File handling is a crucial aspect of COBOL programming, especially given its extensive use in business, finance, and administrative systems for companies and governments. This section will cover the basics of file handling in COBOL, including how to define, open, read, write, and close files.

1. File Control: Specifies the files used in the program.
2. File Description: Describes the structure of the files.
3. File Operations: Includes opening, reading, writing, and closing files.

The FILE-CONTROL paragraph in the ENVIRONMENT DIVISION specifies the files that the program will use. Each file is associated with a file name and a device.

• SELECT INPUT-FILE ASSIGN TO 'input.dat': Associates the logical file name INPUT-FILE with the physical file input.dat.
• ORGANIZATION IS SEQUENTIAL: Specifies that the file is a sequential file.

The FILE SECTION in the DATA DIVISION describes the structure of the files. Each file is defined with a FD (File Description) entry.

• FD INPUT-FILE: Defines the file INPUT-FILE.
• 01 INPUT-RECORD: Describes the structure of each record in the file.
• 05 INPUT-ID PIC 9(5): Defines a field INPUT-ID with a picture clause 9(5) (5-digit numeric).
• 05 INPUT-NAME PIC X(20): Defines a field INPUT-NAME with a picture clause X(20) (20-character alphanumeric).

Files must be opened before they can be read or written. The OPEN statement is used for this purpose.

• OPEN INPUT INPUT-FILE: Opens the INPUT-FILE for reading.
• OPEN OUTPUT OUTPUT-FILE: Opens the OUTPUT-FILE for writing.

The READ statement is used to read records from a file.

• READ INPUT-FILE INTO INPUT-RECORD: Reads a record from INPUT-FILE into INPUT-RECORD.
• AT END MOVE 'YES' TO END-OF-FILE: Sets the END-OF-FILE flag to 'YES' if the end of the file is reached.

The WRITE statement is used to write records to a file.

• MOVE INPUT-ID TO OUTPUT-ID: Copies the value of INPUT-ID to OUTPUT-ID.
• MOVE INPUT-NAME TO OUTPUT-NAME: Copies the value of INPUT-NAME to OUTPUT-NAME.
• WRITE OUTPUT-RECORD: Writes the OUTPUT-RECORD to the OUTPUT-FILE.

Files must be closed after all operations are completed. The CLOSE statement is used for this purpose.

• CLOSE INPUT-FILE: Closes the INPUT-FILE.
• CLOSE OUTPUT-FILE: Closes the OUTPUT-FILE.

Write a COBOL program that reads records from an input file and writes them to an output file.

• The program reads records from input.dat and writes them to output.dat.
• The PERFORM UNTIL loop continues until the end of the input file is reached.
• Each record is read from the input file, and if not at the end of the file, it is written to the output file.

1. Forgetting to Open or Close Files: Always ensure files are opened before any operations and closed after operations are completed.
2. Incorrect File Assignments: Double-check file assignments in the FILE-CONTROL paragraph.
3. Mismatched Record Descriptions: Ensure that the record descriptions in the FILE SECTION match the actual file structure.

In this section, you learned the basics of file handling in COBOL, including how to define, open, read, write, and close files. Understanding these concepts is essential for working with data in COBOL programs. In the next section, we will delve into sequential file processing, which builds on the concepts covered here.