COBOL Procedure Division: Logic, File Handling, and Data Manipulation

Important Note: Once a file is closed, it cannot be read from or written to again until it is opened once more.

The Mechanics of Reading Data

When you read a sequential file, COBOL uses a "pointer" to keep track of your position. When the file is opened, the pointer starts at the beginning. Every time a READ statement is executed, the pointer moves to the start of the next record.

The process continues until the pointer reaches the End of File (EOF) record. The EOF is not a data record itself, but a special marker indicating that no more data is available.

To prevent errors, programmers use the AT END clause within the READ statement to handle the moment the end of the file is reached:

READ STUDENT-FILE 
    AT END MOVE "YES" TO EOF-FLAG 
    NOT AT END ADD 1 TO RECORDS-READ-CTR.

Writing Data to Files

To output data, you use the WRITE statement. A common pattern in COBOL is to define a generic output record in the FILE SECTION (via the FD entry) and then move specific, detailed data from the WORKING-STORAGE SECTION into that generic record before writing it.

Data Manipulation: MOVE and Arithmetic

Data is constantly being transferred and transformed within a program.

The MOVE Statement

The MOVE statement is used to copy data from one field to another. However, the fields must be compatible.

• Numeric fields are moved from right to left.
• Alphabetic and Alphanumeric fields are moved from left to right.

Arithmetic and Conditionals

While this session focuses on the mechanics of movement, the PROCEDURE DIVISION is also the home for arithmetic operations and conditional logic (like IF statements) that allow the program to make decisions based on the data it reads.

Controlling Program Flow with PERFORM

The PERFORM statement is the primary way to execute paragraphs. It can be used in two main ways:

1. Simple Execution: PERFORM paragraph-name executes the paragraph and then returns to the next line in the calling paragraph.
2. Conditional Execution: PERFORM paragraph-name UNTIL condition allows you to create loops. A very common pattern is to loop through a file until an EOF flag is set:

   PERFORM PROCESS-RECORDS UNTIL EOF-FLAG = "YES".
   

Pro-Tip for Processing: When working with sequential files, always perform a "priming read" (an initial read before the loop starts) to set the state of your EOF flag before the main processing loop begins.