Day 18: COBOL Tables and Arrays – Managing In-Memory Data Efficiently for Business Processing

 


1. Introduction: What Are Tables (Arrays) in COBOL and Why Are They Important?

In COBOL, a table (or array) is a structured way to store multiple related data items in memory under a single name. Instead of defining multiple variables for each item (like EMP1, EMP2, EMP3), you can define a table and access items using an index or subscript.

This is essential when working with lists, such as employee IDs, prices, or department codes, where data must be processed repeatedly or compared efficiently.

For example:

  • A payroll system might store all employee salaries in a table for quick averaging.

  • A banking program might hold all transaction amounts in memory before writing them to a file.

By using COBOL tables, you can dramatically reduce repetitive code and improve performance by keeping frequently used data in memory instead of reading from files repeatedly.

2. Declaring and Using Tables in COBOL

COBOL tables are defined using the OCCURS clause inside a data structure. You can access table elements using a subscript or an index.

Here’s a complete example 👇

Program: Managing Employee Names Using Tables

IDENTIFICATION DIVISION.
PROGRAM-ID. EMPARRAY.

DATA DIVISION.
WORKING-STORAGE SECTION.
01 EMPLOYEE-TABLE.
   05 EMP-NAME OCCURS 5 TIMES.
      10 EMP-FIRST PIC A(10).
      10 EMP-LAST  PIC A(10).

01 I            PIC 9 VALUE 1.

PROCEDURE DIVISION.
    DISPLAY "ENTER 5 EMPLOYEE NAMES:"
    PERFORM VARYING I FROM 1 BY 1 UNTIL I > 5
        DISPLAY "ENTER FIRST NAME OF EMPLOYEE " I ":"
        ACCEPT EMP-FIRST(I)
        DISPLAY "ENTER LAST NAME OF EMPLOYEE " I ":"
        ACCEPT EMP-LAST(I)
    END-PERFORM

    DISPLAY "-----------------------------------"
    DISPLAY "EMPLOYEE LIST:"
    DISPLAY "-----------------------------------"
    PERFORM VARYING I FROM 1 BY 1 UNTIL I > 5
        DISPLAY "EMPLOYEE " I ": " EMP-FIRST(I) " " EMP-LAST(I)
    END-PERFORM
STOP RUN.

Explanation:

  • OCCURS 5 TIMES defines a table with 5 entries.

  • EMP-FIRST(I) and EMP-LAST(I) access specific elements using a subscript I.

  • The PERFORM VARYING loop populates and displays the data.

Tables can also be multi-dimensional, allowing more complex structures such as department-wise employee lists or region-wise sales matrices.

3. Advanced Concepts: INDEXED Tables and SEARCH

For large datasets, COBOL allows indexed tables, which are faster than subscripted ones because indexes work with memory positions directly.

Example:

05 EMP-ID OCCURS 100 TIMES
   INDEXED BY EMP-INDEX
   PIC 9(4).

You can then use:

SET EMP-INDEX TO 1
PERFORM UNTIL EMP-INDEX > 100
    DISPLAY EMP-ID (EMP-INDEX)
    SET EMP-INDEX UP BY 1
END-PERFORM

COBOL also supports:

  • SEARCH – For linear searches in unsorted tables.

  • SEARCH ALL – For binary searches in sorted tables (much faster).

This makes COBOL tables powerful for lookup tasks like verifying if a product code exists or fetching employee details quickly.

4. Practical Applications and Best Practices

Common Use Cases:

  • 🧾 Storing monthly sales data before writing reports.

  • 🏦 Holding customer IDs temporarily for fast lookups.

  • 📊 Processing large datasets for statistical summaries.

Best Practices:

  • Always define tables with a logical maximum size.

  • Use INDEXED BY for better efficiency on large datasets.

  • Ensure data is sorted before using SEARCH ALL.

  • Combine table processing with PERFORM loops for flexibility.

With tables, COBOL programs can handle in-memory operations efficiently, reducing file reads and improving batch processing speeds significantly.

🧠 Key Takeaways

  • Tables (OCCURS) store multiple related data items in memory.

  • Access elements using subscripts or indexes.

  • Use PERFORM loops for iteration and SEARCH for lookups.

  • Tables enhance performance for repetitive data operations.

  • Indexed tables enable fast, database-like searching in COBOL.

💻 Practice Task

  1. Create a program SALESTAB.cbl that:

    • Accepts 10 product sales amounts into a table.

    • Displays all values and computes the total and average.

  2. Add a feature to find the highest and lowest sales in the list.

  3. Bonus: Implement a SEARCH ALL to find a specific sales amount in the sorted table.

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