In-depth exploration of LabVIEW's internal and external conditional structures

        The conditional structure (CASE) is one of the commonly used structures in LV. The most typical conditional structure is the conditional structure with error handling, which is often used to avoid the use of sequential structures. Different from the conditional structure of C language, for display controls, all CASE branches must pass data (assignment) to the display controls or directly use the default value, and no output is allowed.

        In a conditional structure, there are two completely different ways to handle display controls. Although these two methods seem to be not much different on the surface and may have been used in specific programming, the operating efficiency varies greatly when processing large data, such as large arrays, and may inadvertently cause the program to run slowly and be difficult to detect errors.

Method 1: Display input controls and indicators outside the case structure. In this method, the input controls and indicators are located outside the case structure branch. As shown in the following figure:

        

 

Method 2: Processing controls and indicators in a case structure. In this method, the controls and indicators are placed in the case structure branch. From the flowchart, it seems to be more concise, but it is very wrong, as shown in the following figure.

In method 2, no processing is performed in the error branch, and the input and display controls are placed in the non-error branch.

OUT.VI and IN.VI implement exactly the same functions, but their running efficiency is very different. The larger the array, the more obvious the difference. Of course, the running time will be different on different computers. The following is the test program flowchart.

 

Below are the test results. The running time of the two differs by more than 20,000 times.

Next, we introduce a "top-level flowchart". The top-level flowchart is often seen in articles written by LV enthusiasts, but there is no such concept in LV-related books. In OUT.VI, the input controls are on the left and the display controls are on the right, both outside of all structures. The area outside of all structures is called the "top-level flowchart", which is similar to the formal parameter part in C language.

In the top-level flowchart, the running efficiency is extremely high. This is because before entering the subVI, LV can easily determine that the cache is reusable, so there is no need to copy the array each time in the loop. However, when the input control and indicator are located inside the conditional branch, LV may not be able to determine that the cache is reusable, so the value copy method is used, which sacrifices efficiency but ensures the correct result.

Method 2 seems more reasonable on the surface, but it is wrong. Therefore, we can conclude that the input controls and display controls in the subVI should be located in the top-level program flowchart and should not be located in any structure.