In COBOL, passing parameters between programs or subprograms is a fundamental concept that allows for modular and reusable code. This section will cover the basics of passing parameters, including the different methods and best practices.

1. CALL Statement: Used to invoke a subprogram.
2. Linkage Section: Defines the parameters that are passed to and from a subprogram.
3. USING Clause: Specifies the parameters to be passed.
4. BY REFERENCE vs. BY CONTENT: Methods of passing parameters.

The CALL statement is used to invoke a subprogram. The syntax is as follows:

In this example, the main program calls the subprogram CALCULATE-TAX and passes two parameters: TAX-RATE and GROSS-INCOME.

The LINKAGE SECTION is used in the subprogram to define the parameters that are passed from the main program. The PROCEDURE DIVISION header must also include a USING clause to specify these parameters.

When parameters are passed BY REFERENCE, the subprogram can modify the original data in the calling program. This is the default method.

When parameters are passed BY CONTENT, the subprogram receives a copy of the data, and any modifications do not affect the original data in the calling program.

In this example, TAX-RATE is passed by reference, and GROSS-INCOME is passed by content.

In this example, the main program calls the subprogram CALCULATE-TAX, passing TAX-RATE, GROSS-INCOME, and NET-INCOME. The subprogram calculates the net income and returns the result to the main program.

Task: Write a main program and a subprogram where the main program passes two numbers to the subprogram, and the subprogram returns their sum.

Solution:

Main Program

Subprogram

Task: Modify the previous exercise to pass the numbers by content and the sum by reference.

Solution:

Main Program

Subprogram

1. Mismatch in Data Types: Ensure that the data types of the parameters in the main program and subprogram match.
2. Incorrect USING Clause: Verify that the USING clause in the PROCEDURE DIVISION header of the subprogram matches the parameters defined in the LINKAGE SECTION.
3. Uninitialized Variables: Initialize variables in the main program before passing them to the subprogram to avoid unexpected results.

Passing parameters in COBOL is a powerful feature that promotes modularity and code reuse. By understanding the CALL statement, LINKAGE SECTION, and the differences between BY REFERENCE and BY CONTENT, you can effectively manage data flow between programs and subprograms. Practice with the provided exercises to reinforce your understanding and avoid common pitfalls.