rpg/400

 rpg/400

RPG (Report Program Generator) is a high-level programming language with a long history, primarily used for business applications on IBM midrange systems. RPG/400 specifically refers to the version of the language designed for the IBM AS/400 (now known as IBM i) system, introduced in the late 1980s.

Unlike "free-form" languages like Python or Java, classic RPG/400 is positional, meaning the column in which you type a character determines what that character does.

Key Characteristics

• Column-Based Logic: Traditional RPG/400 uses fixed-format coding. For example, an operation code (like ADD) must start in a specific column (usually column 28).

• The "Program Cycle": A unique feature where the language automatically handles reading records, updating counters, and printing totals without the programmer writing explicit loops.

• Indicators: These are two-digit switches (01–99) used to control conditioning logic (e.g., if a record is found, turn on indicator 10).

• Data-Centric: It is exceptionally efficient at handling DB2 databases, which are integrated directly into the OS/400 operating system.

The Specification Types (Specs)

In RPG/400, you don't just type code anywhere. You use different "Specification Sheets" to define different parts of the program:

Spec Type - Name -Purpose

H-Spec -Header - Defines program-wide settings and title.

F-Spec - File - Defines the files (databases, displays, printers) the program will use.

E-Spec - Extension - Used for defining arrays and tables.

I-Spec - Input - Defines the data structures and fields within the files.

C-Spec - Calculation - Where the actual logic (addition, moves, comparisons) happens.

O-Spec - Output - Defines the layout of reports or files being written.

Evolution of the Language

RPG has evolved significantly since the RPG/400 era:

1. RPG II / RPG III: Used on older System/34 and System/38 machines.

2. RPG/400: The version standardized for the original AS/400.

3. RPGLE (RPG IV): Introduced in 1994. This allowed for mixed-case, longer field names, and "Bound" programs (Integrated Language Environment - ILE).

4. Free-Form RPG: The modern version used today, which looks much more like C# or Java and has mostly abandoned the strict column-based requirements.

Code snippet to accept 2 numeric inputs, store them, multiple them, display the results on screen and send it to printer: 

     A          R SCREEN1

     A                                  1 30'MULTIPLY TOOL'

     A                                  5  5'ENTER NUM 1:'

     A            NUM1           5S 0I  5 20

     A                                  6  5'ENTER NUM 2:'

     A            NUM2           5S 0I  6 20

     A                                 10  5'RESULT IS:'

     A            RESULT         10S 0O 10 20

     A                                 23  5'CF03 TO EXIT'

     A          R RPTREC

     A                                  1 10'CALCULATION REPORT'

     A            NUM1          10      5  5

     A                                  5 16'*'

     A            NUM2          10      5 18

     A                                  5 29'='

     A            RESULT        15      5 31

     H* Header Spec - Program Name: MULTIPLY

     FWORKDSPFCF  E                    WORKSTN

     FWORKPRTF  O    E             PRINTER

     *

     C* 1. Display the screen and wait for user input

     C           TAG1      TAG

     C                     EXFMTIDSCREEN1

     *

     C* 2. Check if user pressed F3 (Indicator 03) to exit

     C   *IN03     GOTO      END

     *

     C* 3. Multiplication Logic (Factor 1 * Factor 2 = Result)

     C           NUM1      MULT NUM2      RESULT

     *

     C* 4. Output to Printer

     C                     WRITE RPTREC

     *

     C* 5. Loop back to show result on screen

     C                     GOTO  TAG1

     *

     C           END       TAG

     C                     SETON                     LR

ILE RPG

     H DFTACTGRP(*NO) ACTGRP(*NEW)

     * File Specifications

     FMYSCREEN  CF   E             WORKSTN

     FMYPRINTER O    E             PRINTER

     * Data Specifications (Variable Definitions)

     D Input1          S              5P 0

     D Input2          S              5P 0

     D FinalResult     S             10P 0

     * Main Logic

     C     MainLoop      TAG

     * 1. Display Screen (EXFMT)

     C                   EXFMT     SCREEN1

     * 2. Exit if F3 is pressed (Indicator 03)

     C                   IF        *IN03 = *ON

     C                   SETON                                        LR

     C                   RETURN

     C                   ENDIF

     * 3. Calculation using EVAL (Extended Factor 2)

     C                   EVAL      FinalResult = Input1 * Input2

     * 4. Write to Printer

     C                   WRITE     PRT_REC

     * 5. Loop back

     C                   GOTO      MainLoop