Beginner's Guide to COBOL

Introduction to COBOL #

What is COBOL? #

COBOL (Common Business-Oriented Language) is a compiled English-like programming language designed for business applications. It’s known for its readability and its ability to handle large volumes of data efficiently. Unlike languages focused on system-level programming or web development, COBOL excels at tasks such as processing transactions, managing files, and performing complex calculations within a business context. Its structure is highly procedural, making it easy to understand the flow of a program. Key features include strong support for file handling, decimal arithmetic (crucial for financial applications), and robust error handling.

History and Relevance of COBOL #

Developed in the late 1950s, COBOL is one of the oldest high-level programming languages still in widespread use. Its initial goal was to create a standardized language for business data processing, aiming for portability across different computer systems. This portability was a revolutionary concept at the time. Throughout the 1960s and 70s, COBOL became the dominant language for mainframe systems, powering critical applications in finance, government, and other sectors. While newer languages emerged, the vast amount of legacy COBOL code continued to operate, and many organizations remain heavily reliant on COBOL systems.

Why Learn COBOL Today? #

Despite its age, COBOL remains relevant for several reasons:

• Legacy System Maintenance: A significant portion of the world’s business infrastructure runs on COBOL. Maintaining and updating these systems requires COBOL programmers.
• High Demand: The aging workforce of COBOL programmers is leading to a skills shortage, creating high demand for professionals with COBOL expertise.
• Financial Stability: The stability and reliability of COBOL are highly valued in industries where data accuracy and system uptime are critical.
• Career Opportunities: Learning COBOL can open doors to well-paying jobs in stable, established companies.

Setting up your COBOL Environment #

Setting up a COBOL development environment depends on your operating system and preferred compiler. Several options exist, both open-source and commercial:

• OpenCOBOL: A free, open-source COBOL compiler that runs on various platforms (Windows, Linux, macOS). Its website provides detailed installation instructions and documentation. This is a good starting point for beginners due to its ease of use and accessibility.

• GnuCOBOL (OpenCOBOL fork): Another popular open-source compiler. Its community support and features are strong options.

• Commercial Compilers: Several commercial COBOL compilers offer advanced features and support. These often come with IDEs (Integrated Development Environments) that provide enhanced development tools. However, these typically require purchasing licenses.

General Steps (using OpenCOBOL as an example):

1. Download and Install: Download the appropriate OpenCOBOL package for your operating system from the official website. Follow the installation instructions provided.

2. Verify Installation: After installation, compile a simple “Hello, World!” program to verify that OpenCOBOL is correctly configured.

3. Text Editor: You’ll need a text editor or IDE to write your COBOL code. Any plain text editor will work (Notepad, Notepad++, Sublime Text, etc.), although IDEs designed for COBOL provide helpful features like syntax highlighting and debugging tools.

4. Compiler Command-Line: You’ll typically compile your COBOL code using command-line instructions. Consult the OpenCOBOL documentation for specific compilation commands.

Remember to always consult the documentation for your chosen compiler for detailed and specific installation and usage instructions. The process may vary slightly depending on your operating system and compiler version.

Basic COBOL Structure #

Understanding the COBOL Divisions (IDENTIFICATION, ENVIRONMENT, DATA, PROCEDURE) #

A COBOL program is organized into four divisions:

• IDENTIFICATION DIVISION: This division provides descriptive information about the program, such as the program name, author, and date. It’s primarily for documentation purposes and doesn’t contain executable code. The PROGRAM-ID paragraph is mandatory and specifies the program’s name.

• ENVIRONMENT DIVISION: This division describes the program’s relationship to the computer’s hardware and operating system. It’s divided into two sections: CONFIGURATION SECTION (specifies the compiler and other system details) and INPUT-OUTPUT SECTION (describes the files used by the program, connecting internal file names to external file names). The FILE-CONTROL paragraph within the INPUT-OUTPUT SECTION is crucial for file handling.

• DATA DIVISION: This is where you declare all the data structures used by the program. It defines variables, constants, files, and their characteristics. The FILE SECTION describes the structure of the files used (record layouts), while the WORKING-STORAGE SECTION defines variables and constants used for internal calculations and data storage.

• PROCEDURE DIVISION: This division contains the actual executable instructions of the program—the logic that processes the data. It’s structured into paragraphs and sections to organize the code logically. This is where the program’s flow control, input/output operations, and calculations are specified.

Data Types and Declarations (PIC, VALUE) #

COBOL uses the PIC (PICTURE) clause to define the data type and format of a variable. The VALUE clause can be used to initialize a variable with a specific value.

Examples:

• 05 CUSTOMER-NAME PIC X(30). This declares a variable named CUSTOMER-NAME that can store up to 30 alphanumeric characters. X represents an alphanumeric character.

• 05 CUSTOMER-ID PIC 9(5). This declares a variable named CUSTOMER-ID which can hold a 5-digit numeric value. 9 represents a numeric digit.

• 05 AMOUNT PIC 9(7)V99. This declares a variable named AMOUNT for a numeric value with 7 digits before the decimal point and 2 digits after (e.g., 1234567.89). V represents an implied decimal point.

• 05 TAX-RATE PIC 9V99 VALUE 0.06. This declares a variable TAX-RATE initialized to 0.06.

The level numbers (e.g., 05) indicate the hierarchical structure within the data declaration; lower-level numbers represent subordinate elements.

File Handling (SELECT, OPEN, CLOSE, READ, WRITE) #

COBOL provides built-in features for managing files:

• SELECT: This statement associates an internal file name (used within the program) with an external file name (the actual file on the disk). Example: SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.DAT"

• OPEN: This statement prepares a file for processing. OPEN INPUT CUSTOMER-FILE opens CUSTOMER-FILE for reading; OPEN OUTPUT TRANSACTION-FILE opens TRANSACTION-FILE for writing.

• CLOSE: This statement closes a file, releasing it from the program’s control. Example: CLOSE CUSTOMER-FILE, TRANSACTION-FILE.

• READ: This statement reads a record from a file. Example: READ CUSTOMER-FILE AT END MOVE 1 TO EOF-FLAG. The AT END clause specifies what to do if the end of the file is reached.

• WRITE: This statement writes a record to a file. Example: WRITE TRANSACTION-RECORD.

Simple Input and Output #

Basic input and output in COBOL often involves reading from and writing to files (as described above). However, you can also use the DISPLAY statement to output data to the console (screen) and the ACCEPT statement to receive input from the console.

Examples:

• DISPLAY "Hello, world!" This displays the message “Hello, world!” on the console.

• ACCEPT USER-INPUT This accepts input from the console and stores it in the USER-INPUT variable. You’ll need to define USER-INPUT appropriately in the DATA DIVISION. The format of the input will depend on how the variable USER-INPUT is defined using PIC.

Remember that the DISPLAY and ACCEPT statements are generally for simple console interaction; file handling is more common for larger business applications.

Working with Data #

Variables and Constants #

In COBOL, variables and constants are declared in the DATA DIVISION. Variables hold values that can change during program execution, while constants maintain a fixed value throughout.

Variables: Variables are declared using the PIC clause to specify their data type and format (as explained previously). They are assigned values through assignment statements or input operations.

Example:

05  QUANTITY PIC 9(5).       
05  PRICE    PIC 99V99.
05  TOTAL    PIC 9(7)V99.

Constants: Constants are declared using the VALUE clause along with the PIC clause. Their values cannot be changed during program execution.

Example:

05 TAX-RATE PIC V99 VALUE 0.06.
05  PI       PIC 9V99 VALUE 3.14.

Arithmetic Operations #

COBOL supports standard arithmetic operations using the following verbs:

• ADD: Adds two or more numbers. ADD A TO B adds the value of A to B, storing the result in B. ADD A, B GIVING C adds A and B, storing the result in C.

• SUBTRACT: Subtracts one number from another. SUBTRACT A FROM B subtracts A from B, storing the result in B. SUBTRACT A FROM B GIVING C subtracts A from B, storing the result in C.

• MULTIPLY: Multiplies two numbers. MULTIPLY A BY B multiplies A by B, storing the result in B. MULTIPLY A BY B GIVING C multiplies A by B, storing the result in C.

• DIVIDE: Divides one number by another. DIVIDE A INTO B divides B by A, storing the result in B. DIVIDE A INTO B GIVING C divides B by A, storing the result in C. DIVIDE A BY B GIVING C also divides A by B storing the result in C.

• COMPUTE: A more general-purpose arithmetic statement. COMPUTE TOTAL = QUANTITY * PRICE calculates the total. It supports more complex expressions involving multiple operators.

Example:

COMPUTE TOTAL = QUANTITY * PRICE + (QUANTITY * PRICE) * TAX-RATE.

String Manipulation #

COBOL provides limited but sufficient string manipulation capabilities. Common operations include:

• INSPECT: Counts or replaces characters within a string.

• STRING: Concatenates strings together.

• UNSTRING: Separates a string into multiple parts.

Example (STRING):

05  FIRST-NAME PIC X(20).
05  LAST-NAME  PIC X(30).
05  FULL-NAME  PIC X(50).

... some code to populate FIRST-NAME and LAST-NAME ...

STRING FIRST-NAME DELIMITED BY SIZE
       INTO LAST-NAME DELIMITED BY SIZE
       DELIMITED BY SPACE
       INTO FULL-NAME

Note: The specific syntax of string manipulation varies depending on the operations used, and detailed examples are best found in a comprehensive COBOL reference.

Working with Tables and Arrays #

COBOL uses tables (essentially arrays) to store collections of data of the same type. They are declared in the DATA DIVISION using OCCURS clauses.

Example:

01  SALES-DATA.
    05  SALES-RECORD OCCURS 12 TIMES.
        10  MONTH      PIC 99.
        10  SALES-AMOUNT PIC 9(7)V99.

This declares a table called SALES-DATA with 12 elements (SALES-RECORD). Each element contains a month number (MONTH) and a sales amount (SALES-AMOUNT). You can access individual elements using array indexing (subscripting). For example, SALES-AMOUNT(3) refers to the sales amount for the third month. Note that indexing starts from 1, not 0, as is usual in many other languages. Iterating through tables often involves using PERFORM loops (discussed in later sections).

Control Structures #

IF-THEN-ELSE Statements #

COBOL uses IF-THEN-ELSE statements to implement conditional logic. The basic structure is:

IF condition THEN
    statement-1
    statement-2
ELSE
    statement-3
    statement-4
END-IF

Conditions are typically comparisons using relational operators:

• = (equal to)
• > (greater than)
• < (less than)
• >= (greater than or equal to)
• <= (less than or equal to)
• >= (not equal to)

Example:

IF QUANTITY > 100 THEN
    COMPUTE DISCOUNT = PRICE * 0.10
ELSE
    COMPUTE DISCOUNT = 0
END-IF

Multiple conditions can be combined using AND and OR:

IF (QUANTITY > 100) AND (CUSTOMER-TYPE = "PREMIUM") THEN
    COMPUTE DISCOUNT = PRICE * 0.20
ELSE
    COMPUTE DISCOUNT = 0
END-IF

IF statements can also be nested.

PERFORM Loops #

PERFORM statements are used to create loops in COBOL. There are several variations:

• Simple PERFORM: Executes a paragraph or section a specified number of times.

PERFORM PROCESS-SALES 12 TIMES.

This executes the paragraph named PROCESS-SALES twelve times.

• PERFORM UNTIL: Repeats a paragraph or section until a condition becomes true.

PERFORM UNTIL EOF-FLAG = 1
    READ CUSTOMER-FILE AT END MOVE 1 TO EOF-FLAG
END-PERFORM.

This reads records from CUSTOMER-FILE until the end of the file is reached (EOF-FLAG becomes 1).

• PERFORM VARYING: Similar to a for loop in other languages, it iterates through a counter variable.

PERFORM VARYING MONTH FROM 1 BY 1 UNTIL MONTH > 12
    PROCESS-MONTHLY-SALES
END-PERFORM.

This iterates through months from 1 to 12, executing PROCESS-MONTHLY-SALES for each month.

GO TO Statements (and why to avoid them) #

COBOL supports GO TO statements for unconditional branching. However, overuse of GO TO statements leads to “spaghetti code”—programs that are difficult to understand and maintain. The highly structured nature of COBOL’s IF-THEN-ELSE and PERFORM statements is preferred for creating readable and maintainable programs. Modern COBOL programming strongly discourages the use of GO TO.

Nested Control Structures #

Control structures can be nested within each other to create complex logic. For example, you can nest IF-THEN-ELSE statements within PERFORM loops, or PERFORM loops within IF-THEN-ELSE structures.

Example:

PERFORM VARYING I FROM 1 BY 1 UNTIL I > 10
    IF (I MOD 2) = 0 THEN
        DISPLAY "Even number: " I
    END-IF
END-PERFORM.

This code iterates from 1 to 10, displaying only even numbers. The IF-THEN-ELSE statement is nested inside the PERFORM loop. Careful indentation and clear naming conventions are crucial when working with nested structures to maintain code readability.

Nested structures should be used judiciously, as overly complex nesting can make the code harder to understand. Consider refactoring your code into smaller, more manageable modules if the nesting becomes excessive.

Advanced COBOL Concepts #

Subprograms and Modules #

Modularizing COBOL code into subprograms (similar to functions or procedures in other languages) improves organization, readability, and reusability. COBOL uses the CALL statement to invoke a subprogram and the EXIT PROGRAM statement to return control to the calling program. Subprograms are typically compiled separately and linked during the final build process.

Defining a Subprogram: A subprogram is defined as a separate COBOL program. It has its own IDENTIFICATION, ENVIRONMENT, DATA, and PROCEDURE divisions. Data can be passed to and from subprograms using parameters.

Calling a Subprogram: The CALL statement is used to invoke a subprogram, passing parameters as needed. The USING clause specifies the parameters.

Example:

Main Program:

CALL "CALCULATE-TOTAL" USING  QUANTITY, PRICE, TOTAL.
DISPLAY "Total: " TOTAL.

Subprogram (CALCULATE-TOTAL):

IDENTIFICATION DIVISION.
PROGRAM-ID. CALCULATE-TOTAL.
DATA DIVISION.
WORKING-STORAGE SECTION.
01  QUANTITY-PARAM PIC 9(5).
01  PRICE-PARAM    PIC 99V99.
01  TOTAL-PARAM    PIC 9(7)V99.
PROCEDURE DIVISION USING QUANTITY-PARAM, PRICE-PARAM, TOTAL-PARAM.
COMPUTE TOTAL-PARAM = QUANTITY-PARAM * PRICE-PARAM.
EXIT PROGRAM.

Error Handling and Exception Management #

Robust error handling is crucial in COBOL applications. While COBOL doesn’t have explicit exception-handling mechanisms like try-catch blocks in some other languages, it offers several ways to manage errors:

• AT END Clause: Used with READ statements to handle the end-of-file condition.

• FILE STATUS: A special variable that contains a code indicating the success or failure of file operations. Checking the file status after every file operation is a standard practice for identifying errors (e.g., file not found, disk error).

• Program Status Codes: These are return values set by a COBOL program to signal success or failure. Calling programs can check these codes to determine if errors occurred.

• Conditional Logic: Using IF statements to check for error conditions (e.g., invalid input data, division by zero).

Debugging and Troubleshooting #

Debugging COBOL programs can be challenging, as it often involves working with large datasets and complex logic. However, several techniques and tools can help:

• Print Statements: Inserting DISPLAY statements to output variable values at different points in the program is a basic but effective debugging technique.

• Debuggers: Most COBOL compilers provide debuggers that allow you to step through the code, inspect variables, and set breakpoints.

• Logging: Implementing logging mechanisms to record program events, variable values, and errors in a log file can be helpful in tracking down issues.

• Code Review: Having another developer review your code can help identify potential errors and improve overall code quality.

Working with Databases #

COBOL programs frequently interact with databases. Several methods exist, but the most common approaches involve using database APIs and pre-compilers or embedded SQL.

• Database APIs: Many COBOL compilers provide interfaces to access various database systems using specialized APIs. This may involve calling functions or procedures provided by the database system.

• Embedded SQL: This approach allows embedding SQL statements directly within COBOL programs. A pre-compiler translates these SQL statements into calls to the database API.

The specific approach to database interaction depends on the chosen database system and the COBOL compiler’s capabilities. Understanding SQL (Structured Query Language) is essential for effectively working with databases from COBOL. Accessing and manipulating data within a database requires careful consideration of data integrity, transaction management (e.g., ensuring atomicity and consistency), and concurrency control (managing access when multiple users or processes interact with the database simultaneously).

Real-world Examples #

Simple Business Applications #

To solidify your understanding of COBOL’s practical applications, let’s explore some straightforward business scenarios:

1. Inventory Management: A program could track inventory levels for a small retail store. It would read data from an inventory file, process transactions (adding or removing items), update the inventory file, and generate reports showing current stock levels, low-stock warnings, and value of inventory. Key COBOL features used would include file handling (reading and writing inventory data), data structures (to represent inventory items), arithmetic operations (calculating total value), and reporting (generating output to a file or screen).

2. Payroll Processing: A simple payroll program could calculate employee pay based on hours worked and pay rate. It would read employee data from a file, calculate gross pay, deduct taxes and other deductions, and generate paychecks or deposit information. This would involve file handling, arithmetic operations, data structures to hold employee information (name, ID, pay rate, etc.), and potentially decision-making (handling different pay scales or deductions).

3. Customer Account Management: A program could manage customer accounts, allowing for adding new customers, updating existing customer information (address, contact details), and generating customer statements. This would use file handling to store customer data, data structures to represent customer records, and input/output operations for user interaction (adding or updating information). It might also include searching and sorting capabilities.

4. Simple Billing System: A basic billing system could calculate the amount due for a customer based on their purchase history. This involves reading transaction data from a file (or database), applying discounts if applicable, calculating totals, and generating invoices. This showcases COBOL’s strength in handling numeric data and generating formatted reports.

These examples highlight how COBOL’s features directly translate to solving common business problems. While seemingly simple, these applications form the foundation for many more complex business systems.

Case Studies: COBOL in Action #

COBOL’s longevity means it’s involved in many critical systems. Though specifics of proprietary systems are often confidential, here are generalized examples illustrating COBOL’s impact:

1. Financial Transactions: Many banks and financial institutions still rely heavily on COBOL for core banking systems. These systems handle trillions of dollars in transactions daily, processing deposits, withdrawals, loan payments, and other financial operations with extremely high reliability and speed. These systems demonstrate COBOL’s strength in handling large volumes of numeric data with precision and maintaining data integrity under heavy load. The inherent security and stability of these long-standing systems are crucial.

2. Government and Healthcare: Government agencies utilize COBOL for applications ranging from managing social security benefits to tracking public health data. These applications often involve massive datasets and complex regulations, highlighting COBOL’s ability to handle intricate data processing tasks. The accuracy and reliability of these systems directly impact citizens’ well-being and government services.

3. Airline Reservation Systems: Though newer systems exist, many elements of airline reservation systems still utilize COBOL for managing bookings, seat assignments, and flight schedules. The capacity to handle thousands of concurrent transactions efficiently and accurately is a testament to COBOL’s enduring relevance.

These case studies showcase that while COBOL might not be at the forefront of new technologies, its robust capabilities are continually relied upon in mission-critical applications handling sensitive and vast amounts of data. Many modern systems interact with or rely on COBOL-based legacy systems. Understanding COBOL, therefore, remains crucial for maintaining and evolving these systems.

Resources and Further Learning #

Online Courses and Tutorials #

Numerous online resources are available to help you continue your COBOL learning journey, catering to different skill levels and learning styles:

• Massive Open Online Courses (MOOCs): Platforms like Coursera, edX, and Udemy occasionally offer COBOL courses, often focusing on specific aspects like legacy system maintenance or database integration. Search their course catalogs for “COBOL” or “COBOL programming.” The availability of these courses can vary.

• Video Tutorials: YouTube and other video platforms host many COBOL tutorials, ranging from beginner-friendly introductions to more advanced topics. Look for tutorials that clearly explain concepts and provide practical examples. Pay attention to the age of the videos and the COBOL compiler version used, as some techniques might be outdated.

• Interactive Online Compilers: Websites like OnlineGDB offer online COBOL compilers, allowing you to write and execute code directly in your browser without installing any software. This is a convenient way to experiment with small programs and test code snippets.

• Official Documentation: While often dense, the documentation for specific COBOL compilers (like OpenCOBOL or GnuCOBOL) provides comprehensive information on language features, compiler options, and troubleshooting.

COBOL Compilers and IDEs #

Choosing the right compiler and IDE (Integrated Development Environment) is vital for a smooth COBOL development experience.

• OpenCOBOL: A free, open-source COBOL compiler available for various operating systems (Windows, Linux, macOS). It’s a popular choice for beginners due to its accessibility and community support.

• GnuCOBOL: Another open-source COBOL compiler, often considered a fork or improvement of OpenCOBOL, also with a strong community.

• Micro Focus COBOL: A commercially licensed compiler offering advanced features and comprehensive support, commonly used in professional environments. It often comes bundled with a powerful IDE.

• IBM COBOL: IBM’s COBOL compiler and associated tools are popular in mainframe environments.

• IDEs: While many developers use simple text editors, dedicated IDEs can significantly improve productivity. Some IDEs provide COBOL support as plugins, while others offer integrated COBOL capabilities. Examples include Eclipse (with plugins), Visual Studio Code (with extensions), and the IDEs bundled with commercial COBOL compilers.

Remember to carefully review the documentation for your chosen compiler and IDE to learn about installation, configuration, and usage.

Community Forums and Support #

Engaging with the COBOL community is a great way to get help, share knowledge, and stay updated on the latest developments:

• Online Forums: Search for “COBOL forum” or “COBOL community” to find relevant online forums and discussion groups. These provide platforms to ask questions, seek solutions to problems, and participate in discussions with other COBOL developers.

• Stack Overflow: While not exclusively dedicated to COBOL, Stack Overflow often has questions and answers related to COBOL programming. Searching for specific problems or concepts within Stack Overflow can yield valuable insights.

• GitHub: GitHub hosts open-source COBOL projects, libraries, and tools. It also allows you to access source code for various COBOL compilers and contribute to the community.

• Local User Groups: Depending on your location, you might find local user groups or meetups dedicated to COBOL or legacy systems programming. These offer opportunities to network with other COBOL professionals and learn from experienced developers.

By utilizing these resources, you can continue building your COBOL skills and stay engaged with the wider COBOL community. Remember that persistence and practice are key to mastering any programming language.