Logic, files and calling other programs

COBOL's control flow is small: conditionals, PERFORM loops, EVALUATE for multi-way choices, and CALL to run another program. File I/O is verb-based and always tells you what happened through a status code.

EVALUATE: the readable multi-way branch

EVALUATE replaces nested IFs

           EVALUATE TRUE
               WHEN CUST-BALANCE > 100000
                   MOVE 'PREMIUM' TO WS-TIER
               WHEN CUST-BALANCE > 10000
                   MOVE 'STANDARD' TO WS-TIER
               WHEN CUST-CLOSED
                   MOVE 'CLOSED' TO WS-TIER
               WHEN OTHER
                   MOVE 'BASIC' TO WS-TIER
           END-EVALUATE

File verbs and FILE STATUS

Always declare FILE STATUS IS WS-STATUS on the SELECT and check it after every operation. Without it, an I/O failure gives you an abend with no context; with it, you know exactly which file and which condition.

Status	Means
00	Successful
02	Successful, duplicate key on an alternate index
10	End of file
22	Duplicate key — the record already exists
23	Record not found
35	File not found at OPEN — usually a missing DD
37	Open failed on an incompatible device or format
39	Attribute mismatch between program and file
9x	Vendor-specific; check the compiler's list

Tables and subscripts

OCCURS, indexes and a search

       01  WS-RATE-TABLE.
           05  WS-RATE-ENTRY OCCURS 50 TIMES
                             INDEXED BY WS-RATE-IX.
               10  WS-RATE-CODE   PIC X(03).
               10  WS-RATE-VALUE  PIC S9(3)V9(4) COMP-3.

           SET WS-RATE-IX TO 1
           SEARCH WS-RATE-ENTRY
               AT END
                   DISPLAY 'RATE NOT FOUND: ' WS-IN-CODE
                   MOVE 16 TO RETURN-CODE
               WHEN WS-RATE-CODE(WS-RATE-IX) = WS-IN-CODE
                   MOVE WS-RATE-VALUE(WS-RATE-IX) TO WS-RATE
           END-SEARCH

Calling another program

CALL with parameters

* in the caller
       01  WS-PARM-AREA.
           05  WS-IN-CODE     PIC X(03).
           05  WS-OUT-RATE    PIC S9(3)V9(4) COMP-3.
           05  WS-RETURN      PIC S9(4) COMP.

           CALL 'RATELKUP' USING WS-PARM-AREA

* in the called program RATELKUP
       LINKAGE SECTION.
       01  LK-PARM-AREA.
           05  LK-IN-CODE     PIC X(03).
           05  LK-OUT-RATE    PIC S9(3)V9(4) COMP-3.
           05  LK-RETURN      PIC S9(4) COMP.
       PROCEDURE DIVISION USING LK-PARM-AREA.

Static CALL — a literal name; the module is bound in at link time. Fast, but changing the called program means relinking the caller.
Dynamic CALL — a variable name, or compiled with DYNAM; resolved at run time from the STEPLIB. Slower per call, but you can replace one module without relinking everything.
Parameters are passed by reference by default: the called program changes the caller's storage directly.

Example, line by line

COBOLWhat it means

PERFORM UNTIL WS-EOF

Loop until the end-of-file condition (an 88-level) becomes true.

READ INFILE

Read the next record from the file the program calls INFILE.

AT END SET WS-EOF TO TRUE

No more records: set the flag so the loop stops.

NOT AT END PERFORM 2000-PROCESS

A record was read: process it in its own paragraph.

END-READ

Scope terminator: safer than relying on a full stop.

END-PERFORM

End of the loop body.

Try it yourself

TRY IT YOURSELF

Which statement should a called subprogram use to return control to its caller?

Show a hint

STOP RUN would end the whole run unit, not just the subprogram.

Show the solution

GOBACK. It returns to the caller; in a main program it also ends the run.