COBOL by Example: Relative Files

COBOL 2002

Understanding relative file organization for record access by position number, using RELATIVE KEY with Working-Storage variables, implementing array-like disk storage, and managing sparse data efficiently.

Code

       IDENTIFICATION DIVISION.
       PROGRAM-ID. REL-FILES.
       
       ENVIRONMENT DIVISION.
       INPUT-OUTPUT SECTION.
       FILE-CONTROL.
           SELECT DATA-FILE ASSIGN TO "data.dat"
           ORGANIZATION IS RELATIVE
           ACCESS MODE IS RANDOM
           RELATIVE KEY IS REC-NUM.
           
       DATA DIVISION.
       FILE SECTION.
       FD  DATA-FILE.
       01  DATA-REC    PIC X(20).
       
       WORKING-STORAGE SECTION.
       01  REC-NUM     PIC 99.
       
       PROCEDURE DIVISION.
           OPEN INPUT DATA-FILE.
           MOVE 5 TO REC-NUM.
           READ DATA-FILE
               INVALID KEY DISPLAY "Record 5 empty"
               NOT INVALID KEY DISPLAY "Rec 5: " DATA-REC
           END-READ.
           CLOSE DATA-FILE.
           STOP RUN.

Explanation

Relative files are similar to arrays but stored on disk. Records are accessed by their relative position (Record 1, Record 2, etc.). This is faster than indexed files because no index lookup is required; the system calculates the offset directly based on the record length.

You define a RELATIVE KEY variable in Working-Storage (not in the file record itself) to specify which record number you want to read or write. This key acts as the "array index" for the file.

Relative files are ideal for data that is naturally numbered, such as days of the year (1-366) or sequential invoice numbers. However, they can be wasteful of space if the data is sparse (e.g., if you have record 1 and record 1000, space is reserved for the 998 empty slots in between).

Code Breakdown

7

ORGANIZATION IS RELATIVE. Specifies the file type. Records are fixed-length slots.

9

RELATIVE KEY IS REC-NUM. Links the file operation to a variable 'REC-NUM' defined in Working-Storage.

17

01 REC-NUM PIC 99. This variable holds the record number we want to access (e.g., 5 for the 5th record).

21

MOVE 5 TO REC-NUM. Sets the target record number.

22

READ DATA-FILE. Attempts to read the record at the position specified by REC-NUM.

23

INVALID KEY. Triggered if the slot at position 5 is empty or out of bounds.

Samplebadu

COBOL by Example: Relative Files

Code

Explanation

Code Breakdown