Understanding the DATA DIVISION in COBOL

Understanding the DATA DIVISION in COBOL

Introduction

The DATA DIVISION is where a COBOL program comes alive, defining all the data it will use. It’s the engine room, housing everything from variables to complex file structures. If the IDENTIFICATION DIVISION is the title page and the ENVIRONMENT DIVISION sets the stage, the DATA DIVISION is where the real work begins.

What is the DATA DIVISION?

The DATA DIVISION specifies all the variables and file structures a COBOL program needs to perform its tasks. From storing temporary values to managing persistent records, this division ensures that data is organized and accessible. It’s a cornerstone of COBOL's ability to handle business logic with precision.

Here’s a simple structure for a DATA DIVISION with both a WORKING-STORAGE SECTION and a FILE SECTION:

       DATA DIVISION.
       WORKING-STORAGE SECTION.
           01 CUSTOMER-NAME        PIC X(30).
           01 CUSTOMER-ID          PIC 9(5).

       FILE SECTION.
           FD CUSTOMER-FILE.
           01 CUSTOMER-RECORD.
               05 CUSTOMER-ID      PIC 9(5).
               05 CUSTOMER-NAME    PIC X(30).

In this example:

• The WORKING-STORAGE SECTION defines temporary fields for CUSTOMER-NAME and CUSTOMER-ID.
• The FILE SECTION links CUSTOMER-FILE to a structure for handling file records.

Understanding the FD Keyword in the FILE SECTION

In the FILE SECTION, the FD (File Descriptor) keyword is used to define the structure and attributes of a file. Each file the program interacts with requires an FD entry. This keyword bridges the gap between the logical structure of a file (as the program sees it) and the physical file (as defined in the ENVIRONMENT DIVISION).

Let’s take a closer look at how the FD works:

       FILE SECTION.
           FD PAYROLL-FILE.
           01 PAYROLL-RECORD.
               05 EMPLOYEE-ID      PIC 9(5).
               05 EMPLOYEE-NAME    PIC X(30).
               05 SALARY           PIC 9(7)V99.

Here’s what’s happening:

• FD PAYROLL-FILE
  Declares a file named PAYROLL-FILE that the program will use. This ties the physical file (defined in the ENVIRONMENT DIVISION) to the logical structure described here.

• 01 PAYROLL-RECORD
  Specifies the structure of a single record in the file.

• Fields (e.g., EMPLOYEE-ID, EMPLOYEE-NAME)
  Defines the format and size of the individual data fields within the record.

The FD keyword is vital because it ensures that COBOL programs interpret file data consistently, avoiding potential errors during file operations.

Hands-On Example: Combining Sections

Here’s a complete DATA DIVISION setup that includes WORKING-STORAGE, FILE, and LOCAL-STORAGE sections:

       DATA DIVISION.
       WORKING-STORAGE SECTION.
           01 REPORT-DATE       PIC X(10).
           01 TOTAL-PAYROLL     PIC 9(10)V99 VALUE 0.

       FILE SECTION.
           FD EMPLOYEE-FILE.
           01 EMPLOYEE-RECORD.
               05 EMP-ID        PIC 9(5).
               05 EMP-NAME      PIC X(25).
               05 EMP-SALARY    PIC 9(7)V99.

       LOCAL-STORAGE SECTION.
           01 TEMP-VALUE        PIC 9(3).
           01 TEMP-TOTAL        PIC 9(6)V99.

In this example:

• WORKING-STORAGE SECTION
  Defines REPORT-DATE and TOTAL-PAYROLL, which persist throughout the program's execution.

• FILE SECTION
  Maps EMPLOYEE-FILE to a record structure containing employee IDs, names, and salaries.

• LOCAL-STORAGE SECTION
  Adds TEMP-VALUE and TEMP-TOTAL, which are reinitialized each time the program or subprogram begins execution.

This example demonstrates how the three sections can work together to handle both temporary and persistent data while also interacting with external files.

Conclusion

The DATA DIVISION is the heart of COBOL programming, where data is meticulously defined to support the program’s logic. Its optional sections provide flexibility, ensuring that programs include only what they need. By clearly defining every piece of data—and using tools like the FD to map logical and physical file structures—COBOL eliminates ambiguity and enhances reliability. Whether you're working with files or temporary data, the DATA DIVISION equips your program to handle it all with precision.

Image:  Lukas Bieri from Pixabay