Editing IBM RPG (section)

==History==
===Background===
Originally developed by [[International Business Machines|IBM]] in 1959, the name ''Report Program Generator'' was descriptive of the purpose of the language: generation of reports from data files.<ref>[http://www-03.ibm.com/ibm/history/documents/pdf/HistoryofIBMDataProcessing.pdf ''"...IBM 1401 and the Report Program Generator (RPG) contributed significantly to this success..."''] {{Webarchive|url=https://web.archive.org/web/20120124182914/http://www-03.ibm.com/ibm/history/documents/pdf/HistoryofIBMDataProcessing.pdf |date=2012-01-24 }}, IBM 1401 Data Processing System / 1959</ref> [[FOLDOC]] accredits Wilf Hey with work at IBM that resulted in the development of RPG.<ref>but it says 1965, rather than the 1959 mentioned here.</ref> [[FARGO Programming Language|FARGO]] ('''F'''ourteen-o-one '''A'''utomatic '''R'''eport '''G'''eneration '''O'''peration) was the predecessor to RPG on the [[IBM 1401]].

Both languages were intended to facilitate ease of transition for IBM [[tabulating machine]] (Tab) [[unit record equipment]] technicians to the then-new computers. Tab machine technicians were accustomed to plugging wires into [[plugboard|control panels]] to implement input, output, control and counter operations (add, subtract, multiply, divide).  Tab machines programs were executed by impulses emitted in a machine cycle; hence, FARGO and RPG emulated the notion of the machine cycle with the program cycle. RPG was superior to and rapidly replaced FARGO as the report generator program of choice.

IBM later implemented RPG(7070-RG-902), but not FARGO, on the [[IBM 7070]]/72/74.<ref>{{cite book
 | title     = 7070/7074 Compiler Systems: Report Program Generator
 | id        = C28-6113
 | publisher = [[IBM]]
 }}
</ref><ref>{{cite book
 |title        = Programming Systems Analysis Guide - IBM 7070 Series Report Program Generator
 |id           = C28-6192
 |url          = http://bitsavers.org/pdf/ibm/7070/C28-6192_7070_RPG_System_Analysis_Guide_1962.pdf
 |year         = 1962
 |publisher    = [[IBM]]
 |access-date  = January 19, 2024
 |archive-date = January 19, 2024
 |archive-url  = https://web.archive.org/web/20240119173806/http://bitsavers.org/pdf/ibm/7070/C28-6192_7070_RPG_System_Analysis_Guide_1962.pdf
 |url-status   = live
}}</ref>

The alternative languages generally available at the time were [[Assembly language|Assembler]], [[COBOL]] or [[FORTRAN]]. Assembler and COBOL were more common in mainframe business operations ([[System/360]] models 30 and above) and RPG more commonly used by customers who were in transition from tabulating equipment (System/360 model 20).

===RPG II===
{{main|RPG II}}
''RPG II'' was introduced about 1969 with the [[System/3]] series of computers.  It was later used on [[System/32]], [[System/34]], and [[System/36]], with an improved version of the language.  RPG II was also available for larger systems, including the [[IBM System/370]] mainframe running [[DOS/VSE]] (then [[VSE/SP]], [[VSE/ESA]], and [[z/VSE]]). [[International Computers Limited|ICL]] also produced a version on its [[ICL VME|VME/K]] operating system.

In the early days of RPG, its major strength was the ''program cycle''. A programmer would write code to process an individual record, and the program cycle would execute the change against every record of a file, taking care of the control flow. At that time each record (individual punched card) would be compared to each line in the program, which would act upon the record, or not, based upon whether that line had an "indicator" turned "on" or "off". The indicator consisted of a set of logical variables numbered 01–99 for user-defined purposes, or other smaller sets based upon record, field, or report processing functions. The concept of level breaks and matching records is unique to the RPG II language, and was originally developed with card readers in mind. The matching record feature of the cycle enabled easy processing of files having a header-to-detail relationship. RPG programs written to take advantage of the program cycle could produce complex reports with far fewer lines of computer code than programs written in [[COBOL]] and other business-centric languages.  
 
The program File Specifications, listed all files being written to, read from or updated, followed by [[Data definition specification|Data Definition Specification]]s containing program elements such as Data Structures and dimensional arrays, much like a "Working-Storage" section of a COBOL program. This is followed by Calculation Specifications, which contain the executable instructions. Output Specifications can follow which can be used to determine the layout of other files or reports. Alternatively files, some [[data structure]]s and reports can be defined externally, mostly eliminating the need to hand code input and output ("[[Input/output|I/O]]") specifications.

===RPG III===
{{main|RPG III}}
''RPG III'' was created for the [[System/38]] and its successor the [[AS/400]].  RPG III significantly departed from the original language, providing modern structured constructs like IF-ENDIF blocks, DO loops, and [[subroutine]]s.  RPG III was also available for larger systems including the [[IBM System/370]] mainframe running [[OS/VS1]]. It was also available from [[Unisys]] for the [[VS/9]] operating system running on the [[UNIVAC Series 90]] mainframes.

Since the introduction of the [[IBM System/38]] in 1979 most RPG programmers discontinued use of the cycle in favor of controlling program flow with standard looping constructs, although IBM has continued to provide backward compatibility for the cycle.

====DE/RPG====
''DE/RPG'' or Data Entry RPG was exclusively available on the [[IBM 5280]] series of data-entry workstations in the early '80s. It was similar to RPG III but lacking external Data Descriptions (DDS) to describe data(files) like on the System/38 and its successors. Instead, the DDS part had to be included into the RPG source itself.

====RPG/400====

''RPG/400'' was effectively RPG III running on AS/400. IBM renamed the RPG compiler as "RPG/400" but at the time of its introduction it was identical to the RPG III compiler on System/38. Virtually all IBM System/38 products were rebranded as xxx/400 and the RPG compiler was no exception. RPG III compiled with the RPG/400 compiler offered nothing new to the RPG III language until IBM began development of new operation codes, such as SCAN, CAT and XLATE after several years of AS/400 availability. These enhancements to RPG III were not available in the System/38 version of RPG III.

=== RPG IV and ILE RPG===

''RPG IV'', a.k.a. ''RPG ILE'',<ref>[https://archive.today/20121217164248/https://www-304.ibm.com/support/docview.wss?uid=nas1f23d225573c211a186256a240067a3da "Generating Random Numbers in ILE RPG Using the CEERAN0 API"], IBM.com</ref> was released in 1994 as part of the V3R2 release of [[OS/400]] (now known as IBM i).<ref>[http://www.ibmsystemsmag.com/Blogs/iDevelop/April-2015/How-Did-We-Miss-RPG-IV-s-Coming-of-Age-/ "How did we miss RPG IV's Coming of Age?"] {{Webarchive|url=https://web.archive.org/web/20170303042652/http://www.ibmsystemsmag.com/Blogs/iDevelop/April-2015/How-Did-We-Miss-RPG-IV-s-Coming-of-Age-/ |date=2017-03-03 }}, ibmsystemsmag.com</ref> 
<ref>{{cite web |title=RPG/400 |url=https://www.ibm.com/docs/en/rdfi/9.6.0?topic=languages-rpg400 |website=IBM.com | date=8 May 2018 |publisher=International Business Machines Corporation |access-date=4 April 2022 |archive-date=3 April 2022 |archive-url=https://web.archive.org/web/20220403171745/https://www.ibm.com/docs/en/rdfi/9.6.0?topic=languages-rpg400 |url-status=live }}</ref>

With the release of RPG IV, the RPG name was officially no longer an [[initialism]]. RPG IV offered a greater variety of expressions within its Extended Factor-2 Calculation Specification and, later in life, its free-format Calculation Specifications and Procedure syntax.
RPG IV in the [[Integrated Language Environment|Integrated Language Environment framework]] is known as ILE RPG, and user guides explain the nuances of both.<ref>{{cite web |title=ILE RPG Reference |url=https://www.ibm.com/docs/en/rdfi/9.6.0?topic=reference-ile-rpg |website=IBM.com | date=June 2012 |publisher=International Business Machines Corporation |access-date=4 April 2022 |archive-date=3 April 2022 |archive-url=https://web.archive.org/web/20220403170239/https://www.ibm.com/docs/en/rdfi/9.6.0?topic=reference-ile-rpg |url-status=live }}</ref><ref>{{cite web |title=ILE RPG Programmer's Guide |url=https://www.ibm.com/docs/en/rdfi/9.6.0?topic=guide-ile-rpg-programmers |website=IBM.com | date=June 2012 |publisher=International Business Machines Corporation |access-date=4 April 2022 |archive-date=3 April 2022 |archive-url=https://web.archive.org/web/20220403171745/https://www.ibm.com/docs/en/rdfi/9.6.0?topic=guide-ile-rpg-programmers |url-status=live }}</ref><ref>{{cite web |title=Who Should Use This Guide |url=https://www.ibm.com/docs/en/rdfi/9.6.0?topic=airpg-who-should-use-this-guide |website=IBM.com | date=June 2012 |publisher=International Business Machines Corporation |access-date=4 April 2022 |archive-date=3 April 2022 |archive-url=https://web.archive.org/web/20220403171743/https://www.ibm.com/docs/en/rdfi/9.6.0?topic=airpg-who-should-use-this-guide |url-status=live }}</ref> RPG IV and ILE RPG are supported by IBM in the current IBM i platform.<ref>{{cite web |title=High Level Languages |url=https://www.ibm.com/docs/en/rdfi/9.6.0?topic=information-high-level-languages |website=IBM.com | date=8 May 2018 |publisher=International Business Machines Corporation |access-date=4 April 2022 |archive-date=3 April 2022 |archive-url=https://web.archive.org/web/20220403171742/https://www.ibm.com/docs/en/rdfi/9.6.0?topic=information-high-level-languages |url-status=live }}</ref>

In 2001, with the release of [[IBM i|OS/400]] V5R1, RPG IV offered greater freedom for calculations than offered by the Extended Factor-2 Calculation Specification: a ''free-format'' text-capable source entry, as an alternative to the original column-dependent source format. The "/FREE" calculation did not require the operation code to be placed in a particular column; the operation code is optional for the EVAL and CALLP operations; and syntax generally more closely resembles that of mainstream, general-purpose programming languages. Until November 2013, the free format applied exclusively to the calculation specifications.<ref>{{cite web|title=Totally Free RPG|url=http://ibmsystemsmag.blogs.com/idevelop/2013/10/as-you-may-have-already-heard-there-was-a-major-technology-refresh-tr-announcement-yesterday-check-out-fellow-blogger-st.html|publisher=IBM Systems Magazine|access-date=8 March 2014|author=Paris, Jon|author-link=Jon Paris|author2=Gantner, Susan|archive-date=8 March 2014|archive-url=https://web.archive.org/web/20140308153010/http://ibmsystemsmag.blogs.com/idevelop/2013/10/as-you-may-have-already-heard-there-was-a-major-technology-refresh-tr-announcement-yesterday-check-out-fellow-blogger-st.html|url-status=live}}</ref> With the [[IBM i]] V7R1 TR7 upgrade to the language, the "/free" and "/end-free" calculations are no longer necessary, and the language has finally broken the ties to punched cards.

IBM Rational Developer for i (RDi),<ref>{{cite web |title=IBM Rational Developer for i |url=https://www.ibm.com/products/rational-developer-for-i |website=IBM.com |publisher=International Business Machines Corporation |access-date=3 April 2022 |archive-date=3 April 2022 |archive-url=https://web.archive.org/web/20220403060839/https://www.ibm.com/products/rational-developer-for-i |url-status=live }}</ref> an [[Eclipse (software)|Eclipse]]-based [[Integrated Development Environment]], is recommended by IBM for RPG development.<ref>{{cite web |title=5770-WDS IBM Rational Development Studio for i V7.4 |url=https://www.ibm.com/common/ssi/ShowDoc.wss?docURL=/common/ssi/rep_sm/s/877/ENUS5770-WDS/index.html |website=IBM.com |publisher=International Business Machines Corporation |access-date=3 April 2022 |archive-date=3 April 2022 |archive-url=https://web.archive.org/web/20220403060839/https://www.ibm.com/common/ssi/ShowDoc.wss?docURL=/common/ssi/rep_sm/s/877/ENUS5770-WDS/index.html |url-status=live }}</ref> The Source Entry Utility (SEU) text editor is no longer recommended for RPG development, and development ceased after IBM i 6.1.<ref>{{cite web |title=5770-WDS IBM Rational Development Studio for i V7.4 |url=https://www.ibm.com/common/ssi/ShowDoc.wss?docURL=/common/ssi/rep_sm/s/877/ENUS5770-WDS/index.html |website=IBM.com |publisher=International Business Machines Corporation |access-date=4 April 2022 |archive-date=3 April 2022 |archive-url=https://web.archive.org/web/20220403060843/https://www.ibm.com/common/ssi/ShowDoc.wss?docURL=/common/ssi/rep_sm/s/877/ENUS5770-WDS/index.html |url-status=live }}</ref> Other legacy developer tools include CODE/400 (based on IBM WorkFrame/2) and [[VisualAge|VisualAge for RPG]].

====Continuing language enhancements====

IBM continues to enhance the RPG language<ref>{{cite web |last1=IBM |title=RPG Cafe |url=https://www.ibm.com/support/pages/node/1106229 |website=IBM.com |publisher=International Business Machines Corporation |access-date=3 April 2022 |archive-date=3 April 2022 |archive-url=https://web.archive.org/web/20220403042552/https://www.ibm.com/support/pages/node/1106229 |url-status=live }}</ref> via software releases and intra-release “technology refreshes” (TRs).<ref>{{cite web |last1=IBM |title=IBM i Technology Updates |url=https://www.ibm.com/support/pages/node/1119129/ |website=IBM.com |publisher=International Business Machines Corporation |access-date=3 April 2022 |archive-date=12 April 2022 |archive-url=https://web.archive.org/web/20220412041649/https://www.ibm.com/support/pages/node/1119129 |url-status=live }}</ref><ref>{{cite web |last1=IBM |title=IBM i Technology Refresh |url=https://www.ibm.com/support/pages/ibm-i-technology-refresh |website=IBM.com |publisher=International Business Machines Corporation |access-date=3 April 2022 |archive-date=3 April 2022 |archive-url=https://web.archive.org/web/20220403042552/https://www.ibm.com/support/pages/ibm-i-technology-refresh |url-status=live }}</ref>  More built-in functions (BIFs) have been added.  It has the ability to link to [[Java (programming language)|Java]] objects,<ref>{{cite web|title=RPG/400 User's Guide|url=https://www.ibm.com/support/knowledgecenter/en/ssw_i5_54/books/c0918160.pdf|website=Ibm.com|access-date=6 August 2017|archive-date=7 August 2017|archive-url=https://web.archive.org/web/20170807070650/https://www.ibm.com/support/knowledgecenter/en/ssw_i5_54/books/c0918160.pdf|url-status=live}}</ref> and IBM i [[Application programming interface|API]]s; it can be used to write [[Common Gateway Interface|CGI]] programs with the help of IBM's [[Cgidev2]] Web toolkit,<ref>{{cite web|url=https://www.easy400.net/cgidev2/start|title=EASY400 CGIDEV2|website=Easy400.net|access-date=26 March 2019|archive-date=26 March 2019|archive-url=https://web.archive.org/web/20190326190652/https://www.easy400.net/cgidev2/start|url-status=live}}</ref> the [[Linoma Software|RPG Toolbox]], and other commercial Web-enabled packages. Even with the changes, it retains a great deal of backward compatibility, so an RPG program written 37 years ago could run today with little or no modification.

The SQL precompiler allows current RPG developers to take advantage of IBM's cost-based SQE (SQL Query Engine).  With the traditional F-Spec approach a developer had to identify a specific access path to a data set, now they can implement standard embedded SQL statements directly in the program.  When compiled, the SQL precompiler transforms SQL statements into RPG statements which call the database manager programs that ultimately implement the query request.

The RPG IV language is based on the [[EBCDIC]] character set, but also supports UTF-8, UTF-16 and many other character sets. The threadsafe aspects of the language are considered idiosyncratic by some as the compiler team has addressed threads by giving each thread its own static storage, rather than make the RPG run-time environment re-entrant. This has been noted to muddle the distinction between a thread and a process (making RPG IV threads a kind of hybrid between threads and processes).

In 2010, IBM launched ''RPG Open Access'', also known as ''Rational Open Access: RPG Edition''. It allows new I/O handlers to be defined by a programmer - enabling data to be read from and written to sources which RPG does not provide inbuilt support for.<ref>{{cite web|url=https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_72/rzasm/roaIntro.htm|title=Rational Open Access: RPG Edition|publisher=IBM|access-date=2021-03-17|archive-date=2024-02-29|archive-url=https://web.archive.org/web/20240229162204/https://www.ibm.com/docs/en/i/7.2?topic=edition-rational-open-access-rpg|url-status=live}}</ref>