We discussed the VALUE and CONST keywords in previous TechTips. However, that's not all you need to know about parameters. Keep reading to find out more!
The title of this TechTip has a double meaning: in order to make the best use of a procedure's parameters, you should be familiar with all the keywords at your disposal. This includes not only the aforementioned VALUE and CONST keywords, but also the OPTIONS keyword.
While the VALUE and CONST keywords are rigid, in the sense that you cannot "tweak" them. They are what they are, and you choose to use them or not. The OPTIONS keyword is a whole different story: this keyword is more of a placeholder for one or more…well, options that you can specify. The available choices are *NOPASS, *OMIT, *VARSIZE, *STRING, and *RIGHTADJ.
In this and the next TechTips, I'll discuss only the first three options in detail, mainly because I never came across a practical situation in which the last two could be of some use.
Let's start with *NOPASS. This option allows you to indicate that a certain parameter in your procedure's parameter list is optional. However, there's a catch: all the parameters that follow the first one with OPTIONS(*NOPASS) must also include this definition. I know that this may sound a bit confusing, so let's look at an example.
A few TechTips ago, I mentioned a simple function that converts USD to EUR. This function had only one parameter (the USD amount) and returned the corresponding EUR amount, using the latest available exchange rate. What if you needed to use that exchange rate most of the times the function is called, but sometimes needed the same functionality but for an earlier date? Simple: just include a parameter to hold the exchange rate date to use, right? However, if most of the times the function is called that date is the current date, or the latest available, it would be a waste of time to include that parameter in every single call. By using OPTIONS(*NOPASS), you can avoid that and specify the date only when you actually need it. The revamped prototype definition looks like this:
*------------------------------------------------------------------------*
* USD_To_Eur: Convert USD to Eur at the current exchange rate
* This function accepts an USD Amount (11, 2)
* And an optional date (8, 0) as input parameters.
* It returns an EUR Amount (11, 2)
*------------------------------------------------------------------------*
D USD_to_Eur PR 11 2
* Input parameter: Amount in US Dollars
D P_USD_Amt 11 2 Value
* Input parameter: exchange rate date (latest available if not specified)
D P_Rate_Date 8 0 OPTIONS(*NOPASS)
Even though it's not shown here, because it would be an almost exact copy of the Prototype Definition, imagine that I also changed the Prototype Interface of the function in the same way. The next step is preparing the code to deal with the additional parameter and, in particular, with the situations in which that parameter is not passed. This is achieved with the %PARMS built-in function (BIF). This BIF returns the number of parameters that were passed when the procedure or function was called. Here's an example of its use:
* Check if the date was passed and fill W_Rate_Date accordingly
C IF %PARMS >= 2
C EVAL W_Rate_Date = P_Rate_Date
C ELSE
C EVAL W_Rate_Date = *HIVAL
C ENDIF
* Then use the W_Rate_Date to look for the appropriate rate
In this case, I've used an IF %PARMS >= 2 statement to determine which exchange rate date to use: if the second parameter was passed, %Parms will return 2 and I'll use its value to retrieve the exchange rate. Otherwise, I'll just look for the latest available USD-to-EUR exchange rate.
A quick note about this piece of code: you probably noticed that I used %PARMS >= 2, but I only have two parameters. I usually do this because if I add more parameters later (all of them with the OPTIONS(*NOPASS) keyword, because it's mandatory to do so), %PARMS will no longer return 2; it will return the total number of parameters passed. This way, I just need to check and fill new variables (one for each new parameter, copying its type and size) accordingly and don't have to change the existing code.
If your code tries to do something with a parameter that was not passed, your procedure will crash because the variable was not initialized. In order to use OPTIONS(*NOPASS), you need to think about and write your code carefully, using the %PARMS BIF to determine how many parameters are available and make it act accordingly, using, for instance, a series of IF statements to test the number of parameters and fill variables with the passed or default values.
The next TechTip will continue to discuss the OPTIONS keyword. Meanwhile, feel free to comment on this TechTip here or in one of the LinkedIn groups where it usually ends up.
Rafael Victória-Pereira has more than 20 years of IBM i experience as a programmer, analyst, and manager. Over that period, he has been an active voice in the IBM i community, encouraging and helping programmers transition to ILE and free-format RPG. Rafael has written more than 100 technical articles about topics ranging from interfaces (the topic for his first book, Flexible Input, Dazzling Output with IBM i) to modern RPG and SQL in his popular RPG Academy and SQL 101 series on mcpressonline.com and in his books Evolve Your RPG Coding and SQL for IBM i: A Database Modernization Guide. Rafael writes in an easy-to-read, practical style that is highly popular with his audience of IBM technology professionals.
Rafael is the Deputy IT Director - Infrastructures and Services at the Luis Simões Group in Portugal. His areas of expertise include programming in the IBM i native languages (RPG, CL, and DB2 SQL) and in "modern" programming languages, such as Java, C#, and Python, as well as project management and consultancy.
MC Press books written by Rafael Victória-Pereira available now on the MC Press Bookstore.
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