Reading From a Sequential File

Welcome back! You've already learned how to create and write to a sequential file in COBOL. Now, it's time to take the next step: reading from a sequential file. This lesson is crucial because it allows you to retrieve and utilize stored data, which is a fundamental aspect of building dynamic applications, especially in sectors like finance, where data retrieval is frequent.

In this lesson, you will learn how to read records from an existing sequential file in COBOL. Understanding how to read files will help you build applications that can interact with stored data, enabling you to view, process, and analyze the information.

Let's consider this simple file with two customer details:

John Doe                      1234567890
John Smith                    1111111111

Here's a glimpse of the code you'll be working with:

IDENTIFICATION DIVISION.
PROGRAM-ID. ReadFromFile.
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.

FILE-CONTROL.
    SELECT InputFile ASSIGN TO 'accounts.dat'
    ORGANIZATION IS LINE SEQUENTIAL. *> Since each record is in a new line

DATA DIVISION.
FILE SECTION.
FD  InputFile.
01  InputRecord.
    05 Customer-Name PIC X(30).
    05 Account-Number PIC 9(10).

WORKING-STORAGE SECTION.
01  WS-EOF          PIC X VALUE 'N'.

PROCEDURE DIVISION.
    OPEN INPUT InputFile.
    PERFORM UNTIL WS-EOF = 'Y'
        READ InputFile INTO InputRecord
            AT END
                MOVE 'Y' TO WS-EOF
                DISPLAY "End of file reached."
            NOT AT END
                DISPLAY "Customer Name: " Customer-Name
                DISPLAY "Account Number: " Account-Number
        END-READ
    END-PERFORM
    CLOSE InputFile.
    STOP RUN.

The output of the code will be:

Customer Name: John Doe
Account Number: 1234567890
Customer Name: John Smith
Account Number: 1111111111
End of file reached.

Let's analyze the code snippet:

1. First, we use FILE-CONTROL to define the file we want to read from. We specify the file name accounts.dat and set the organization as LINE SEQUENTIAL, since the file contains records separated by new lines. Note, that similarly you can use the LINE SEQUENTIAL organization if you want to write each record on a new line.
2. In the DATA DIVISION FILE SECTION, we define the file structure using FD InputFile InputRecord. We specify the fields Customer-Name and Account-Number.
3. We define a working storage variable WS-EOF to check if we have reached the end of the file.
4. Next, we move to the PROCEDURE DIVISION. We open the file using OPEN INPUT InputFile.
5. We use a PERFORM UNTIL loop to read the file until we reach the end of the file. We read the file using READ InputFile INTO InputRecord.
6. If we reach the end of the file (When AT END returns true), we set WS-EOF to 'Y' and display a message. If we haven't reached the end of the file, we display the customer name and account number.

Finally, we close the file using CLOSE InputFile and stop the program using STOP RUN.