2009 for loop zero iteration bug

[yenknip]

This may have been posted before, but when a variable is wired straight through a for loop (queue references for example) and the loop iteration is set to zero, or there are no elements in an auto indexed array, the wire going straight through results in a null or zero value.

 

[Thoric · Accepted solution]

This is not a bug, and is the behaviour of LV as far back as I can remember. To avoid this, use a shift register instead of loop nodes - this will ensure the data is passed through correctly even when iteration count = 0.

[F._Schubert]

You always get the default value for that data type. There was a small discussion in one of Darrens nuggets (if I remember correctly).

Felix 

[Norbert_B]

This behavior is as old as LabVIEW's For Loop and it is no bug. It's simply how LV works.

Please note that there have been quite many KBs and similar around. The latest KB regarding this behavior can be found here. 

hope this helps,

Norbert 

[yenknip]

I suppose it's because tunnels are not 'paired' in the same way SRs are.

I can't believe I haven't come across this before 🙂

Message Edited by yenknip on 09-08-2009 10:43 AM

[Norbert_B]

It is no "pair" when using shift registers (SR).

The SR is always the same memory location (left and right node) whereas tunnels are always seperate memory locations (aka as copy of value).

Norbert 

[yenknip]

Thanks, Norbert. I was under the impression that SRs were buffered as well.

[d.w.b]

> There was a small discussion in one of Darrens nuggets (if I remember correctly).

I didn't find it: https://forums.ni.com/t5/Community-Documents/Darren-s-Nuggets/ta-p/3524320.

Using a shift register is not always appropriate. Take an error wire for example; it is likely that as many of the for loop iterations should work as possible (e.g. read all calibration tables into memory). If one at the beginning fails the later ones should not. In this case, the error wire Tunnel Mode should be Indexing instead, and the zero N / no iterations scenario handled by routing the error wire around the loop and merging the array into it, sort of like this: https://forums.ni.com/t5/LabVIEW-Idea-Exchange/Add-Merge-Errors-on-Loop-Terminal/idi-p/3202506?light...