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Auto-Indexing Tunnels with For Loops and While Loops in LabVIEW

Updated Jun 10, 2019

Reported In


  • LabVIEW

Issue Details

  • When wiring through a While Loop or For Loop, the tunnels for the wires are either solid or have square brackets on them. What is the difference in these tunnels and how can I change them?
  • When wiring through a While Loop or For Loop, the data type entering is different than the data type exiting. Why is this?


What is Auto-Indexing?
              Auto-index enabled                    Auto-index disabled

Output examples from top to bottom
Auto-index tunnel mode
Auto-index with conditional tunnel mode
Last value tunnel mode
When you have small brackets inside the tunnels, auto-indexing is enabled. This is the default setting when you wire in or out of For Loops. If you enable auto-indexing on an array wired to a For Loop input terminal, LabVIEW sets the count terminal to the array size so you do not need to wire the count terminal.

How to enable or disable Auto-Indexing?
To switch between auto-indexing and non-indexing tunnels for an input terminal, right-click a tunnel and select Enable Indexing or Disable Indexing
To switch between auto-indexing and non-indexing tunnels for an output terminal, right click the tunnel and select. Tunnel Mode>>Last Value or Tunnel Mode>>Indexing.

Auto-Indexing and For Loops
Auto-indexing is enabled by default for every array you wire to a For Loop because a common application of a For Loop is to process individual array elements. Disable auto-indexing if you do not need to process the elements of an array individually. When you disable auto-indexing input, the For Loop handles all the elements in the array at once, instead of handling an element per iteration. 

Likewise, when you auto-index an array output tunnel, the output array receives a new element from every iteration of the loop. Therefore, auto-indexed output arrays are always equal in size to the number of iterations unless you select the conditional terminal mode. If you disable auto-indexing in the output tunnel, you receive the value of data from the last iteration, not the elements from every iteration.

Auto-Indexing and While Loops
Auto-Indexing is disabled by default when wiring in or out of While Loops so the wiring tunnel will be solid. If you enable auto-indexing for an array entering a While Loop, the While Loop indexes the array the same way a For Loop does. However, the number of iterations a While Loop executes is not limited by the size of the array because the While Loop iterates until a specific condition is met. Additionally, when you auto-index an array output tunnel, the output array receives a new element from every iteration of the loop.

Additional Information

  • If two arrays of different lengths are auto-indexed into a For Loop, then the smaller size array determines the total number of iterations. Example: two numeric arrays of sizes 4 and 8 are auto-indexed into a for loop, the total number of iterations will be limited to 4. 
  • If two arrays of different length are auto-indexed into a While Loop, then they will both be indexed until they are out of data and the default value of that data type will pad the rest of the array every subsequent iteration. A while loop has a stop condition that is independent of the arrays, so the program must take care to stop the while loop when appropriate to avoid erroneous data being added to the input and output arrays.