Case structure & event structure

1.What are the disadvantages of case strucure?
2.What are the advantages of event structure?

Are these test questions? I could name 10 for each, but why are you asking?
While there are often many ways to accomplish the same thing, Case structures and Event structures serve different purposes. Straight from the Help file:
Case Structure
Has one or more subdiagrams, or cases, exactly one of which
executes when the structure executes. The value wired to the selector terminal
determines which case to execute and can be Boolean, string, integer, or
enumerated type. Right-click the structure border to add or delete cases. Use
the Labeling tool to enter value(s) in the case selector label and configure the
value(s) handled by each case.
Event Structure (Not in Base Package)
Has one or more
subdiagrams, or event cases, exactly one of which executes when the structure
executes. The Event structure waits until an event happens, then executes the
appropriate case to handle that event.
Right-click the structure border to add new event cases and configure which
events to handle. Wire a value to the Timeout
terminal at the top left of the Event structure to specify the number of
milliseconds the Event structure should wait for an event to occur. The default
is –1, indicating never to time out.

Similar Messages

Transfering a case structure to event structure

Hi, I am trying to transfer case structures that are currently set up to a control that if on prompt the user where to save and then save all following files to that location till end of program and if off the do nothing. I would like to enable these to be switched on and off at any point and know to use an event structure. The problem is I am new to coding with an event structure and have been able to get the button to work but not continuously save. I attached a screen shot of the original case structure and was wondering if someone could show me how to change it to the event structure format with the same functionality. I'm getting a little lost and confused. Thanks!
Attachments:
alltruecases.jpg ‏534 KB

I can't attach the vi due to the size and I do not think a simplified piece of code is going to help. I did however go through and stitch two jpegs together of the main two sequences. hopefully this gives a better idea of what I am trying to do and where to go from here. thank you.
Attachments:
1.jpg ‏669 KB
2.jpg ‏317 KB

Event structure vs case structure?

I am developing an application that is fairly user input based... lots of boolean control buttons to do different things.
In the past, I have avoided "value change" event structures, and instead went with a while loop, stacked sequence, and series of case structures. After having done it, I decided this was a terrible idea. This time, I intend to build an array from the booleans, convert it to a number, and feed it in to a single case structure. This will give me the option to add cases for different combinations of the booleans being pressed.
I guess the question is: What is the best method for doing something like this? Does one have advantages over the other?
Edit: I wasn't that clear. I would like a comparison between the "value change" event structure method, and just feeding everything in to an array -> case structure. I also am worried that with the bool array -> case structure that I will run in to problems where I end up with too many control variables, and the value of the resulting number gets ugly very quickly. The application has several buttons... next, last, save, load, several different configure setup buttons etc.
Thanks!
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Aalenox wrote:
I am developing an application that is fairly user input based... lots of boolean control buttons to do different things.
In the past, I have avoided "value change" event structures, and instead went with a while loop, stacked sequence, and series of case structures. After having done it, I decided this was a terrible idea. This time, I intend to build an array from the booleans, convert it to a number, and feed it in to a single case structure. This will give me the option to add cases for different combinations of the booleans being pressed.
I guess the question is: What is the best method for doing something like this? Does one have advantages over the other?
Edit: I wasn't that clear. I would like a comparison between the "value change" event structure method, and just feeding everything in to an array -> case structure. I also am worried that with the bool array -> case structure that I will run in to problems where I end up with too many control variables, and the value of the resulting number gets ugly very quickly. The application has several buttons... next, last, save, load, several different configure setup buttons etc.
Thanks!
Event structure. Among other things, it will allow your loop to "sleep." then wake up when an event happens. With a case structure it's going to be constantly polling. Event structure is pretty much always the right way to go in newer versions of LabVIEW that have it available. I assure you, your first solution was aboslutely wrong and you noticed that, your second is better and was common the way before event structures existed, now that event structures exist, you should go with those.
CLA, LabVIEW Versions 2010-2013

Event structure or case structure

I have some case structures wired to buttons. When each button is true, code inside the corresponding case structure executes. `What are the advantages/disadvantages of using an event structure instead of a case structure?
Ron Deavers, CLD

Place an indicator for the iteration terminal of the main loop to see an obvious difference
Your case structures must be in a loop that constanty runs. If you don't put a delay in the loop, it consumes all CPU, but if your delay is too long, it takes a moment to react to the button changes. The loop spins all the time, no matter if you press buttons or not.
Your event structure also sits in a loop, but nothing happens until you press a button. Now the event structure triggers IMMEDIATELY, executes the event case, and then waits for the next event. The loop does not spin unless needed.
You can even mix the functions by using a timeout event. Code in the timeout event occurs at regular intervals, but at any time, any of the other events can take over.
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Usage of Events over Case Structure

Hi,
I want to Know what are the advantages if using Events over case strutures.
Is memory useage less in case of events.
With regards,
Suresh Thangappan
Suresh Thangappan

Hi Suresh
I think you can't just compare these two structure types, as they have a different purpose.
I assume you're about to create an application, so I'd suggest you to study the producer consumer design pattern. There are both of these structures used to separate program-logic and reaction to user-inputs.
Thomas
Using LV8.0
Don't be afraid to rate a good answer...

Switching from case to event structure

I once down converted some LV 7.0 code to 6.0 and it (automatically) converted an event structure to a case structure. When trying to upgrade some code to change a case structure to an event structure, I didn't see any means for doing that (a flat sequence, yes... event structure, no).
Is there a way to convert a case to an event structure -- if not, why not?
...Dan

No, the question is very valid!
The resulting code does not need to make sense immediately, but I have desired this functionality (=convert case structure to event structure) at several occasions. I have many legacy programs that were made before the introduction of the event structure. Some contain big case structures with many cases, one is the "idle" case which only contains a small wait statement, while others are executed if certain controls change their value. (Sort of a poor man's event structure emulation ). Upgrading this old code would require a simple replacement of the case structure with an event structure so each event case contains the code of one of the original cases, followed by a quick manual cleanup (assignment of the event triggers, etc.).
Without such a conversion tool, my code upgrade is major work, especially if there are many cases and dozens of shift registers crossing the case structure. Basically, I need to rebuild things from scratch.
Summary: It would be useful to be able to easily convert between event structure and case structure and vice versa using a "replace" command. I even remember asking for this functionality. I would use it!
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Event structure inside a case structure

Hi,
I've got a problem w/ my porgram, and managed to boil it down to this
simple example. The program basically hangs if you change the
Numeric value when the boolean is false. I don't understand
why. I suspect it has to do with the way event structures are
treated, but if someone cou please explain this I'd be greatful.
Also, now imagine I have 7 numeric controls, handled by 7 cases in that
event structure, and I actually want to do things in response to the
value change for each. Can someone suggest an elegant way to do
that? I thought what I had here was a good design, but apparently
not.
Bjorn
(I'm using labview 7.1)
Attachments:
EventInCase.vi ‏24 KB

It is generally a bad idea to place event structures into cases. Events structures need to be able to breathe.
Maybe you can redesign your program and place the case structure inside the event structure, for example? (Also have a look at the online help for the event structure, especially the link "caveats and recommendations for using events ".)
Detailed analysis:
Your event case for the numeric is set to "lock front panel until event completes". If you change the numeric while the case is false, the evet structure cannot execute, so the front panel locks forever. The observed behavior is as expected. All clear?
In addition, if the case is false, the main loop spins at nearly infinite rate, consuming all available CPU. Place a small wait to be more cosiderate to everything else running on your rig.
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How can I determine whether to use an event structure or a case structure?

I'm starting a large project and need a state machine. I can't decide on whether to use a case structure or event structure. Is there an article or other information that describes criteria for selecting between the two approaches?
Thank you,
Chuck
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Hi Chuck,
Well case structures and event structures differ quite alot. Here's a link for indepth information on Event Structures, and using them in state machines: http://zone.ni.com/devzone/cda/tut/p/id/2962. Hopefuly this will help you make up your mind.
Let me know if you have any questions after reading it.
Regards,
Dominic Walker
Cardiff University
Electrical and Electronic Engineering Student

Case structure/event counting

Thanks so much everyone for your help. I was
able to get it to work. I now have a second question that I have been
wrestling with on my program for some time. I am hoping to be able to
count the number of times each event happens, be it >0, <0 or 0.
So far, I haven't been having a lot of luck on figuring this out.
The
way my program works is that the case structures control the opening of
solenoid valves. I am running a while loop to keep the program running,
but would like to be able to at least count the number of times each of
the cases are true in an effort to figure out the amount of time each
of the solenoids is open during a run.
What I really need
to do is be able to run each case for a maximum of 10 minutes each, and
then stop the case from running. I know this is an in depth problem,
but simply counting the number of times each of the cases run would be
a huge start for me.
Thanks again for the help on the other problem.
Tim

You need three different counters, one for each event. Right now you increment the same number, irrespective of case, so you might as well do this outside the case structure or even just use the iteration terminal. Also, the counter should be blue (I32). You also need to initialize the shift registers.
Here's a simple example (LabVIEW 8.0) that uses an array of three elements to keep count of the three cases. See if this makes sense.
Message Edited by altenbach on 08-27-2008 09:30 AM
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Attachments:
countCases.vi ‏13 KB

Event in case structure

Dear all,
I am using Event Structure in a Tab Control.
On page 1, there is a Table Control and when the mouse is clicked on it, it returns the Cell Value. The event is working ablsolutely fine, but the problem is i have no idea how to Stop the Event.
I am attaching my Vi here. Pls have a look at it and let me know of a possible solution.
Thanks,
Ritesh
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Attachments:
Event in Tab.vi ‏18 KB

ritesh024 wrote:
But, it means that the Events in Page 1 are getting triggered even when the Control is on Page 2.
That would take the unnecessary memory and time as there is no need for the events on Page 1 to run when the Control is on a different page.
No, the event will not get "triggered", because if you are on the other tab it is impossible to fire the table event and a mouse up cannot occur. The event structure is in memory no matter what and placing it inside a case structure only complicates the code and can will lead to problems.
The event structure should always be in the dataflow, the tab position is irrelevant. Never hide an event structures inside a case structure!
Here's a quick example (LV 8.5). Tab 1 has your table and senses the cell position. Tab 2 has a light switch. As you can see there is no case structure needed.
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Attachments:
Event_in_TabMOD.vi ‏19 KB

Run only once in case structure

Hello,
I have a menu set up that is wired to a case structure. Of course the menu is some boolean buttons that are set to switch when pressed so the case will stay open as long as the button is true.
inside that case I have another menu and a case structure that operates the same. In one of these cases I have a piece of code that I only want to run once. I have tried the while loop that runs once and a for loop that only runs once, but the section of code seems to run continuosly instead of just once. It works ok if I use any of the latch functions but not the switch when pressed functions. However, I need the button to stay pressed until the user is finished with that case.
I really don't want to use an event structure as I was saving that for another piece of code that monitors inputs.
Anyone have a suggestion?

Typically, this works easiest with a shift register and and a boolean "implies" function.
In the code example (see image), the button is set to "switch until released". (Of course you probably don't need the NOT, simply place your one-time code into the FALSE case )
Message Edited by altenbach on 01-26-2007 01:52 PM
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Attachments:
implies.png ‏8 KB

How to get a case structure to execute only once in a loop

I have a while loop that is monitoring temperature. Once the temperature meets or exceeds a given setpoint I want to start a timer. At this point I don't want to monitor the temperature anymore. That is, if the temperature should drop below the setpoint, I don't want to execute the case structure again when the temperture meets or exceeds the setpoint(the temp may oscillate about the setpoint for a given period of time). In any event, I need the case structure to execute only one time, not every iteration of the loop.

You can put a local Boolean variable "Flag" inside that case structure and
set "Flag" to False. Outside the case structure, use an "AND" function
output to control the case structure. The "AND" function has two inputs. One
goes to the comparison results between real temp and setpoint. The other
inputs connects to the "Flag" variable. In this way, once you entered that
case structure, the "Flag" will be turned to False, and then in next
iteration, you won't get into the case again because the "AND" function will
be False as your "Flag" is False now.
Hope this helps.
Rentian
1. Inside the case structure, put a
"BB Herman" wrote in message
news:[email protected]..
>I have a while loop that is monitori
ng temperature. Once the
> temperature meets or exceeds a given setpoint I want to start a timer.
> At this point I don't want to monitor the temperature anymore. That
> is, if the temperature should drop below the setpoint, I don't want to
> execute the case structure again when the temperture meets or exceeds
> the setpoint(the temp may oscillate about the setpoint for a given
> period of time). In any event, I need the case structure to execute
> only one time, not every iteration of the loop.

I want to create an array that goes into a case structure where each element in the array is an individual case and gets done in order

I want to create an array that goes into a case structure where each element in the array is an individual case and gets done in order. Any ideas, I've been playing with the idea but have had no luck, is this even possible?

Hi,
Please check it out the attached Vi.. Is this you need?
Sasi.
Certified LabVIEW Associate Developer
If you can DREAM it, You can DO it - Walt Disney
Attachments:
Event.vi ‏11 KB

Multiple booleans controlling one boolean input for a case structure

Hello,
I have four boolean buttons and a case structure with a boolean input. I would like to write the program so that whenever any one of the four boolean button changes value, the case structure reads true for one loop interation, and then returns to false.
I have tried all sorts of things and can't seem to figure it out.
Ultimately, the ideal function of the program is so that whenever you push any one of the four boolean voltage controllers, that voltage case is chosen in the case structure, the time in seconds is written to a file, and the elapsed time timer is reset.
I have attached my program. Let me know if you have suggestions. Thanks.
Attachments:
Volt_Control_And_Time_Counter.vi ‏71 KB

Zappow wrote:
However, with this setup, the time counter still doesn't work.
The event structure for all four buttons has a true boolean constant connected to my file-saving case structure. If I connect this to my elapsed time express VI reset button, the timer doesn't count.
The timer does not count because you have not wired a timeout to the lower event structure. The default timeout is -1, so the lower loop never spins via timeout
It is useless to have a wait in a loop that also has a timeout case. Just wire an approriate timeout value to the timeout terminal and remove all waits. It is also not necessary to have a 1ms ttimeout if your timeout values only displays to 100ms resolution. Can you explain why you think you need two loops and all that extra code?
Attached is a quick rewrite that solves some of the problem. Since I don't have any hardware, I replace the voltage out with a meter.
You have way too many FP controls and indicators. Use arrays!
Let me know if yo have any questions. Modify as needed.
LabVIEW Champion . Do more with less code and in less time .
Attachments:
Volt_Control_And_Time_CounterMOD.vi ‏63 KB

Case structure problem with default 0, 1, 2

Good afternoon,
I have a question about the case structure. If you use 3 cases, default 0, 1 and 2, like in the following VI.
How can you fix that the sum is never 3? Because when you turn on the switch 1 and 2 simultaneously it gives the sum 3 and the case will go in the default stand.
Thanks for the help.
Solved!
Go to Solution.
Attachments:
case_structure_question.vi ‏11 KB

I recommend changing to using an Event Structure. That way you react to when a switch is toggled instead of constantly polling.
What I would do is, with the Event Structure, have a case for each switch's value change. In the event case, you use a property node to set the Disabled property of the other switch based on the current switches value (Switch 1 goes true, so disable switch 2)
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