Master The Enigma: How To Read Control Card In Cobol With Clarity

In the realm of COBOL programming, control cards play a crucial role in steering the execution flow and providing essential parameters. Understanding how to read control cards is paramount for effectively utilizing this powerful feature. This comprehensive guide will delve into the intricacies of reading control cards in COBOL programs, empowering you with the knowledge to harness their full potential.

What are Control Cards?

Control cards are special records that contain instructions and parameters for the COBOL program. They are typically placed at the beginning of the program and provide information such as:

• Program name
• Execution mode
• Input and output file names
• Environment settings

Reading Control Cards

COBOL provides two primary methods for reading control cards:

1. Using the READ statement

The `READ` statement reads a control card from the system input device (usually the terminal) and assigns its contents to a data item. The syntax is:

“`cobol
READ control-card-name
“`

where `control-card-name` is the name of the control card to be read.

2. Using the ACCEPT statement

The `ACCEPT` statement is similar to `READ`, but it allows you to specify a specific format for the control card. The syntax is:

“`cobol
ACCEPT control-card-name FROM system-name
“`

where:

• `control-card-name` is the name of the control card to be read.
• `system-name` is the name of the system device from which to read the control card.

Processing Control Card Data

Once a control card is read, its contents can be processed using standard COBOL statements. The data can be stored in variables, used to set program parameters, or trigger specific actions.

Example

Consider the following COBOL program:

“`cobol
IDENTIFICATION DIVISION.
PROGRAM-ID. CONTROL-CARD-EXAMPLE.

DATA DIVISION.
WORKING-STORAGE SECTION.
01 CONTROL-CARD-NAME PIC X(80).

PROCEDURE DIVISION.
READ CONTROL-CARD-NAME.
DISPLAY CONTROL-CARD-NAME.
STOP RUN.
“`

This program reads a control card and displays its contents.

Handling Control Card Errors

It is important to handle errors that may occur while reading control cards. COBOL provides the following error indicators:

• `AT END` occurs when the end of the input file is reached without finding the expected control card.
• `INVALID KEY` occurs when a control card with an invalid key is encountered.
• `NOT FOUND` occurs when a control card with a specified name cannot be found.

Advanced Techniques

In addition to the basic methods described above, COBOL also offers advanced techniques for reading control cards:

• Using the VALUE clause: The `VALUE` clause can be used to assign a default value to a control card if it is not found.
• Using the DELIMITER clause: The `DELIMITER` clause can be used to specify a delimiter character that separates the fields in a control card.
• Using the REPLACING clause: The `REPLACING` clause can be used to replace specific characters in a control card with other characters.

Final Note

Mastering the art of reading control cards in COBOL programs is essential for effective program execution. By understanding the concepts and techniques discussed in this guide, you can harness the power of control cards to enhance the functionality and flexibility of your programs.

What You Need to Know

Q1. What is the difference between `READ` and `ACCEPT` statements for reading control cards?

A1. `READ` reads a control card without any format checking, while `ACCEPT` allows you to specify a format for the control card.

Q2. How do I handle errors that occur while reading control cards?

A2. COBOL provides error indicators such as `AT END`, `INVALID KEY`, and `NOT FOUND` to handle errors.

Q3. Can I use advanced techniques to read control cards more efficiently?

A3. Yes, COBOL offers advanced techniques such as using the `VALUE`, `DELIMITER`, and `REPLACING` clauses to enhance control card reading.