17
Fri, May
7 New Articles
  1. You are here:  
  2. Home
  3. Programming
  4. RPG
  5. Building a Nest out of Your Data Structures

Building a Nest out of Your Data Structures

RPG
Typography
  • Smaller Small Medium Big Bigger
  • Default Helvetica Segoe Georgia Times

In the last issue, I talked about qualified data structures in RPG IV. Qualified data structures give you the ability to create multiple data structures with identical subfield names, a capability previously unavailable to the RPG world.

Qualified data structures were introduced with OS/400 V5R1. When V5R2 was announced, IBM included several enhancements to RPG IV, including an enhancement to qualified data structures. Surprisingly, this slight variation to qualified data structures makes them even more useable than they are under V5R1.

Since qualified data structures can contain subfield names that are the same as the subfield names of other data structures, IBM instituted nested data structures. IBM calls this new feature "complex data structures." To me, the only thing complex about them is calling them "complex data structures," so I choose to call them "nested data structures."

In addition, RPG IV now allows data structures to be declared and used just like arrays. That is, you can use the DIM keyword instead of the OCCURS keyword when defining a multiple-occurrence data structure. The benefit of data structures as arrays is that the syntax is much less complicated when you refer to a data structure as an array vs. when you need to use the OCCURS keyword and operation code.

Nested Data Structures

Nested data structures are data structures within data structures. Strictly speaking, nested data structures are qualified data structures within qualified data structures. In other words, a subfield of a qualified data structure may also be another qualified data structure.

Nested data structures allow you to move beyond using the OVERLAY keyword in your definition specification by providing you with the ability to declare a data structure subfield as a data structure itself. How do you do this? Simply add the LIKEDS keyword to the subfield, and it becomes a nested, qualified data structure. Figure 1 shows an example:

.....DDame+++++++++++EUDS.......Length+TDc.Functions++++++++++++++
     D CLDate          DS                  
     D  Century                       1S 0
     D  Year                          4S 0
     D  Month                         2S 0
     D  Day                           2S 0
  
     D CustOrder       DS                  Qualified Inz
     D  CustNo                        7P 0
     D  OrdNbr                        7P 0
     D  OrdDate                            LikeDS(CLDate)
     D  BalDue                       11P 2

Figure 1: Example of a nested data structure

When this code is compiled, it is expanded to include the QUALIFIED keyword for the ORDDATE subfield, and the four subfields of CLDate are inserted into the CUSTORDER data structure. See Figure 2.

.....DDame+++++++++++EUDS.......Length+TDc.Functions++++++++++++++
     D CustOrder       DS                  Qualified Inz
     D  CustNo                        7P 0
     D  OrdNbr                        7P 0
     D  OrdDate                            Qualified
     D   Century                      1S 0
     D   Year                         4S 0
     D   Month                        2S 0
     D   Day                          2S 0
     D  BalDue                       11P 2

Figure 2: Compiler-expanded nested data structure

The ORDDATE subfield actually becomes a nested data structure. It is a regular data structure that also happens to be a subfield of the CUSTORDER data structure. In Figure 2, the ORDDATE subfield contains the CENTURY, YEAR, MONTH, and DAY subfields. You'll recall from the last isue that the way you access qualified data structure subfields is by qualifying the subfields to the data structure name. For example, in the Calculation specifications, in order to retrieve the DAY subfield, the following code is required:

.....C..n01..............OpCode(ex)Extended-factor2+++++++++++++
     C                   eval      nDay = CustOrder.OrdDate.Day

In this example, the DAY subfield is qualified to its parent data structure, ORDDATE. ORDDATE is qualified to its parent data structure, CUSTORDER. If the subfield DAY was not used (as shown below) then the value of ORDDATE would be returned by this statement. That is, all seven positions of the ORDDATE subfield would be returned instead of only the DAY subfield's value.

.....C..n01..............OpCode(ex)Extended-factor2+++++++++++++
     C                   eval      CLDate = CustOrder.OrdDate

More Than LIKEDS: LIKEREC

With V5R2, in addition to the LIKEDS keyword, IBM introduced a new keyword that allows you to declare one data structure "like" another. This new keyword does not use an existing data structure as the format for the new data structure; rather, it inherits its data structure layout from input file format.

The LIKEREC (Like Record) keyword is used to declare a data structure or a subfield of a data structure (in the case of nested data structures) the same as the layout of record format declared elsewhere in the source member. This means that if you have a database file declared in the source member, its record format name may be used to declare a data structure. This is very similar to the LIKEDS keyword, except it uses a file format instead of a data structure.

The LIKEREC keyword has an additional parameter that LIKEDS does not. The second parameter is optional and allows you to specify which fields from the record format are going to be included in the data structure you're declaring. These are the special values:

  • *INPUT--Only input-capable fields are included.
  • *ALL--All fields of the externally defined record format are included.
  • *OUTPUT--Only output-capable fields are included.
  • *KEY--The key fields of the externally described database file are included. They are included in the same order that they appear in in the key.

If the second parameter of LIKEREC is not specified, then *INPUT is the default. Most of the options for LIKEREC are obvious: *INPUT includes only input fields, and *OUTPUT includes only output fields. The one that is interesting is the *KEY option, which causes the key fields to be included in the data structure.

Why do I care about *KEY? Another new feature of RPG IV is the ability to use a data structure as a key list. To do this, you declare a data structure with the key field as subfields. The LIKEREC keyword and its *KEY option make this incredibly easy! Then, in free-format RPG IV, you can use the new %KDS operation code function to identify that the data structure is a key list. For example, see Figure 3.

0001 FCustMast  IF A E           K DISK    Rename(Customer:CustRec)
0002 D UndoBuffer      DS                  LikeRec(CustRec) DIM(20)
0003 D CustKeys        DS                  LikeRec(CustRec:*KEY)

      /FREE
         Chain  %KDS(CustKeys) CustMast;
         if     %Found();
           Write    UpdData;
         else;
           Write    NotFound;
         Endif;
      /END-FREE

Figure 3: LIKEREC Keyword and Key Lists

Granted, my free-format RPG IV skills are not the best (I'm still being stubborn about the need for a semicolon after the IF, ELSE, ELSEIF, ENDIF, and ENDDO statements), so the code in Figure 3 may not be perfect. But Figure 3 declares an externally described file named CUSTMAST. That file's record format (CUSTREC) is being used on lines 2 and 3 to declare two different data structures. The first one is named UNDOBUFFER. It occurs 20 times and is probably being used in the program as a way to enable an "undo" function in the program.

The second data structure (line 3) is also based on the CUSTREC format, but the LIKEREC keyword includes the *KEY option. This causes the data structure to be declared using only the key fields from the CUSTMAST file.

Down in the free-format specifications, the first statement is a CHAIN operation that uses the new %KDS operation function. This tells the CHAIN opcode three things about CUSTKEYS:
1. It's a data structure.
2. It includes only key fields.
3. It is to be used as the key list to chain out to the file.

The %KDS function also accepts an optional second parameter that indicates the number of key fields to be used. For example, if the data structure contains five key fields, but you want to use only three of them, the following CHAIN operation and %KDS function would be used.

         Chain  %KDS(CustKeys : 3) CustMast;

This tells the %KDS function to use the first three subfields of the CUSTKEYS data structure as the key list for the CHAIN operation.

You can use the LIKEREC keyword just as you'd use the LIKEDS keyword. The data structure is a qualified data structure and may also be a nested data structure or the parameter of a prototype or procedure interface. It may also be the return value of a procedure.

Next time, I'll review the new "Data Structures as Array" feature that makes multiple occurrence data structures obsolete.

BOB COZZI

Bob Cozzi is a programmer/consultant, writer/author, and software developer. His popular RPG xTools add-on subprocedure library for RPG IV is fast becoming a standard with RPG developers. His book The Modern RPG Language has been the most widely used RPG programming book for more than a decade. He, along with others, speaks at and produces the highly popular RPG World conference for RPG programmers.

MC Press books written by Robert Cozzi available now on the MC Press Bookstore.

RPG TnT RPG TnT
Get this jam-packed resource of quick, easy-to-implement RPG tips!
List Price $65.00

Now On Sale

The Modern RPG IV Language The Modern RPG IV Language
 Cozzi on everything RPG! What more could you want?
List Price $99.95

Now On Sale
BLOG COMMENTS POWERED BY DISQUS

LATEST COMMENTS

Support MC Press Online

$0.00 Raised:
$

Book Reviews

Resource Center

  • SB Profound WC 5536 Have you been wondering about Node.js? Our free Node.js Webinar Series takes you from total beginner to creating a fully-functional IBM i Node.js business application. You can find Part 1 here. In Part 2 of our free Node.js Webinar Series, Brian May teaches you the different tooling options available for writing code, debugging, and using Git for version control. Brian will briefly discuss the different tools available, and demonstrate his preferred setup for Node development on IBM i or any platform. Attend this webinar to learn:

  • SB Profound WP 5539More than ever, there is a demand for IT to deliver innovation. Your IBM i has been an essential part of your business operations for years. However, your organization may struggle to maintain the current system and implement new projects. The thousands of customers we've worked with and surveyed state that expectations regarding the digital footprint and vision of the company are not aligned with the current IT environment.

  • SB HelpSystems ROBOT Generic IBM announced the E1080 servers using the latest Power10 processor in September 2021. The most powerful processor from IBM to date, Power10 is designed to handle the demands of doing business in today’s high-tech atmosphere, including running cloud applications, supporting big data, and managing AI workloads. But what does Power10 mean for your data center? In this recorded webinar, IBMers Dan Sundt and Dylan Boday join IBM Power Champion Tom Huntington for a discussion on why Power10 technology is the right strategic investment if you run IBM i, AIX, or Linux. In this action-packed hour, Tom will share trends from the IBM i and AIX user communities while Dan and Dylan dive into the tech specs for key hardware, including:

  • Magic MarkTRY the one package that solves all your document design and printing challenges on all your platforms. Produce bar code labels, electronic forms, ad hoc reports, and RFID tags – without programming! MarkMagic is the only document design and print solution that combines report writing, WYSIWYG label and forms design, and conditional printing in one integrated product. Make sure your data survives when catastrophe hits. Request your trial now!  Request Now.

  • SB HelpSystems ROBOT GenericForms of ransomware has been around for over 30 years, and with more and more organizations suffering attacks each year, it continues to endure. What has made ransomware such a durable threat and what is the best way to combat it? In order to prevent ransomware, organizations must first understand how it works.

  • SB HelpSystems ROBOT GenericIT security is a top priority for businesses around the world, but most IBM i pros don’t know where to begin—and most cybersecurity experts don’t know IBM i. In this session, Robin Tatam explores the business impact of lax IBM i security, the top vulnerabilities putting IBM i at risk, and the steps you can take to protect your organization. If you’re looking to avoid unexpected downtime or corrupted data, you don’t want to miss this session.

  • SB HelpSystems ROBOT GenericCan you trust all of your users all of the time? A typical end user receives 16 malicious emails each month, but only 17 percent of these phishing campaigns are reported to IT. Once an attack is underway, most organizations won’t discover the breach until six months later. A staggering amount of damage can occur in that time. Despite these risks, 93 percent of organizations are leaving their IBM i systems vulnerable to cybercrime. In this on-demand webinar, IBM i security experts Robin Tatam and Sandi Moore will reveal:

  • FORTRA Disaster protection is vital to every business. Yet, it often consists of patched together procedures that are prone to error. From automatic backups to data encryption to media management, Robot automates the routine (yet often complex) tasks of iSeries backup and recovery, saving you time and money and making the process safer and more reliable. Automate your backups with the Robot Backup and Recovery Solution. Key features include:

  • FORTRAManaging messages on your IBM i can be more than a full-time job if you have to do it manually. Messages need a response and resources must be monitored—often over multiple systems and across platforms. How can you be sure you won’t miss important system events? Automate your message center with the Robot Message Management Solution. Key features include:

  • FORTRAThe thought of printing, distributing, and storing iSeries reports manually may reduce you to tears. Paper and labor costs associated with report generation can spiral out of control. Mountains of paper threaten to swamp your files. Robot automates report bursting, distribution, bundling, and archiving, and offers secure, selective online report viewing. Manage your reports with the Robot Report Management Solution. Key features include:

  • FORTRAFor over 30 years, Robot has been a leader in systems management for IBM i. With batch job creation and scheduling at its core, the Robot Job Scheduling Solution reduces the opportunity for human error and helps you maintain service levels, automating even the biggest, most complex runbooks. Manage your job schedule with the Robot Job Scheduling Solution. Key features include:

  • LANSA Business users want new applications now. Market and regulatory pressures require faster application updates and delivery into production. Your IBM i developers may be approaching retirement, and you see no sure way to fill their positions with experienced developers. In addition, you may be caught between maintaining your existing applications and the uncertainty of moving to something new.

  • LANSAWhen it comes to creating your business applications, there are hundreds of coding platforms and programming languages to choose from. These options range from very complex traditional programming languages to Low-Code platforms where sometimes no traditional coding experience is needed. Download our whitepaper, The Power of Writing Code in a Low-Code Solution, and:

  • LANSASupply Chain is becoming increasingly complex and unpredictable. From raw materials for manufacturing to food supply chains, the journey from source to production to delivery to consumers is marred with inefficiencies, manual processes, shortages, recalls, counterfeits, and scandals. In this webinar, we discuss how:

  • The MC Resource Centers bring you the widest selection of white papers, trial software, and on-demand webcasts for you to choose from. >> Review the list of White Papers, Trial Software or On-Demand Webcast at the MC Press Resource Center. >> Add the items to yru Cart and complet he checkout process and submit

  • Profound Logic Have you been wondering about Node.js? Our free Node.js Webinar Series takes you from total beginner to creating a fully-functional IBM i Node.js business application.

  • SB Profound WC 5536Join us for this hour-long webcast that will explore:

  • Fortra IT managers hoping to find new IBM i talent are discovering that the pool of experienced RPG programmers and operators or administrators with intimate knowledge of the operating system and the applications that run on it is small. This begs the question: How will you manage the platform that supports such a big part of your business? This guide offers strategies and software suggestions to help you plan IT staffing and resources and smooth the transition after your AS/400 talent retires. Read on to learn: