loop exits. When the notifier is released, the "Waiting on Notifier" returns an error because it can't find the reference so it kills the bottom loop. In your picture, you reversed everything and it is not going to exit.

You are way to cavalier with your use of event structures. I have yet to come across a case where you need more than one. But if you do use more than one, you better know what you're doing.

If you need to "pause" a for loop, you need to think about your design more. Why are you writing to a file and then pausing your for loop waiting for that write to complete, only to read the data back from the file again. At least that is what I understand from your requirements.