Build and Configure a For Loop in LabVIEW

A For Loop is a structure you use to execute a block of code a set number of times. When the VI runs, the iteration count is evaluated, and then the code is executed. For Loops are used in many programming languages when you want code to execute for a set number of times.

This tutorial walks you through how to build and configure a For Loop to LabVIEW. This introduction material is good for those new to LabVIEW and looking to familiarize themselves with this basic programming structure.

Build a For Loop

1. Launch LabVIEW and open a new VI. You can open a blank VI by selecting File»New VI.
2. Right-click on the block diagram to display a temporary version of the Functions palette.
   • Optional: You can click the thumbtack in the upper left corner of the Functions palette to pin the palette so it is no longer temporary.
3. Navigate to Structures and select the For Loop. 
   • To place the loop on the block diagram, left-click and drag the mouse until the loop is your desired size.

4. Right-click on the count terminal and select Create Constant to display a numeric constant. 
5. The count terminal dictates how many times the subdiagram is executed. Input the number of times you want the loop to execute into the count terminal constant.
   • For example, by inserting 100 into the numeric constant, the for loop executes 100 times before stopping.

6. Monitor the number of iterations the loop has undergone by right-clicking on the iteration terminal and selecting Create Indicator.

The image below shows a For Loop in LabVIEW. The count terminal is set to 100, so the code will execute 100 times before exiting the loop. In each iteration of the loop, a random number between 0 and 100 is displayed to the user every 500 ms. The iteration terminal increments with each execution of the loop.   

Adding a Conditional Terminal to a For Loop

If necessary, you can add a conditional terminal to configure a for loop to stop when a Boolean condition is met or an error occurs. A for loop with a conditional terminal executes until the condition occurs or until all iterations are complete, whichever happens first.

1. Right-click on the edge of the for loop and select Conditional Terminal.
   • You will notice that count terminal now has a red glyph and a conditional terminal (i.e. stop glyph) was added in the lower right corner.
2. Add in the code for which you want the for loop to stop executing.
   • This could be based on user input (e.g. a stop button, front panel value change, etc.) or based on programmatic analysis (e.g. input value measurement reaches a threshold, an error occurs, time elapsed, etc.)

The snippet below shows the code as described in the Build a For Loop section above with one exception - a conditional terminal is added. This code stop executing if the random number generated is zero or if the loop has gone through 100 iterations -- whichever comes first. 

Next Steps

• Once you have familiarized yourself with the For Loop, learn how you can pass information from one loop iteration to the next using shift registers. You can use the Learn LabVIEW Basics online educational material as a resource to get started.
• Process elements of an array individually using auto-indexing