TexAS400 Tutorial


RPG III - Adding a Loop, IF and Subroutine

In a previous topic, you should have written a very short RPG III program to change the first record in the CUST file.

Let's add so more statements to the program so that it will read all records in the file and change the zip code for everyone in Tennessee.

You may want to read this very short article on data navigation first:

Data Navigation

Your program will still reference the CUST file, so, use DSPLIBL to display your library list. If you don't see your user library in the list, use the CHGCURLIB USER999 command. This will change your current library to USER999. Remember that when I say USER999, I mean your user ID.

You can look at your data file with the command: RUNQRY QRYFILE(CUST)

Now, use PDM to copy your old program. Start PDM (STRPDM). You should see something like:

 
--------------------------------------------------------------------- 
                 AS/400 Programming Development Manager (PDM)  
                                                               
Select one of the following:                                   
                                                               
     1. Work with libraries                                    
     2. Work with objects                                      
     3. Work with members                                      
     4. Work with projects                                     
     5. Work with groups                                       
     6. Work with parts                                        
                                                               
     9. Work with user-defined options                         
--------------------------------------------------------------------- 
Key in 3 and hit ENTER. Fill in the screen to look like:

                      Specify Members to Work With                        
                                                                          
Type choices, press Enter.                                                
                                                                          
  File  . . . . . . . . . .   QRPGSRC      Name, F4 for list              
                                                                          
    Library . . . . . . . .     user999    *LIBL, *CURLIB, name         
                                                                          
  Member:                                                                 
    Name  . . . . . . . . .   *ALL         *ALL, name, *generic*          
    Type  . . . . . . . . .   *ALL         *ALL, type, *generic*,  
                                                                          
----------------------------------------------------------------------- 
Of course, change the USER999 to your user ID. Hit ENTER now.

Your screen should look like:


                                                                              
File  . . . . . .   QRPGSRC                                                   
  Library . . . .     USER999              Position to  . . . . .             
                                                                              
Type options, press Enter.                                                    
 2=Edit         3=Copy  4=Delete 5=Display       6=Print     7=Rename         
 8=Display description  9=Save  13=Change text  14=Compile  15=Create module..
                                                                              
Opt  Member      Type        Text                                             
        TUTR001     RPG         Change the first record in CUST file                                                
Key a 3 as the option next to TUTR001. Hit ENTER and then fill in the screen to copy the program and name it TUTR004:

                                 Copy Members                     
                                                                  
 From file . . . . . . . :   QRPGSRC                              
   From library  . . . . :     USER999                            
                                                                  
 Type the file name and library name to receive the copied members
                                                                  
   To file . . . . . . . .   QRPGSRC      Name, F4 for list       
     To library  . . . . .     USER999                            
                                                                  
 To rename copied member, type New Name, press Enter.             
                                                                  
 Member         New Name                                          
 TUTR001        TUTR004                                           
Hit ETER and you should now see the copied program in your list. You may want to change the description of the program so you'll remember which program does what. Your screen should look like:

Remember from your PDM lesson that this is trying to show you a list of all of the RPG

Work with Members Using PDM

                                                                    
 File  . . . . . .   QRPGSRC                                        
   Library . . . .     USER000              Position to  . . . . .  
                                                                    
 Type options, press Enter.                                         
  2=Edit         3=Copy  4=Delete 5=Display       6=Print     7=Rename
  8=Display description  9=Save  13=Change text  14=Compile  15=Create
                                                                    
 Opt  Member      Type        Text                                  
      TUTR001     RPG         Change the first record in CUST file  
      TUTR004     RPG         Change all TN records in CUST file    
Key a 2 next to TUTR004 so you can change the source statements to the program.

Before we make the changes to the program we need to discuss different ways to write a loop. We want a loop that will read every record in the CUST file and change the records that have TN (Tennessee) as the state.

I always think of a loop as saying:


	Try to Read a record from the file
	If there was a record then
		change it
		update it
	Go back and try to read the next record
Remember that when RPG III reads a record, you must specify which indicator to turn on it there are no more records; that is, it is END OF FILE.

You could write the instructions above in RPG III like this:


FCUST    UF  E                    DISK                 
C           READIT    TAG                              
C                     READ CUST                     90 
C           *IN90     IFEQ *OFF                        
C                     MOVE *BLANK    CSADR1            
C                     MOVEL'456 OAK' CSADR1            
C                     UPDATCSREC                       
C                     GOTO READIT                      
C                     ENDIF                            
C                     MOVE *ON       *INLR             
C                     RETRN                            
This program says to define a file named CUST.

Then, name the first line of the Calculations READIT. The, try to read a record (if END OF FILE, move ON to indicator# 90) If indicator# 90 is *OFF Move … Move … Update … Go back to the first line of the program Turn on the "magic" LR indicator End the program

This will work. The only problem is that for about 15 years, it has been considered bad style to use GOTO statements. This is an industry trend that includes all languages and all computers.

The reason is that long programs are hard to understand when there are a lot of GOTO statements. Programs with lots of GOTO's are nicknamed spaghetti programs because the statements can be as hard to follow as a strand of spaghetti.

SO, the industry prefers that we use DO statements. We can use a DO WHILE or a DO UNTIL. This still leaves us with 2 good ways to write the loop. I prefer the first because it looks a lot like the GOTO loop but without the GOTO:


FCUST    UF  E                    DISK                
C           *IN90     DOWEQ*OFF                       
C                     READ CUST                     90
C           *IN90     IFEQ *OFF                       
C                     MOVE *BLANK    CSADR1           
C                     MOVEL'456 OAK' CSADR1           
C                     UPDATCSREC                      
C                     ENDIF                           
C                     ENDDO                           
C                     MOVE *ON       *INLR            
C                     RETRN                           
This program defines the file and then starts a DO loop.

The DOWEQ statement says that as long as indicator # 90 is OFF, execute all of the statements until the ENDDO (End DO) statement.

Then, start back at the top (a lot like a GOTO). If indicator # 90 is still OFF, execute the statements again. Eventually, the program will read all 1,000+ records in the file. When it does, indicator # 90 will be turned ON.

When indicator # 90 is ON, the program will not update the record and it will not continue to loop.

The other way to code the loop is with what is called a priming read statement. This method needs 2 READ statements but does not need the IF statement.

If looks like:


FCUST    UF  E                    DISK                    
C                     READ CUST                     90    
C           *IN90     DOWEQ*OFF                           
C                     MOVE *BLANK    CSADR1               
C                     MOVEL'456 OAK' CSADR1               
C                     UPDATCSREC                          
C                     READ CUST                     90    
C                     ENDDO                               
C                     MOVE *ON       *INLR                
C                     RETRN                               
Here, the first record is read before the DO begins. The statements are executed and the READ statement is at the end of the DO loop.

Either of these 2 methods is considered good code. I prefer the first.

Using the first acceptable loop, add the IF statement to test to see if the state is TN. Also, replace the code so that it changes the ZIP to 987650000.

First, do this by putting in an IF statement like this:


C           CSSTE     IFEQ 'TN'                      
C                     MOVE 987650000 CSZIP           
C                     UPDATCSREC                     
C                     ENDIF                          

So, the whole program looks like:

FCUST    UF  E                    DISK                
C           *IN90     DOWEQ*OFF                       
C                     READ CUST                     90
C           *IN90     IFEQ *OFF                       
C           CSSTE     IFEQ 'TN'                       
C                     MOVE 987650000 CSZIP            
C                     UPDATCSREC                      
C                     ENDIF                           
C                     ENDIF                           
C                     ENDDO                           
C                     MOVE *ON       *INLR            
C                     RETRN 
This is considered good code by most programmers. But it really helps to move the update logic to a separate place in the program as a subroutine. This is done here only to help readability. It doesn't change the outcome.

In RPG, subroutines begin with a BEGSR (BEGIN SUBROUTINE) statement which also has the name of the subroutine. They end with ENDSR (END SUBROUTINE). They are run with the EXSR (EXECUTE SUBROUTINE) command.

If we move the update logic to a subroutine named UPD, the program looks cleaner. Also, we can add "COMMENTS" records. Any statement with a * after the record type, is a "COMMENT". In fact you can even remove the record type (the C or F in column 6).

With a few comment lines added to define the sections of the program, we get a pretty readable program:


FCUST    UF  E                    DISK                 
 *-----------------------------------------------------
C           *IN90     DOWEQ*OFF                        
C*
C                     READ CUST                     90 
C           *IN90     IFEQ *OFF                        
C                     EXSR UPD                         
C                     ENDIF                            
C                     ENDDO                            
C*                                                     
C                     MOVE *ON       *INLR             
C                     RETRN                            
 *-----------------------------------------------------
C           UPD       BEGSR                            
C*                                                     
C           CSSTE     IFEQ 'TN'                        
C                     MOVE 987650000 CSZIP             
C                     UPDATCSREC  
C                     ENDIF       
C*                                
C                     ENDSR       	
 *------------------------------------------------------
Now, compile this (remember… option 14) and run it (CALL TUTR004).

Then use RUNQRY QRYFILE(CUST) to see the data. If you scroll right by hitting F20 (actually Shift and F8) you will see that you changed the zip in all of the records with TN as CSSTE.

Are you starting to have fun? I hope so.

The next lesson will be short. It will put these statements in the RPG ILE version. You will see how much prettier ILE code is than RPG III.

 

 

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