12
Tue, Nov
5 New Articles
  1. You are here:  
  2. Home
  3. Programming
  4. RPG
  5. Declare Two Files with the Same Format

Declare Two Files with the Same Format

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

With OS/400 Version 5 Release 1, IBM introduced QUALIFIED data structures into RPG IV. This functionality gives the programmer the ability to refer to the subfield names of the data structures, using the data structure name as a prefix or qualifier of the subfield.

In the example that's shown below, the data structure MYDS is declared with seven subfields. Note the QUALIFIED keyword; this is what causes the data structure to be considered qualified. 

     D MyDs            DS                  QUALIFIED Inz
     D  Name                         30A
     D  Address                      30A
     D  City                         30A
     D  State                         2A
     D  ZipCode                      10A
     D  Phone                        11P 0
     D  email                        64A

The only way to access the subfields of this data structure is through what is called "qualified syntax." Qualified syntax is as follows:

  DS.subfield

"DS" is the name of the data structure, and "subfield" is the name of a subfield. They're separated by a period (.). For example, the following EVAL operation illustrates how to access the CITY subfield in the MYDS data structure.

     C                   eval      MyDS.City = 'Chicago'

This statement copies the word "Chicago" into the CITY subfield of the MYDS data structure.

When a data structure is qualified, the subfield names may only be referred to using this qualified syntax. The following traditional statement will not compile:

     C                   eval      City = 'Chicago'

This will not compile because the field named CITY no longer exists; only MYDS.CITY exists in the program. But it's possible for you to declare an additional, standalone field named "CITY" in the program and refer to it independently of the CITY subfield in the MYDS data structure. However, you'd be creating some very confusing code.

IBM also allows qualified data structures to be used for input files. There isn't a direct syntax that says "make this input file use qualified names," but it is somewhat simple.

Duplicate I/O Formats

One challenge for RPG programmers has been what to do when the same field name is in two or more files. Specifically, what happens when you have two files with identical formats, and hence all field names are identical?

If there were a way to isolate these formats so that their fields were stored in different buffers, wouldn't that be cool? Well, there is a way.

The PREFIX keyword has been a part of RPG IV since it was introduced. Since the beginning, you could specify PREFIX(XYZ), and the compiler would automatically rename every input field by adding the XYZ prefix to the field name.

In V5R1, when IBM added qualified data structures, they also enhanced the PREFIX keyword by allowing you to include the name of a qualified data structure in the prefix. To do this, you include the period in the prefix itself. For example:

PREFIX('MYDS.')

This causes all the fields to be renamed to MYDS.XXXX, where MYDS. is a constant and XXXX is the input field name.

Note that since a period is not a character allowed in normal RPG IV field names, the prefix that includes a qualifier must be enclosed in quotation marks. In addition, it must be specified in all uppercase because RPG IV does not automatically translate source code segments that are enclosed in quotes.

But the PREFIX keyword is only half the solution. The other half is declaring a qualified data structure. You have to declare the data structure with the same subfields as that of the input file, and it must be a qualified data structure.

Here's an example:

     FCUSTMAST  UF   E           K DISK    PREFIX('MYDS.') 

This declares the CUSTMAST file as usual but indicates that the input fields are mapped to a data structure named CUST1 and that the data structure is declared as a qualified data structure.

The second half is the qualified data structure. You declare the qualified data structure as follows:

     D MYDS         E DS                  EXTNAME(CUSTMAST) QUALIFIED

The data structure name is the same name as the character pattern specified on the PREFIX keyword on the File specification. I used the EXTNAME keyword to import the data structure's subfields from the record format of the CUSTMAST file. This makes it extremely easy to declare the data structure with the appropriate subfields (versus hand-coding each subfield).

With this syntax, all the fields in the CUSTMAST file must now be referred to using qualified syntax. So, for example, the CUSTNO field is referred to as MYDS.CUSTNO (with a qualifier) rather than CUSTNO (without the qualifier).

Now, suppose you want to add another file to the program. This time, the file is CUSTNAME, which happens to be a logical view over the CUSTMAST database file. Like most logical files, the only thing this logical is used for is to access the CUSTMAST file by an alternate key. Therefore, the record format is identical to the CUSTMAST file. While identical formats are not a specific requirement for this example, this is a typical situation.

A logical file with a custom format will also include fields with the same name as the physical, and this technique applies to that situation as well.

To declare the second file, you need to add the following two lines of code to the above examples:

     FCUSTNAME  IF   E           K DISK    PREFIX('MYDS2.')
     
     D MYDS2         E DS                  EXTNAME(CUSTNAME) QUALIFIED

This creates the same qualified data structure as the first example, but it changes the name to MYDS2. Therefore, the subfield names are referred to as MYDS2.XXXX instead of MYDS1.XXXX as in the first example.

The complete RPG IV source code for these two files would be as follows:

     FCUSTMAST  UF   E           K DISK    PREFIX('MYDS1.')
     FCUSTNAME  IF   E           K DISK    PREFIX('MYDS2.')
     
     D MYDS1         E DS                  EXTNAME(CUSTMAST) QUALIFIED
     D MYDS2         E DS                  EXTNAME(CUSTNAME) QUALIFIED

When we use the PREFIX keyword to cause the input fields to be qualified, they are mapped to the corresponding qualified data structure. Therefore, any I/O operations that are performed access the subfields in this qualified data structure.

The following gives a slightly more complete example of using a qualified data structure to allow you to declare two files with the same field names in the same program and avoid field name collision.

     FEDITCUST  CF   E             WORKSTN INFDS(WSDS)
     FCUSTMAST  UF   E           K DISK    PREFIX('CUST1.')
     FCUSTNAME  IF   E           K DISK    PREFIX('CUST2.')
     
     DCUST1          E DS                  EXTNAME(CUSTMAST) QUALIFIED
     DCUST2          E DS                  EXTNAME(CUSTNAME) QUALIFIED

     C                   EXFMT     GETCUST
     C                   if        CSTNBR > 0
     C     CSTNBR        Chain     CustMast       
     C                   endif
     C                   if        NOT %Found or CSTNBR = 0
     C     SearchName    SetLL     CustName        
     C                   if        %Equal
     C                   Read      CustName
     C     Cust2.CustNo  Chain     CustMast
     C                   endif
     C                   endif          
     C                   if        %Found(CustMast)  
      **  Got Customer record!                   
     C                   endif

 

Bob Cozzi is a programmer/consultant, writer/author, and software developer of the RPG xTools, a popular add-on subprocedure library for RPG IV. His book The Modern RPG Language has been the most widely used RPG programming book for nearly two decades. He, along with others, speaks at and runs the highly-popular RPG World conference for RPG programmers.

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

$

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: