I know this idea has been coined already in A new head-controlled while loop, though it was closed becasue it didn't receive more then 4 kudos. That was 3 years ago and I didn't get any vote because I wasn't aware this post was out there. I'd like to re-open the discussion. Or for how long do we close suggested topics because at the time, there was not enough interest? It is not al that difficult and yet we are 2019 and NI still didn't solve this (as well as taking the square of a united input doesn't square the unit).
I have been programming a lot in LabVIEW and generally love it, but I also find it quite frustrating that you can not do the classic "check condition first, then run". I alos find the classic arguments not valid.
Some argue that it is easily solved using a case structure and while loop, which is not equivalent. One solution is to put the while loop inside the conditional loop, which forces you duplicate the conditional code and good programmers don't like duplicate code. (You could create subVI though, but that is also creating a lot of overhead for both the programmer as the program at run-time). Another solution, as some suggested here, is to put the case structure in the while loop, but then if you are indexing the output, you output a array with one element, whereas you probably wanted to output an empty array. In this case you can work with conditional terminals but that is also a lot of overhead and there is even a bug in LabVIEW when you output multidimensional arrays in this way that are empty and later on try to iterate over it, a for loop will execute the inside of the for loop, although the array is "empty".
I also find the argument that it is a rare case not compelling, I myself experienced this often while programming and the whole sense of a while (or conditional for) loop is that you don't know beforehand how many times you will have to iterate it. So it might be as well that you don't even have to iterate it a single time. That sounds reasonable, no? Otherwise you would use for loop. It is actually the other way round: the classic "do while" is a rare case (that's why textbooks first learn the while loop).
I also don't agree with the fact that the way LabVIEW works, it is not possible. I understand that you need a value for non-indexing terminals, but that would simply be not supported in this type of loop. A "check first then run" kind of loop would only work with iterating terminals (which can be empty) and shift registers (which can take up the value it was initialized to).
A seperate part of the loop's body must be reserved for the conditional code (for instance a rectangle that is fixed at the left underside of the while loop rectangle), just like in most languages where you have to write your conditional statement in brackets as part of the loop statement...
This type of loop would also solve the problem that you often have to delete the last element of your indexed array because the condition for which you probably wanted it, was no longer true (but the loop has excuted first after deciding that).
Lastly, it would give new LabVIEW programmers that come from any other language more familiarity with what they know. As this is reasonably a rather big flaw in LabVIEW (see argumentation above) that is some common and basic in other languages and you are likely to stumble on quite soon when you start experimenting in LabVIEW, it might just make you turn around right away, as it was almost the case when I first started programming in LabVIEW and didn't know how the rest of the language and environment is very beautifull and well designed.
