Fundamental questions before starting next project

Question

Last project I put together received scathing criticism - and I have to say that I did not put any effort at all to clean the code up or optimize it. The reason is I started with small projects and within couple of months it just grew out of hand with requirements matrix gradually increasing.
So this time around I want to take advice from you guys on architecture issues before I jump into programming. The two fundamental questions:
1) I always thought that SubVi is like a method in C++. If I need to use that code multiple times, then I better put it in a SubVi. But after going through some of the posts on this forum it seems that people use SubVis to reduce # of wires,blocks,etc. on the main Vi too. Is this a standard practice?
2) If I want to have more than 1 VI in the software, what is the best way to send data from one VI to the other VI? SubVI has input and output wires. So it is easy. But VIs do not. Basically I want to have Main VI where I get all the information from the use through a front panel and then go off doing my data acquisition. I want to keep the Main VI as clean as possible. There will be some operations which will need only 1 input and will have only 1 output. But this operation will be used only once in the who operation. So can I have this operation all in a VI and then call it in the Main VI?
3) If I have 3 different VIs in my application, and I define something (say an array) in the Main VI or for that matter second_VI.vi in my application, will that be available in the third_VI.vi ?
Thanks in advance.

Answer 1

labview_beginner wrote:
1) I always thought that SubVi is like a method in C++. If I need to use that code multiple times, then I better put it in a SubVi. But after going through some of the posts on this forum it seems that people use SubVis to reduce # of wires,blocks,etc. on the main Vi too. Is this a standard practice?
You could call it similar to a method.  I would say a subroutine or function is a better terminology since the term method is more applicable to object oriented based programming.  Basically any way to encapsulate a portion of code, particularly if you use it over and over.  It might have input and/or outputs, or neither.
2) If I want to have more than 1 VI in the software, what is the best way to send data from one VI to the other VI? SubVI has input and output wires. So it is easy. But VIs do not. Basically I want to have Main VI where I get all the information from the use through a front panel and then go off doing my data acquisition. I want to keep the Main VI as clean as possible. There will be some operations which will need only 1 input and will have only 1 output. But this operation will be used only once in the who operation. So can I have this operation all in a VI and then call it in the Main VI?
There is no difference between a VI and a subVI other than semantics.  Both are completely independent excecutable portions of code.  Your main or top-level VI can have inputs and outputs as well.  You could execute it stand alone, or it could become a subVI of an even higher portion of code in your hierarchy.  The only difference between the two is that a subVI is just a term to distinguish it as being a portion of a higher level of code.  There are multiple ways of passing data between VI's.  One is wires.  When a subVI ends, the data on its indicator that is connected to the panel is available to travel down the wire to another portion of code.  Another is global or shared variables which stores data to be used by multiple locations.  Another is a function global variable, a LV2 style variable, or action engine.  All of these use special characteristics of uninitialized shift registers to store data between calls of that subVI.  Other methods of sharing data are on the Synchronization pallette such as queues and notifiers to store and pass data.
Using a subVI, even it used in only 1 location can help keep a portion of your block diagram clear.  It gives you the ability to hide some details such as a complicated algorithm.  You can write an algorithm as a subVI and it gives you the ability to test the inputs and outpus as a standalone.  Once you have that. You can drop it in and use it even if it is only at 1 location.  If you need to change it, you can modify the subVI and test it without risking the corruption of the rest of your main VI
3) If I have 3 different VIs in my application, and I define something (say an array) in the Main VI or for that matter second_VI.vi in my application, will that be available in the third_VI.vi ?
Possibly, it all depends on which of the methods listed in number 2 you would like to use.
Thanks in advance.