Case Structures

As noted, the case structure is somewhat analogous to the IF-THEN-ELSE statement. In fact, the programming languages C, C++ and Java contained a SWITCH CASE statement which is a closer analogy to the functionality of LabVIEW’s Case Structure, however these structures have somewhat fallen out of popular usage. In general, the SWITCH CASE statement used the following basic syntax:

The SWITCH CASE statement generally tests the value of a variable (the ‘Conditional Control’) and compares it with a set of case identifiers (‘Case 1’, ‘Case 2’, etc.). If the Conditional Control matches a specific case identifier then the block of code associated with that case would be executed. Generally, if the Conditional Control did not match a specific case, a default block of code would be executed instead.

We can illustrate this by creating a simple Case Structure that outputs one of two specific messages to a String Indicator determined by a Boolean Control.

Note that we will be running the VI continuously, so will place the Case Structure inside a WHILE loop terminated by a STOP button.

Create the VI as illustrated, remembering to input a different message into the False Case.

When the Case Selector is controlled by a Boolean Control, only two cases are available to the programmer. However, the input data can also be a string, integer, enumerated data type or even an error cluster. The data type you wire to the case selector determines the number of allowed cases you can enter in the selector label.

Exercise 1: Case Structures and Error Handling

In this exercise, you should create a VI that takes two numbers (A and B) and computes the calculation .

• If B is non-zero, the VI should return the result of the calculation.
• If B = 0, the VI should return a value of 999.99 and insert an error into the error cluster.

To create this VI, use a Case Structure, in which the Case Selector is governed by a Boolean Control evaluating whether value B is equal to zero.

A suitable Block Diagram structure is shown below, which uses the following functions (although you may achieve the same functionality using a different method):

• ‘Equal to 0?’ function to govern the Case Selector
• ‘Divide’ function for calculation when valid
• ‘Bundle by Name’ function for error handling

Sequence Structures

In text-based programming languages, the code executes in the order in which it is written, i.e., executing line-by-line through the code. In data flow based programming environment, a function executes when all its inputs become valid. This can sometimes lead to the developer having difficulty determining the exact order in which code is being executed. This can cause errors as it may be required that some events in code take place before other events. In LabVIEW, the Sequence Structure adds control to the order of execution of the VI

The Sequence Structure (‘Structures >> Flat Sequence Structure’ from the Functions Palette) appears in the form of a film strip, each frame of which executes independently only when the one prior to it passes control. The Sequence Structure executes from left to right.

Note, additional ‘frames’ can be added by right-clicking on the border and selecting ‘Add frame after’.

Formula Nodes

When implementing complex mathematical equations, it is sometimes desirable (e.g. for neatness or ease of writing) to use a text-based programming medium instead of creating a complex Block Diagram. The ‘Formula Node’ in LabVIEW allows the programmer to do this.