How to Read Control Card in COBOL Program: Master the Essential Skills Today!

In the world of legacy systems, COBOL remains a powerful language, handling complex business logic and managing vast amounts of data. One of the intriguing aspects of COBOL programming involves working with control cards, which act as instructions for the program’s execution. Understanding how to read these control cards is crucial for anyone working with COBOL applications. This blog post will guide you through the process of reading control cards in COBOL programs, explaining the underlying concepts and providing practical examples.

What are Control Cards?

Control cards, often referred to as job control language (JCL) cards, are essentially commands that direct the execution of a COBOL program. They provide instructions to the operating system, specifying things like:

• Program name: Identifying the COBOL program to be executed.
• Input files: Defining the data sources for the program.
• Output files: Specifying where the program should write its results.
• Program parameters: Passing specific values to the program for customized execution.

These cards are typically written in a specific format and are read by the operating system before the COBOL program itself starts running.

The Importance of Control Cards in COBOL

Control cards play a pivotal role in COBOL programming for several reasons:

• Program control: They act as the bridge between the operating system and the COBOL program, directing its execution flow.
• Data management: Control cards define the input and output files, ensuring the program accesses the correct data.
• Parameterization: They allow passing parameters to the program, enabling flexible execution based on specific needs.
• Error handling: Control cards can be used to specify error handling procedures, ensuring robust program execution.

Reading Control Cards in a COBOL Program

The process of reading control cards in a COBOL program involves the following steps:
1. Defining the Control Card File:

• Use the `SELECT` statement to define the control card file as an input file.
• Assign a file name to the control card file, which will typically be a sequential file.
• Specify the file organization as `SEQUENTIAL` in the `SELECT` statement.

2. Opening the Control Card File:

• Use the `OPEN INPUT` statement to open the control card file for reading.

3. Reading Control Card Data:

• Employ the `READ` statement to read data from the control card file.
• The `READ` statement will read a single record (control card) at a time.
• Use the `INTO` clause to specify a working storage variable where the control card data will be stored.

4. Processing Control Card Data:

• Analyze the data read from the control card using COBOL’s string manipulation functions.
• Extract relevant information, such as program parameters, file names, and other instructions.
• Use the extracted information to control the program’s execution.

5. Closing the Control Card File:

• Use the `CLOSE` statement to close the control card file after processing all the control cards.

Example COBOL Code for Reading Control Cards

“`cobol
IDENTIFICATION DIVISION.
PROGRAM-ID. READ-CONTROL-CARDS.
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
SELECT CONTROL-CARD-FILE ASSIGN TO ‘CONTROL.DAT’
ORGANIZATION IS SEQUENTIAL.
DATA DIVISION.
FILE SECTION.
FD CONTROL-CARD-FILE.
01 CONTROL-CARD-RECORD PIC X(80).
WORKING-STORAGE SECTION.
01 CONTROL-CARD-DATA.
05 PROGRAM-NAME PIC X(10).
05 INPUT-FILE-NAME PIC X(10).
05 OUTPUT-FILE-NAME PIC X(10).
PROCEDURE DIVISION.
OPEN INPUT CONTROL-CARD-FILE.
READ CONTROL-CARD-FILE INTO CONTROL-CARD-DATA.
IF NOT AT END-OF-FILE
PERFORM PROCESS-CONTROL-CARD
UNTIL AT END-OF-FILE.
CLOSE CONTROL-CARD-FILE.
STOP RUN.
PROCESS-CONTROL-CARD.
READ CONTROL-CARD-FILE INTO CONTROL-CARD-DATA.
IF NOT AT END-OF-FILE
DISPLAY ‘Program Name: ‘ PROGRAM-NAME
DISPLAY ‘Input File: ‘ INPUT-FILE-NAME
DISPLAY ‘Output File: ‘ OUTPUT-FILE-NAME.
“`
This example demonstrates how to read control cards from a file named ‘CONTROL.DAT’. The program defines a file description (FD) for the control card file and a working storage area to store the control card data. It then reads the control cards one by one, extracts the relevant information, and displays it on the console.

Advanced Control Card Handling Techniques

• Conditional Processing: Use `IF` statements to process different control card types based on specific values.
• Looping: Employ `PERFORM` statements to iterate through multiple control cards and process them accordingly.
• Subroutine Calls: Use `CALL` statements to invoke subroutines for specialized control card processing.
• Error Handling: Implement error handling routines to handle situations where control card data is invalid or missing.

Beyond Basic Control Card Reading

While reading control cards is a fundamental aspect of COBOL programming, there are more advanced concepts to explore:

• JCL (Job Control Language): Understanding JCL is crucial for managing complex batch jobs, including submitting COBOL programs, allocating resources, and controlling program execution.
• Parameter Passing: Control cards can be used to pass parameters to COBOL programs, enabling dynamic execution based on specific requirements.
• Control Card Validation: Implement validation routines to ensure that control cards adhere to the specified format and contain valid data.

The End of an Era: Modern Alternatives to Control Cards

In contemporary software development, control cards are gradually fading into the past. Modern programming languages and environments offer more sophisticated ways to manage program execution and data input:

• Command-line arguments: Programs can receive input parameters directly from the command line, eliminating the need for separate control card files.
• Configuration files: Modern applications often use configuration files (e.g., JSON, YAML) to store program settings and parameters.
• Environmental variables: Operating system environment variables provide a flexible way to pass information to programs.

A Look Back, A Step Forward

While control cards may be a relic of a bygone era in software development, understanding how to read them remains valuable for anyone working with legacy COBOL systems. As you delve deeper into the intricacies of COBOL programming, you’ll appreciate the importance of control cards in managing program execution and data flow.

Top Questions Asked

Q: What are some common control card formats?
A: Control cards typically follow a specific format, including:

• Fixed-length records: Each control card has a fixed number of characters, often 80 characters per line.
• Key-value pairs: Control cards may contain key-value pairs, where a keyword identifies the parameter, and a value specifies its setting.
• Command-based: Some control cards may consist of specific commands, such as `START`, `STOP`, or `RUN`.

Q: How do I handle errors when reading control cards?
A: Implement error handling routines to address potential issues with control cards, such as:

• Invalid format: Check if the control card adheres to the expected format.
• Missing data: Handle cases where required data is missing from the control card.
• Invalid values: Validate the data values against predefined rules or ranges.

Q: Are there any tools for creating and managing control cards?
A: While there are no dedicated tools specifically for control cards, you can use general-purpose text editors or IDEs to create and manage control card files.
Q: What are some best practices for using control cards in COBOL programs?
A:

• Clear documentation: Document the format and purpose of each control card.
• Error handling: Implement robust error handling routines to deal with invalid or missing control cards.
• Modular design: Break down complex control card processing into smaller, manageable routines.
• Code readability: Use clear and concise coding practices to enhance code readability and maintainability.

Understanding how to read control cards in COBOL programs is a valuable skill, particularly when working with legacy systems. By mastering this technique, you gain control over program execution, data management, and error handling, ensuring the smooth operation of your COBOL applications. As you venture deeper into the world of COBOL, remember that control cards, though a remnant of the past, offer a glimpse into the history and evolution of software development.