MainframeMaster

COBOL Tutorial

COBOL READ Statement

Progress0 of 0 lessons

READ Statement Overview

The READ statement is one of the most fundamental file operations in COBOL. It retrieves the next logical record from a file and makes it available to the program for processing. The behavior of READ varies depending on the file organization and access mode.

Basic READ Operations

COBOL supports several types of READ operations:

  1. Sequential READ - Read records in order from beginning to end
  2. Random READ - Read a specific record by key value
  3. Dynamic READ - Combine sequential and random access
  4. Relative READ - Read records by relative record number

Each type has specific syntax and error handling requirements.

File Organization Compatibility

File OrganizationSequential READRandom READDynamic READ
SEQUENTIAL✓ Yes✗ No✗ No
INDEXED✓ Yes✓ Yes✓ Yes
RELATIVE✓ Yes✓ Yes✓ Yes
LINE SEQUENTIAL✓ Yes✗ No✗ No

Basic READ Syntax

The basic READ statement syntax varies depending on the access mode and file organization. Here are the most common forms:

Sequential READ Syntax

cobol
1
2
3
4
5
READ file-name
    [NEXT RECORD]
    [AT END imperative-statement]
    [NOT AT END imperative-statement]
    [END-READ]

The NEXT RECORD clause is optional and is the default behavior for sequential reads.

Random READ Syntax

cobol
1
2
3
4
5
READ file-name
    [RECORD KEY IS data-name]
    [INVALID KEY imperative-statement]
    [NOT INVALID KEY imperative-statement]
    [END-READ]

Random READ requires a RECORD KEY for indexed files or a RELATIVE KEY for relative files.

Basic Examples

cobol
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
* Simple sequential read
READ CUSTOMER-FILE
    AT END
        MOVE "Y" TO EOF-FLAG
    NOT AT END
        PERFORM PROCESS-CUSTOMER
END-READ.
 
* Random read with key
MOVE "12345" TO CUSTOMER-ID
READ CUSTOMER-FILE
    INVALID KEY
        DISPLAY "Customer not found"
    NOT INVALID KEY
        PERFORM PROCESS-CUSTOMER
END-READ.
 
* Sequential read with explicit NEXT RECORD
READ TRANSACTION-FILE NEXT RECORD
    AT END
        PERFORM END-OF-FILE-PROCESSING
    NOT AT END
        PERFORM PROCESS-TRANSACTION
END-READ.

These examples show the basic patterns for sequential and random READ operations.

Sequential File READ Operations

Sequential files are read from beginning to end, one record at a time. This is the most common and straightforward type of file processing.

Sequential READ with Error Handling

cobol
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
* Complete sequential read example
IDENTIFICATION DIVISION.
PROGRAM-ID. SEQREAD.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.DAT"
        ORGANIZATION IS SEQUENTIAL
        ACCESS MODE IS SEQUENTIAL
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  CUSTOMER-FILE
    LABEL RECORDS ARE STANDARD
    RECORD CONTAINS 80 CHARACTERS.
01  CUSTOMER-RECORD.
    05  CUSTOMER-ID       PIC 9(5).
    05  CUSTOMER-NAME     PIC X(30).
    05  CUSTOMER-ADDRESS  PIC X(35).
    05  CUSTOMER-BALANCE  PIC 9(7)V99.
 
WORKING-STORAGE SECTION.
01  CUSTOMER-STATUS       PIC XX.
01  EOF-FLAG              PIC X VALUE "N".
    88  END-OF-FILE         VALUE "Y".
01  RECORD-COUNT          PIC 9(5) VALUE ZERO.
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    PERFORM INITIALIZATION
    PERFORM READ-PROCESS UNTIL END-OF-FILE
    PERFORM FINALIZATION
    STOP RUN.
 
INITIALIZATION.
    OPEN INPUT CUSTOMER-FILE
    IF CUSTOMER-STATUS NOT = "00"
        DISPLAY "Error opening file: " CUSTOMER-STATUS
        STOP RUN
    END-IF.
 
READ-PROCESS.
    READ CUSTOMER-FILE
        AT END
            MOVE "Y" TO EOF-FLAG
            DISPLAY "End of file reached"
        NOT AT END
            IF CUSTOMER-STATUS = "00"
                ADD 1 TO RECORD-COUNT
                PERFORM PROCESS-CUSTOMER
            ELSE
                DISPLAY "Read error: " CUSTOMER-STATUS
            END-IF
    END-READ.
 
PROCESS-CUSTOMER.
    DISPLAY "Processing customer: " CUSTOMER-ID
    * Add your processing logic here.
 
FINALIZATION.
    CLOSE CUSTOMER-FILE
    DISPLAY "Total records processed: " RECORD-COUNT.

This example shows a complete sequential file processing program with proper error handling and status checking.

Sequential READ Options

ClausePurposeWhen to Use
NEXT RECORDExplicitly specify sequential readFor clarity in dynamic access mode
AT ENDHandle end-of-file conditionAlways for sequential reads
NOT AT ENDProcess successful readsWhen you need explicit success handling
END-READEnd the READ statementFor clarity and scope control

Indexed File READ Operations

Indexed files support both sequential and random access. The READ operation behavior depends on the access mode and whether you're using keys for positioning.

Indexed File READ Syntax

cobol
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
* Sequential read (ACCESS MODE IS SEQUENTIAL)
READ file-name
    [NEXT RECORD]
    [AT END imperative-statement]
    [NOT AT END imperative-statement]
    [END-READ]
 
* Random read (ACCESS MODE IS RANDOM)
READ file-name
    [RECORD KEY IS data-name]
    [INVALID KEY imperative-statement]
    [NOT INVALID KEY imperative-statement]
    [END-READ]
 
* Dynamic read (ACCESS MODE IS DYNAMIC)
READ file-name
    [NEXT RECORD]
    [RECORD KEY IS data-name]
    [AT END imperative-statement]
    [INVALID KEY imperative-statement]
    [NOT AT END imperative-statement]
    [NOT INVALID KEY imperative-statement]
    [END-READ]

The syntax varies based on the access mode specified in the SELECT statement.

Indexed File READ Examples

cobol
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
* Sequential access example
IDENTIFICATION DIVISION.
PROGRAM-ID. IDXSEQ.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.IDX"
        ORGANIZATION IS INDEXED
        ACCESS MODE IS SEQUENTIAL
        RECORD KEY IS CUSTOMER-ID
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  CUSTOMER-FILE
    LABEL RECORDS ARE STANDARD
    RECORD CONTAINS 80 CHARACTERS.
01  CUSTOMER-RECORD.
    05  CUSTOMER-ID       PIC 9(5).
    05  CUSTOMER-NAME     PIC X(30).
    05  CUSTOMER-ADDRESS  PIC X(35).
    05  CUSTOMER-BALANCE  PIC 9(7)V99.
 
WORKING-STORAGE SECTION.
01  CUSTOMER-STATUS       PIC XX.
01  EOF-FLAG              PIC X VALUE "N".
    88  END-OF-FILE         VALUE "Y".
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    OPEN INPUT CUSTOMER-FILE
    PERFORM READ-SEQUENTIAL UNTIL END-OF-FILE
    CLOSE CUSTOMER-FILE
    STOP RUN.
 
READ-SEQUENTIAL.
    READ CUSTOMER-FILE
        AT END
            MOVE "Y" TO EOF-FLAG
        NOT AT END
            IF CUSTOMER-STATUS = "00"
                DISPLAY "Customer: " CUSTOMER-ID " - " CUSTOMER-NAME
            ELSE
                DISPLAY "Read error: " CUSTOMER-STATUS
            END-IF
    END-READ.

This example shows sequential access to an indexed file, reading records in key order.

Random Access Example

cobol
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
* Random access example
IDENTIFICATION DIVISION.
PROGRAM-ID. IDXRAND.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.IDX"
        ORGANIZATION IS INDEXED
        ACCESS MODE IS RANDOM
        RECORD KEY IS CUSTOMER-ID
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  CUSTOMER-FILE
    LABEL RECORDS ARE STANDARD
    RECORD CONTAINS 80 CHARACTERS.
01  CUSTOMER-RECORD.
    05  CUSTOMER-ID       PIC 9(5).
    05  CUSTOMER-NAME     PIC X(30).
    05  CUSTOMER-ADDRESS  PIC X(35).
    05  CUSTOMER-BALANCE  PIC 9(7)V99.
 
WORKING-STORAGE SECTION.
01  CUSTOMER-STATUS       PIC XX.
01  SEARCH-ID             PIC 9(5).
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    OPEN INPUT CUSTOMER-FILE
    PERFORM SEARCH-CUSTOMER
    CLOSE CUSTOMER-FILE
    STOP RUN.
 
SEARCH-CUSTOMER.
    DISPLAY "Enter customer ID to search: "
    ACCEPT SEARCH-ID
    
    MOVE SEARCH-ID TO CUSTOMER-ID
    READ CUSTOMER-FILE
        INVALID KEY
            DISPLAY "Customer " SEARCH-ID " not found"
        NOT INVALID KEY
            DISPLAY "Customer found:"
            DISPLAY "  ID: " CUSTOMER-ID
            DISPLAY "  Name: " CUSTOMER-NAME
            DISPLAY "  Address: " CUSTOMER-ADDRESS
            DISPLAY "  Balance: " CUSTOMER-BALANCE
    END-READ.

This example demonstrates random access to an indexed file using a key value.

Dynamic Access with START

cobol
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
* Dynamic access with START example
IDENTIFICATION DIVISION.
PROGRAM-ID. IDXDYN.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.IDX"
        ORGANIZATION IS INDEXED
        ACCESS MODE IS DYNAMIC
        RECORD KEY IS CUSTOMER-ID
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  CUSTOMER-FILE
    LABEL RECORDS ARE STANDARD
    RECORD CONTAINS 80 CHARACTERS.
01  CUSTOMER-RECORD.
    05  CUSTOMER-ID       PIC 9(5).
    05  CUSTOMER-NAME     PIC X(30).
    05  CUSTOMER-ADDRESS  PIC X(35).
    05  CUSTOMER-BALANCE  PIC 9(7)V99.
 
WORKING-STORAGE SECTION.
01  CUSTOMER-STATUS       PIC XX.
01  START-ID              PIC 9(5) VALUE 10000.
01  EOF-FLAG              PIC X VALUE "N".
    88  END-OF-FILE         VALUE "Y".
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    OPEN INPUT CUSTOMER-FILE
    PERFORM START-AND-READ
    CLOSE CUSTOMER-FILE
    STOP RUN.
 
START-AND-READ.
    * Position file at customer ID 10000
    MOVE START-ID TO CUSTOMER-ID
    START CUSTOMER-FILE
        KEY IS GREATER THAN CUSTOMER-ID
        INVALID KEY
            DISPLAY "No customers found with ID > " START-ID
        NOT INVALID KEY
            PERFORM READ-FROM-POSITION
    END-START.
 
READ-FROM-POSITION.
    PERFORM UNTIL END-OF-FILE
        READ CUSTOMER-FILE NEXT RECORD
            AT END
                MOVE "Y" TO EOF-FLAG
            NOT AT END
                IF CUSTOMER-STATUS = "00"
                    DISPLAY "Customer: " CUSTOMER-ID " - " CUSTOMER-NAME
                ELSE
                    DISPLAY "Read error: " CUSTOMER-STATUS
                    MOVE "Y" TO EOF-FLAG
                END-IF
        END-READ
    END-PERFORM.

This example shows dynamic access using START to position the file, then reading sequentially from that position.

Relative File READ Operations

Relative files allow access by relative record number. The READ operation can be sequential or random, depending on the access mode.

Relative File READ Syntax

cobol
1
2
3
4
5
6
7
8
9
10
11
12
13
* Sequential read
READ file-name
    [NEXT RECORD]
    [AT END imperative-statement]
    [NOT AT END imperative-statement]
    [END-READ]
 
* Random read by relative record number
READ file-name
    [RELATIVE KEY IS data-name]
    [INVALID KEY imperative-statement]
    [NOT INVALID KEY imperative-statement]
    [END-READ]

For random access, you must set the RELATIVE KEY before the READ operation.

Relative File READ Example

cobol
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
* Relative file read example
IDENTIFICATION DIVISION.
PROGRAM-ID. RELREAD.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.REL"
        ORGANIZATION IS RELATIVE
        ACCESS MODE IS RANDOM
        RELATIVE KEY IS RELATIVE-RECORD-NUMBER
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  CUSTOMER-FILE
    LABEL RECORDS ARE STANDARD
    RECORD CONTAINS 80 CHARACTERS.
01  CUSTOMER-RECORD.
    05  CUSTOMER-ID       PIC 9(5).
    05  CUSTOMER-NAME     PIC X(30).
    05  CUSTOMER-ADDRESS  PIC X(35).
    05  CUSTOMER-BALANCE  PIC 9(7)V99.
 
WORKING-STORAGE SECTION.
01  CUSTOMER-STATUS           PIC XX.
01  RELATIVE-RECORD-NUMBER    PIC 9(5).
01  SEARCH-RECORD-NUMBER      PIC 9(5).
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    OPEN INPUT CUSTOMER-FILE
    PERFORM SEARCH-BY-RECORD-NUMBER
    CLOSE CUSTOMER-FILE
    STOP RUN.
 
SEARCH-BY-RECORD-NUMBER.
    DISPLAY "Enter record number to read: "
    ACCEPT SEARCH-RECORD-NUMBER
    
    MOVE SEARCH-RECORD-NUMBER TO RELATIVE-RECORD-NUMBER
    READ CUSTOMER-FILE
        INVALID KEY
            DISPLAY "Record " SEARCH-RECORD-NUMBER " not found"
        NOT INVALID KEY
            DISPLAY "Record found:"
            DISPLAY "  Record #: " RELATIVE-RECORD-NUMBER
            DISPLAY "  Customer ID: " CUSTOMER-ID
            DISPLAY "  Name: " CUSTOMER-NAME
            DISPLAY "  Address: " CUSTOMER-ADDRESS
            DISPLAY "  Balance: " CUSTOMER-BALANCE
    END-READ.

This example shows random access to a relative file using record numbers.

Error Handling and File Status

Proper error handling is crucial for robust file operations. COBOL provides file status codes and specific clauses to handle different error conditions.

Common File Status Codes

Status CodeMeaningAction Required
"00"Successful operationContinue processing
"02"Duplicate key (write operations)Handle duplicate key
"04"Record length errorCheck record format
"10"End of fileNormal end condition
"22"Duplicate key (indexed files)Handle duplicate key
"23"Record not foundHandle missing record
"30"Permanent errorCheck file/system
"35"File not foundCheck file existence
"37"File not openOpen file first
"39"File conflictCheck file access
"41"File already openClose file first
"42"File not open for inputOpen file for input
"43"File not open for outputOpen file for output
"44"File not open for I/OOpen file for I/O
"46"No next recordEnd of file reached
"47"File not open for extendOpen file for extend
"48"File not open for readOpen file for read
"49"File not open for writeOpen file for write
"90"File information unavailableCheck file system
"91"File not availableCheck file access
"92"Logical file not foundCheck file name
"93"File access not allowedCheck permissions
"94"File organization conflictCheck file type
"95"File access method conflictCheck access mode
"96"File record format conflictCheck record format
"97"File record length conflictCheck record length
"98"File key conflictCheck key definition
"99"File not availableCheck file system

Error Handling Example

cobol
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
* Comprehensive error handling example
IDENTIFICATION DIVISION.
PROGRAM-ID. READERROR.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.DAT"
        ORGANIZATION IS INDEXED
        ACCESS MODE IS DYNAMIC
        RECORD KEY IS CUSTOMER-ID
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  CUSTOMER-FILE
    LABEL RECORDS ARE STANDARD
    RECORD CONTAINS 80 CHARACTERS.
01  CUSTOMER-RECORD.
    05  CUSTOMER-ID       PIC 9(5).
    05  CUSTOMER-NAME     PIC X(30).
    05  CUSTOMER-ADDRESS  PIC X(35).
    05  CUSTOMER-BALANCE  PIC 9(7)V99.
 
WORKING-STORAGE SECTION.
01  CUSTOMER-STATUS       PIC XX.
01  EOF-FLAG              PIC X VALUE "N".
    88  END-OF-FILE         VALUE "Y".
01  ERROR-FLAG             PIC X VALUE "N".
    88  ERROR-OCCURRED      VALUE "Y".
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    PERFORM INITIALIZATION
    IF NOT ERROR-OCCURRED
        PERFORM READ-PROCESS UNTIL END-OF-FILE OR ERROR-OCCURRED
    END-IF
    PERFORM FINALIZATION
    STOP RUN.
 
INITIALIZATION.
    OPEN INPUT CUSTOMER-FILE
    EVALUATE CUSTOMER-STATUS
        WHEN "00"
            DISPLAY "File opened successfully"
        WHEN "35"
            DISPLAY "ERROR: File not found"
            MOVE "Y" TO ERROR-FLAG
        WHEN "37"
            DISPLAY "ERROR: File not open"
            MOVE "Y" TO ERROR-FLAG
        WHEN "39"
            DISPLAY "ERROR: File access conflict"
            MOVE "Y" TO ERROR-FLAG
        WHEN OTHER
            DISPLAY "ERROR: File open failed - Status: " CUSTOMER-STATUS
            MOVE "Y" TO ERROR-FLAG
    END-EVALUATE.
 
READ-PROCESS.
    READ CUSTOMER-FILE
        AT END
            MOVE "Y" TO EOF-FLAG
            DISPLAY "End of file reached"
        NOT AT END
            PERFORM CHECK-READ-STATUS
    END-READ.
 
CHECK-READ-STATUS.
    EVALUATE CUSTOMER-STATUS
        WHEN "00"
            PERFORM PROCESS-CUSTOMER
        WHEN "10"
            MOVE "Y" TO EOF-FLAG
            DISPLAY "End of file reached"
        WHEN "23"
            DISPLAY "WARNING: Record not found"
        WHEN "30"
            DISPLAY "ERROR: Permanent file error"
            MOVE "Y" TO ERROR-FLAG
        WHEN "90"
            DISPLAY "ERROR: File information unavailable"
            MOVE "Y" TO ERROR-FLAG
        WHEN OTHER
            DISPLAY "ERROR: Read failed - Status: " CUSTOMER-STATUS
            MOVE "Y" TO ERROR-FLAG
    END-EVALUATE.
 
PROCESS-CUSTOMER.
    DISPLAY "Processing customer: " CUSTOMER-ID " - " CUSTOMER-NAME.
 
FINALIZATION.
    IF NOT ERROR-OCCURRED
        CLOSE CUSTOMER-FILE
        IF CUSTOMER-STATUS NOT = "00"
            DISPLAY "WARNING: File close error - Status: " CUSTOMER-STATUS
        END-IF
    END-IF
    DISPLAY "Program completed".

This example shows comprehensive error handling for file operations, checking status codes at each step.

Practical Examples

These examples demonstrate real-world applications of READ operations for common business scenarios.

Example 1: Customer Lookup System

cobol
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
* Customer lookup with multiple access methods
IDENTIFICATION DIVISION.
PROGRAM-ID. CUSTLOOK.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.IDX"
        ORGANIZATION IS INDEXED
        ACCESS MODE IS DYNAMIC
        RECORD KEY IS CUSTOMER-ID
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  CUSTOMER-FILE
    LABEL RECORDS ARE STANDARD
    RECORD CONTAINS 80 CHARACTERS.
01  CUSTOMER-RECORD.
    05  CUSTOMER-ID       PIC 9(5).
    05  CUSTOMER-NAME     PIC X(30).
    05  CUSTOMER-ADDRESS  PIC X(35).
    05  CUSTOMER-BALANCE  PIC 9(7)V99.
 
WORKING-STORAGE SECTION.
01  CUSTOMER-STATUS       PIC XX.
01  SEARCH-ID             PIC 9(5).
01  SEARCH-OPTION         PIC X.
01  EOF-FLAG              PIC X VALUE "N".
    88  END-OF-FILE         VALUE "Y".
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    OPEN INPUT CUSTOMER-FILE
    PERFORM MAIN-MENU
    CLOSE CUSTOMER-FILE
    STOP RUN.
 
MAIN-MENU.
    PERFORM UNTIL SEARCH-OPTION = "X"
        DISPLAY "Customer Lookup System"
        DISPLAY "1. Search by Customer ID"
        DISPLAY "2. List all customers"
        DISPLAY "3. List customers with balance > 1000"
        DISPLAY "X. Exit"
        DISPLAY "Enter option: "
        ACCEPT SEARCH-OPTION
        
        EVALUATE SEARCH-OPTION
            WHEN "1"
                PERFORM SEARCH-BY-ID
            WHEN "2"
                PERFORM LIST-ALL-CUSTOMERS
            WHEN "3"
                PERFORM LIST-HIGH-BALANCE
            WHEN "X"
                DISPLAY "Goodbye"
            WHEN OTHER
                DISPLAY "Invalid option"
        END-EVALUATE
    END-PERFORM.
 
SEARCH-BY-ID.
    DISPLAY "Enter customer ID: "
    ACCEPT SEARCH-ID
    
    MOVE SEARCH-ID TO CUSTOMER-ID
    READ CUSTOMER-FILE
        INVALID KEY
            DISPLAY "Customer " SEARCH-ID " not found"
        NOT INVALID KEY
            PERFORM DISPLAY-CUSTOMER
    END-READ.
 
LIST-ALL-CUSTOMERS.
    MOVE "N" TO EOF-FLAG
    PERFORM READ-SEQUENTIAL UNTIL END-OF-FILE.
 
READ-SEQUENTIAL.
    READ CUSTOMER-FILE NEXT RECORD
        AT END
            MOVE "Y" TO EOF-FLAG
        NOT AT END
            IF CUSTOMER-STATUS = "00"
                PERFORM DISPLAY-CUSTOMER
            END-IF
    END-READ.
 
LIST-HIGH-BALANCE.
    MOVE "N" TO EOF-FLAG
    PERFORM READ-SEQUENTIAL-HIGH UNTIL END-OF-FILE.
 
READ-SEQUENTIAL-HIGH.
    READ CUSTOMER-FILE NEXT RECORD
        AT END
            MOVE "Y" TO EOF-FLAG
        NOT AT END
            IF CUSTOMER-STATUS = "00" AND CUSTOMER-BALANCE > 1000
                PERFORM DISPLAY-CUSTOMER
            END-IF
    END-READ.
 
DISPLAY-CUSTOMER.
    DISPLAY "Customer ID: " CUSTOMER-ID
    DISPLAY "Name: " CUSTOMER-NAME
    DISPLAY "Address: " CUSTOMER-ADDRESS
    DISPLAY "Balance: " CUSTOMER-BALANCE
    DISPLAY "---".

This example shows a customer lookup system with multiple access methods and search options.

Example 2: Transaction Processing

cobol
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
* Transaction processing with master file updates
IDENTIFICATION DIVISION.
PROGRAM-ID. TRANPROC.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT TRANSACTION-FILE ASSIGN TO "TRANS.DAT"
        ORGANIZATION IS SEQUENTIAL
        ACCESS MODE IS SEQUENTIAL
        FILE STATUS IS TRANS-STATUS.
        
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.IDX"
        ORGANIZATION IS INDEXED
        ACCESS MODE IS RANDOM
        RECORD KEY IS CUSTOMER-ID
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  TRANSACTION-FILE
    LABEL RECORDS ARE STANDARD
    RECORD CONTAINS 50 CHARACTERS.
01  TRANSACTION-RECORD.
    05  TRANS-CUSTOMER-ID PIC 9(5).
    05  TRANS-TYPE        PIC X.
        88  DEBIT           VALUE "D".
        88  CREDIT          VALUE "C".
    05  TRANS-AMOUNT      PIC 9(7)V99.
    05  TRANS-DATE        PIC 9(8).
    05  TRANS-DESCRIPTION PIC X(30).
 
FD  CUSTOMER-FILE
    LABEL RECORDS ARE STANDARD
    RECORD CONTAINS 80 CHARACTERS.
01  CUSTOMER-RECORD.
    05  CUSTOMER-ID       PIC 9(5).
    05  CUSTOMER-NAME     PIC X(30).
    05  CUSTOMER-ADDRESS  PIC X(35).
    05  CUSTOMER-BALANCE  PIC 9(7)V99.
 
WORKING-STORAGE SECTION.
01  FILE-STATUSES.
    05  TRANS-STATUS      PIC XX.
    05  CUSTOMER-STATUS   PIC XX.
01  EOF-FLAG              PIC X VALUE "N".
    88  END-OF-FILE         VALUE "Y".
01  PROCESSED-COUNT       PIC 9(5) VALUE ZERO.
01  ERROR-COUNT           PIC 9(5) VALUE ZERO.
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    PERFORM INITIALIZATION
    PERFORM PROCESS-TRANSACTIONS UNTIL END-OF-FILE
    PERFORM FINALIZATION
    STOP RUN.
 
INITIALIZATION.
    OPEN INPUT TRANSACTION-FILE
    OPEN I-O CUSTOMER-FILE
    IF TRANS-STATUS NOT = "00" OR CUSTOMER-STATUS NOT = "00"
        DISPLAY "Error opening files"
        STOP RUN
    END-IF.
 
PROCESS-TRANSACTIONS.
    READ TRANSACTION-FILE
        AT END
            MOVE "Y" TO EOF-FLAG
        NOT AT END
            IF TRANS-STATUS = "00"
                PERFORM PROCESS-TRANSACTION
            ELSE
                ADD 1 TO ERROR-COUNT
                DISPLAY "Transaction read error: " TRANS-STATUS
            END-IF
    END-READ.
 
PROCESS-TRANSACTION.
    MOVE TRANS-CUSTOMER-ID TO CUSTOMER-ID
    READ CUSTOMER-FILE
        INVALID KEY
            ADD 1 TO ERROR-COUNT
            DISPLAY "Customer " TRANS-CUSTOMER-ID " not found"
        NOT INVALID KEY
            PERFORM UPDATE-CUSTOMER-BALANCE
    END-READ.
 
UPDATE-CUSTOMER-BALANCE.
    IF DEBIT
        SUBTRACT TRANS-AMOUNT FROM CUSTOMER-BALANCE
    ELSE
        ADD TRANS-AMOUNT TO CUSTOMER-BALANCE
    END-IF
    
    REWRITE CUSTOMER-RECORD
        INVALID KEY
            ADD 1 TO ERROR-COUNT
            DISPLAY "Error updating customer " CUSTOMER-ID
        NOT INVALID KEY
            ADD 1 TO PROCESSED-COUNT
            DISPLAY "Processed transaction for customer " CUSTOMER-ID
    END-REWRITE.
 
FINALIZATION.
    CLOSE TRANSACTION-FILE
    CLOSE CUSTOMER-FILE
    DISPLAY "Processing complete"
    DISPLAY "Transactions processed: " PROCESSED-COUNT
    DISPLAY "Errors encountered: " ERROR-COUNT.

This example demonstrates transaction processing with master file updates, showing both sequential and random READ operations.

Best Practices and Tips

Following these best practices ensures efficient and reliable READ operations in COBOL programs.

General Best Practices

  • Always check file status codes after READ operations
  • Use AT END clause for sequential reads
  • Use INVALID KEY clause for random reads
  • Initialize key fields before random READ operations
  • Handle all possible error conditions appropriately
  • Close files properly when processing is complete
  • Use meaningful variable names for clarity

Performance Considerations

  • Use sequential access for processing entire files
  • Use random access for specific record lookups
  • Use dynamic access when you need both patterns
  • Position files with START before sequential reads
  • Minimize file opens/closes in loops
  • Use appropriate buffer sizes for optimal I/O performance

Error Handling Guidelines

  • Always include error handling for file operations
  • Log errors appropriately for debugging
  • Provide meaningful error messages to users
  • Handle end-of-file conditions gracefully
  • Check file status after OPEN operations
  • Validate input data before file operations
  • Use structured error handling with EVALUATE statements

Common Pitfalls to Avoid

  • Forgetting to check file status after READ operations
  • Not handling end-of-file conditions in sequential reads
  • Using wrong access mode for the intended operation
  • Not initializing key fields before random reads
  • Ignoring INVALID KEY conditions in random access
  • Not closing files after processing
  • Using sequential access for random lookups
  • Not positioning files before dynamic sequential reads

Test Your Knowledge

1. What is the primary purpose of the READ statement in COBOL?

  • To write data to a file
  • To retrieve the next record from a file
  • To delete records from a file
  • To create a new file

2. Which clause is used to handle end-of-file conditions in a READ statement?

  • ON OVERFLOW
  • AT END
  • ON ERROR
  • WHEN END

3. What is the purpose of the INVALID KEY clause in READ statements?

  • To handle end-of-file conditions
  • To handle record not found or duplicate key conditions
  • To handle file open errors
  • To handle syntax errors

4. Which file organization requires a START statement before random READ operations?

  • Sequential files
  • Indexed files
  • Relative files
  • Line sequential files

5. What does the NEXT RECORD clause specify in a READ statement?

  • Read the previous record
  • Read the next record in sequence
  • Read a specific record by key
  • Read the last record in the file

Frequently Asked Questions

Related Concepts

File Organization Types

Understanding different file organizations for READ operations.

Sequential File Operations

How to work with sequential files and READ operations.

Indexed File Operations

READ operations with indexed files and key-based access.

File Status Error Handling

Handling READ operation errors and status codes.

Data Movement Operations

How data moves during READ operations.

String OperationsOPEN Statement