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COBOL SEQUENTIAL Clause - Quick Reference

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Overview

The SEQUENTIAL clause is used to define sequential file organization and access in COBOL. It specifies that records must be processed in the order they appear in the file, from the first record to the last, providing efficient sequential processing.

Purpose and Usage

  • Sequential processing - Process records in order
  • File organization - Define sequential file structure
  • Access control - Control how records are accessed
  • Performance optimization - Optimize for sequential operations
  • Batch processing - Support batch processing operations

Syntax

The SEQUENTIAL clause is used in the FILE-CONTROL paragraph of the ENVIRONMENT DIVISION.

Basic Syntax

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* Basic SEQUENTIAL syntax
SELECT file-name
ASSIGN TO physical-file-name
ORGANIZATION IS SEQUENTIAL
ACCESS MODE IS SEQUENTIAL.
 
* Complete example
IDENTIFICATION DIVISION.
PROGRAM-ID. SEQUENTIAL-EXAMPLE.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT SEQUENTIAL-FILE
    ASSIGN TO 'DATA/SEQUENTIAL.DAT'
    ORGANIZATION IS SEQUENTIAL
    ACCESS MODE IS SEQUENTIAL
    FILE STATUS IS WS-FILE-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  SEQUENTIAL-FILE.
01  FILE-RECORD.
    05  RECORD-DATA    PIC X(80).
 
WORKING-STORAGE SECTION.
01  WS-FILE-STATUS     PIC XX.
 
PROCEDURE DIVISION.
MAIN-LOGIC.
    OPEN INPUT SEQUENTIAL-FILE
    READ SEQUENTIAL-FILE
    CLOSE SEQUENTIAL-FILE
    STOP RUN.

SEQUENTIAL defines sequential file organization and access.

Practical Examples

Examples of using the SEQUENTIAL clause in different scenarios.

Input Sequential File

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* Input sequential file
SELECT INPUT-FILE
ASSIGN TO 'INPUT.DAT'
ORGANIZATION IS SEQUENTIAL
ACCESS MODE IS SEQUENTIAL
FILE STATUS IS WS-INPUT-STATUS.
 
* Processing loop
PROCEDURE DIVISION.
PROCESS-FILE.
    OPEN INPUT INPUT-FILE
    READ INPUT-FILE
        AT END
            MOVE 'Y' TO END-OF-FILE-FLAG
        NOT AT END
            PERFORM PROCESS-RECORD
    END-READ
    
    PERFORM UNTIL END-OF-FILE-FLAG = 'Y'
        READ INPUT-FILE
            AT END
                MOVE 'Y' TO END-OF-FILE-FLAG
            NOT AT END
                PERFORM PROCESS-RECORD
        END-READ
    END-PERFORM
    
    CLOSE INPUT-FILE.

SEQUENTIAL for input file processing.

Output Sequential File

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* Output sequential file
SELECT OUTPUT-FILE
ASSIGN TO 'OUTPUT.DAT'
ORGANIZATION IS SEQUENTIAL
ACCESS MODE IS SEQUENTIAL
FILE STATUS IS WS-OUTPUT-STATUS.
 
* Writing records
PROCEDURE DIVISION.
WRITE-RECORDS.
    OPEN OUTPUT OUTPUT-FILE
    
    MOVE "Record 1" TO RECORD-DATA
    WRITE FILE-RECORD
    
    MOVE "Record 2" TO RECORD-DATA
    WRITE FILE-RECORD
    
    MOVE "Record 3" TO RECORD-DATA
    WRITE FILE-RECORD
    
    CLOSE OUTPUT-FILE.

SEQUENTIAL for output file creation.

Input-Output Sequential File

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* Input-output sequential file
SELECT UPDATE-FILE
ASSIGN TO 'UPDATE.DAT'
ORGANIZATION IS SEQUENTIAL
ACCESS MODE IS SEQUENTIAL
FILE STATUS IS WS-UPDATE-STATUS.
 
* Updating records
PROCEDURE DIVISION.
UPDATE-RECORDS.
    OPEN I-O UPDATE-FILE
    READ UPDATE-FILE
        AT END
            MOVE 'Y' TO END-OF-FILE-FLAG
        NOT AT END
            PERFORM UPDATE-RECORD
    END-READ
    
    PERFORM UNTIL END-OF-FILE-FLAG = 'Y'
        READ UPDATE-FILE
            AT END
                MOVE 'Y' TO END-OF-FILE-FLAG
            NOT AT END
                PERFORM UPDATE-RECORD
        END-READ
    END-PERFORM
    
    CLOSE UPDATE-FILE.
 
UPDATE-RECORD.
    * Modify record data
    ADD 1 TO RECORD-COUNTER
    REWRITE FILE-RECORD.

SEQUENTIAL for file updates.

Best Practices

Understanding best practices ensures effective use of the SEQUENTIAL clause.

Best Practices

  • Use for batch processing - Choose SEQUENTIAL for batch operations
  • Process entire files - Read files from beginning to end
  • Handle end-of-file - Always check for end-of-file conditions
  • Optimize I/O - Use buffered I/O for better performance
  • Validate file status - Check file status after operations

Test Your Knowledge

1. What is the primary purpose of the SEQUENTIAL clause in COBOL?

  • To sequence program execution
  • To define sequential file organization and access
  • To sequence data types
  • To sequence memory allocation

2. In which COBOL division is the SEQUENTIAL clause typically used?

  • IDENTIFICATION DIVISION
  • ENVIRONMENT DIVISION
  • DATA DIVISION
  • PROCEDURE DIVISION

3. What does SEQUENTIAL organization mean for file access?

  • Random access to any record
  • Records must be accessed in order from beginning to end
  • Access to records by key value
  • Access to records by position

4. Can SEQUENTIAL files be updated in place?

  • Yes, any record can be updated
  • No, sequential files cannot be updated
  • Yes, but only the current record being read
  • Only with special permissions

5. What is the relationship between SEQUENTIAL and RANDOM access modes?

  • They are the same thing
  • SEQUENTIAL processes records in order, RANDOM allows direct access
  • SEQUENTIAL is faster than RANDOM
  • They cannot be used together

Frequently Asked Questions

Related Concepts

File Organization Types

Different file organization types in COBOL.

File Processing

File processing techniques in COBOL.

Batch Processing

Batch processing concepts in COBOL.

SEQUENCE ClauseSERVICE Clause