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COBOL Tutorial

COBOL I-O Operations

I-O operations in COBOL represent the fundamental Input/Output processing mechanisms that enable comprehensive file handling, record manipulation, and data transfer capabilities within enterprise applications. These operations provide sophisticated file access patterns including sequential processing, random access, and update operations that support complex business data management requirements while maintaining data integrity, ensuring reliable transaction processing, and delivering high-performance file operations across diverse mainframe and distributed computing environments with robust error handling and comprehensive status reporting.

File Opening Modes

I-O Mode Syntax
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*> Basic I-O file operations
OPEN I-O file-name
READ file-name
REWRITE record-name
CLOSE file-name
 
*> File control and access
SELECT file-name ASSIGN TO external-name
       ORGANIZATION IS INDEXED
       ACCESS MODE IS DYNAMIC
       RECORD KEY IS primary-key
       FILE STATUS IS file-status.
 
*> Example I-O operations
OPEN I-O CUSTOMER-FILE
READ CUSTOMER-FILE
    KEY IS CUSTOMER-ID
    INVALID KEY DISPLAY "Customer not found"
    NOT INVALID KEY PERFORM UPDATE-CUSTOMER
END-READ
CLOSE CUSTOMER-FILE.
File Processing
Record Update
Data Access

Comprehensive I-O Examples

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IDENTIFICATION DIVISION.
PROGRAM-ID. IO-OPERATIONS-DEMO.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT EMPLOYEE-FILE ASSIGN TO "EMPLOYEE.DAT"
           ORGANIZATION IS INDEXED
           ACCESS MODE IS DYNAMIC
           RECORD KEY IS EMP-ID
           ALTERNATE RECORD KEY IS EMP-NAME WITH DUPLICATES
           FILE STATUS IS EMP-FILE-STATUS.
    
    SELECT TRANSACTION-FILE ASSIGN TO "TRANS.DAT"
           ORGANIZATION IS SEQUENTIAL
           ACCESS MODE IS SEQUENTIAL
           FILE STATUS IS TRANS-FILE-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD EMPLOYEE-FILE.
01 EMPLOYEE-RECORD.
   05 EMP-ID          PIC 9(6).
   05 EMP-NAME        PIC X(30).
   05 EMP-DEPT        PIC X(10).
   05 EMP-SALARY      PIC 9(7)V99.
   05 EMP-STATUS      PIC X.
 
FD TRANSACTION-FILE.
01 TRANSACTION-RECORD.
   05 TRANS-TYPE      PIC X.
   05 TRANS-EMP-ID    PIC 9(6).
   05 TRANS-DATA      PIC X(50).
 
WORKING-STORAGE SECTION.
01 WS-FILE-STATUS.
   05 EMP-FILE-STATUS    PIC XX.
   05 TRANS-FILE-STATUS  PIC XX.
 
01 WS-COUNTERS.
   05 WS-RECORDS-READ    PIC 9(5) VALUE 0.
   05 WS-RECORDS-UPDATED PIC 9(5) VALUE 0.
   05 WS-RECORDS-ADDED   PIC 9(5) VALUE 0.
 
88 EOF-TRANSACTIONS     VALUE "10".
 
PROCEDURE DIVISION.
MAIN-PROCESSING.
    PERFORM OPEN-FILES
    PERFORM PROCESS-TRANSACTIONS
    PERFORM CLOSE-FILES
    PERFORM DISPLAY-STATISTICS
    STOP RUN.
 
OPEN-FILES.
    OPEN I-O EMPLOYEE-FILE
    IF EMP-FILE-STATUS NOT = "00"
       DISPLAY "Error opening employee file: " EMP-FILE-STATUS
       STOP RUN
    END-IF
    
    OPEN INPUT TRANSACTION-FILE
    IF TRANS-FILE-STATUS NOT = "00"
       DISPLAY "Error opening transaction file: " TRANS-FILE-STATUS
       STOP RUN
    END-IF.
 
PROCESS-TRANSACTIONS.
    PERFORM UNTIL EOF-TRANSACTIONS
        READ TRANSACTION-FILE
            AT END SET EOF-TRANSACTIONS TO TRUE
            NOT AT END PERFORM PROCESS-TRANSACTION
        END-READ
    END-PERFORM.
 
PROCESS-TRANSACTION.
    ADD 1 TO WS-RECORDS-READ
    
    EVALUATE TRANS-TYPE
        WHEN "A" PERFORM ADD-EMPLOYEE
        WHEN "U" PERFORM UPDATE-EMPLOYEE  
        WHEN "D" PERFORM DELETE-EMPLOYEE
        WHEN OTHER DISPLAY "Invalid transaction type: " TRANS-TYPE
    END-EVALUATE.
 
ADD-EMPLOYEE.
    MOVE TRANS-EMP-ID TO EMP-ID
    MOVE TRANS-DATA(1:30) TO EMP-NAME
    MOVE TRANS-DATA(31:10) TO EMP-DEPT
    MOVE TRANS-DATA(41:10) TO EMP-SALARY
    MOVE "A" TO EMP-STATUS
    
    WRITE EMPLOYEE-RECORD
        INVALID KEY 
            DISPLAY "Employee " EMP-ID " already exists"
        NOT INVALID KEY
            ADD 1 TO WS-RECORDS-ADDED
            DISPLAY "Added employee: " EMP-ID
    END-WRITE.
 
UPDATE-EMPLOYEE.
    MOVE TRANS-EMP-ID TO EMP-ID
    
    READ EMPLOYEE-FILE
        KEY IS EMP-ID
        INVALID KEY
            DISPLAY "Employee " EMP-ID " not found for update"
        NOT INVALID KEY
            PERFORM APPLY-UPDATE
    END-READ.
 
APPLY-UPDATE.
    *> Update fields based on transaction data
    IF TRANS-DATA(1:1) NOT = SPACE
       MOVE TRANS-DATA(1:30) TO EMP-NAME
    END-IF
    
    IF TRANS-DATA(31:1) NOT = SPACE
       MOVE TRANS-DATA(31:10) TO EMP-DEPT
    END-IF
    
    IF TRANS-DATA(41:1) NOT = SPACE
       MOVE TRANS-DATA(41:10) TO EMP-SALARY
    END-IF
    
    REWRITE EMPLOYEE-RECORD
        INVALID KEY
            DISPLAY "Error updating employee " EMP-ID
        NOT INVALID KEY
            ADD 1 TO WS-RECORDS-UPDATED
            DISPLAY "Updated employee: " EMP-ID
    END-REWRITE.
 
DELETE-EMPLOYEE.
    MOVE TRANS-EMP-ID TO EMP-ID
    
    READ EMPLOYEE-FILE
        KEY IS EMP-ID
        INVALID KEY
            DISPLAY "Employee " EMP-ID " not found for deletion"
        NOT INVALID KEY
            DELETE EMPLOYEE-FILE
                INVALID KEY
                    DISPLAY "Error deleting employee " EMP-ID
                NOT INVALID KEY
                    DISPLAY "Deleted employee: " EMP-ID
            END-DELETE
    END-READ.
 
CLOSE-FILES.
    CLOSE EMPLOYEE-FILE
    CLOSE TRANSACTION-FILE.
 
DISPLAY-STATISTICS.
    DISPLAY "Processing Statistics:"
    DISPLAY "  Transactions read: " WS-RECORDS-READ
    DISPLAY "  Records added: " WS-RECORDS-ADDED
    DISPLAY "  Records updated: " WS-RECORDS-UPDATED.

I-O Mode Capabilities

Read Operations
  • • Sequential record reading
  • • Random access by key
  • • Dynamic access modes
  • • Record validation
Update Operations
  • • Record rewriting
  • • Field modifications
  • • Conditional updates
  • • Status checking

Best Practices

Knowledge Check

Test Your Understanding

Question 1: I-O Mode Purpose

What is the primary purpose of I-O mode in COBOL?

Answer: I-O mode allows both reading from and writing to the same file, enabling record updates and modifications while maintaining data integrity.

Question 2: File Requirements

What are the requirements for opening a file in I-O mode?

Answer: The file must already exist, be properly defined in FILE-CONTROL, and support the organization type (typically indexed or relative files).

Related Concepts

File Processing

File handling and record management techniques

Record Operations

Reading, writing, and updating record data

Data Transfer

Data movement and manipulation methods

Related Pages