Modular programming is a software design technique that emphasizes separating the functionality of a program into independent, interchangeable modules. Each module contains everything necessary to execute only one aspect of the desired functionality. This approach enhances code readability, maintainability, and reusability. In COBOL, modular programming is achieved through the use of subprograms and the CALL statement.

1. Modularity: Breaking down a program into smaller, manageable, and reusable pieces.
2. Subprograms: Independent program units that can be called from other parts of the program.
3. CALL Statement: Used to invoke a subprogram.
4. Parameter Passing: Mechanism to pass data between the main program and subprograms.

• Improved Readability: Smaller, well-defined modules are easier to understand.
• Easier Maintenance: Changes in one module do not affect others.
• Reusability: Modules can be reused across different programs.
• Parallel Development: Different modules can be developed simultaneously by different teams.

A subprogram in COBOL is typically a separate COBOL program that can be called from a main program. It has its own IDENTIFICATION DIVISION, ENVIRONMENT DIVISION, DATA DIVISION, and PROCEDURE DIVISION.

In the main program, you use the CALL statement to invoke the subprogram.

• Subprogram: CALCULATE-TAX computes the tax amount based on a fixed tax rate.
• Main Program: MAIN-PROGRAM calls CALCULATE-TAX and passes INCOME and TAX-AMOUNT as parameters.

Parameters can be passed by reference or by value. In COBOL, parameters are typically passed by reference, meaning the subprogram can modify the original data.

• BY REFERENCE: The subprogram can modify VAR1.
• BY VALUE: The subprogram receives a copy of VAR2 and cannot modify the original variable.

Create a COBOL program that calculates the area of a rectangle using a subprogram.

• Main Program: Initializes LENGTH and WIDTH, calls CALCULATE-AREA, and displays the results.
• Subprogram: CALCULATE-AREA computes the area of the rectangle and returns the result.

• Incorrect Parameter Order: Ensure the order of parameters in the CALL statement matches the order in the subprogram's USING clause.
• Uninitialized Variables: Always initialize variables before passing them to subprograms.
• Data Type Mismatch: Ensure the data types of parameters match between the main program and subprogram.

Modular programming in COBOL enhances code organization, readability, and maintainability. By breaking down a program into subprograms, you can manage complexity more effectively and promote code reuse. Practice creating and using subprograms to become proficient in modular programming techniques in COBOL.