MainframeMaster

COBOL Tutorial

Progress0 of 0 lessons

COBOL WHEN Clause

The WHEN clause is used in EVALUATE statements for conditional execution.

Overview

The WHEN clause provides a structured way to handle multiple conditional branches within EVALUATE statements. It allows you to specify different conditions and the corresponding code that should be executed when each condition is met.

This approach is more readable and maintainable than nested IF statements, especially when dealing with multiple possible conditions or values.

Syntax

cobol
1
2
3
4
5
6
7
8
EVALUATE subject
    WHEN condition-1
        statements-1
    WHEN condition-2
        statements-2
    WHEN OTHER
        statements-3
END-EVALUATE

Basic syntax for the WHEN clause within an EVALUATE statement. The subject is evaluated against each WHEN condition, and the corresponding statements are executed when a match is found.

Parameters

  • subject: The value or expression to be evaluated
  • condition-1, condition-2: Specific conditions to test against
  • statements-1, statements-2: Code to execute when conditions match
  • WHEN OTHER: Optional catch-all for unmatched conditions

Practical Examples

Example 1: Basic Value Evaluation

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
       IDENTIFICATION DIVISION.
       PROGRAM-ID. WHEN-EXAMPLE.
       
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01  USER-CHOICE         PIC 9(1).
       01  MESSAGE-TEXT        PIC X(50).
       
       PROCEDURE DIVISION.
       MAIN-PROCESS.
           DISPLAY "Enter your choice (1-4): "
           ACCEPT USER-CHOICE
           
           EVALUATE USER-CHOICE
               WHEN 1
                   MOVE "You selected Option 1" TO MESSAGE-TEXT
                   DISPLAY MESSAGE-TEXT
               WHEN 2
                   MOVE "You selected Option 2" TO MESSAGE-TEXT
                   DISPLAY MESSAGE-TEXT
               WHEN 3
                   MOVE "You selected Option 3" TO MESSAGE-TEXT
                   DISPLAY MESSAGE-TEXT
               WHEN 4
                   MOVE "You selected Option 4" TO MESSAGE-TEXT
                   DISPLAY MESSAGE-TEXT
               WHEN OTHER
                   MOVE "Invalid choice entered" TO MESSAGE-TEXT
                   DISPLAY MESSAGE-TEXT
           END-EVALUATE
           
           STOP RUN.

This example demonstrates basic value evaluation using WHEN clauses. The program evaluates the user's choice and displays an appropriate message based on the selected option. The WHEN OTHER clause handles any invalid input.

Example 2: File Status 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
       IDENTIFICATION DIVISION.
       PROGRAM-ID. FILE-STATUS-HANDLER.
       
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01  FILE-STATUS         PIC X(2).
       01  STATUS-MESSAGE      PIC X(100).
       
       PROCEDURE DIVISION.
       HANDLE-FILE-STATUS.
           * Simulate different file status codes
           MOVE "00" TO FILE-STATUS
           
           EVALUATE FILE-STATUS
               WHEN "00"
                   MOVE "File operation successful" TO STATUS-MESSAGE
                   DISPLAY STATUS-MESSAGE
               WHEN "10"
                   MOVE "End of file reached" TO STATUS-MESSAGE
                   DISPLAY STATUS-MESSAGE
               WHEN "22"
                   MOVE "Duplicate key error" TO STATUS-MESSAGE
                   DISPLAY STATUS-MESSAGE
               WHEN "23"
                   MOVE "Record not found" TO STATUS-MESSAGE
                   DISPLAY STATUS-MESSAGE
               WHEN "30"
                   MOVE "Permanent error occurred" TO STATUS-MESSAGE
                   DISPLAY STATUS-MESSAGE
               WHEN OTHER
                   MOVE "Unknown file status code" TO STATUS-MESSAGE
                   DISPLAY STATUS-MESSAGE
           END-EVALUATE
           
           STOP RUN.

This example shows how WHEN clauses can be used to handle different file status codes. Each status code has its own handling logic, making the code more organized and easier to maintain than multiple IF statements.

Example 3: Data Type 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
       IDENTIFICATION DIVISION.
       PROGRAM-ID. DATA-TYPE-PROCESSOR.
       
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01  DATA-TYPE           PIC X(1).
       01  PROCESSING-RESULT   PIC X(50).
       
       PROCEDURE DIVISION.
       PROCESS-DATA.
           * Simulate different data types
           MOVE "N" TO DATA-TYPE
           
           EVALUATE DATA-TYPE
               WHEN "A"
                   MOVE "Processing alphabetic data" TO PROCESSING-RESULT
                   DISPLAY PROCESSING-RESULT
                   PERFORM PROCESS-ALPHABETIC
               WHEN "N"
                   MOVE "Processing numeric data" TO PROCESSING-RESULT
                   DISPLAY PROCESSING-RESULT
                   PERFORM PROCESS-NUMERIC
               WHEN "X"
                   MOVE "Processing alphanumeric data" TO PROCESSING-RESULT
                   DISPLAY PROCESSING-RESULT
                   PERFORM PROCESS-ALPHANUMERIC
               WHEN OTHER
                   MOVE "Unknown data type" TO PROCESSING-RESULT
                   DISPLAY PROCESSING-RESULT
           END-EVALUATE
           
           STOP RUN.
       
       PROCESS-ALPHABETIC.
           DISPLAY "Alphabetic processing complete".
       
       PROCESS-NUMERIC.
           DISPLAY "Numeric processing complete".
       
       PROCESS-ALPHANUMERIC.
           DISPLAY "Alphanumeric processing complete".

This example demonstrates processing different data types using WHEN clauses. Each data type has its own processing routine, and the EVALUATE statement provides a clean way to route to the appropriate processing logic.

Example 4: Range Evaluation

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
       IDENTIFICATION DIVISION.
       PROGRAM-ID. RANGE-EVALUATOR.
       
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01  SCORE               PIC 9(3).
       01  GRADE               PIC X(1).
       
       PROCEDURE DIVISION.
       EVALUATE-SCORE.
           DISPLAY "Enter test score (0-100): "
           ACCEPT SCORE
           
           EVALUATE TRUE
               WHEN SCORE >= 90
                   MOVE "A" TO GRADE
                   DISPLAY "Grade: A (Excellent)"
               WHEN SCORE >= 80
                   MOVE "B" TO GRADE
                   DISPLAY "Grade: B (Good)"
               WHEN SCORE >= 70
                   MOVE "C" TO GRADE
                   DISPLAY "Grade: C (Average)"
               WHEN SCORE >= 60
                   MOVE "D" TO GRADE
                   DISPLAY "Grade: D (Below Average)"
               WHEN OTHER
                   MOVE "F" TO GRADE
                   DISPLAY "Grade: F (Failing)"
           END-EVALUATE
           
           STOP RUN.

This example shows how WHEN clauses can be used with EVALUATE TRUE to handle range conditions. The program evaluates a test score and assigns an appropriate grade based on the score range.

Best Practices and Considerations

Best Practices

  • Always include a WHEN OTHER clause for unexpected values
  • Order WHEN clauses from most specific to least specific
  • Use meaningful condition names and descriptions
  • Keep WHEN clause code blocks concise and focused
  • Test all possible conditions thoroughly

Considerations

  • Performance implications of multiple WHEN clauses
  • Maintainability of complex conditional logic
  • Error handling and recovery procedures
  • Code readability and documentation
  • Testing coverage for all conditions

Test Your Knowledge

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

  • To define file operations
  • To specify conditions in EVALUATE statements
  • To control program flow
  • To handle errors

2. In which statement is the WHEN clause most commonly used?

  • IF statement
  • EVALUATE statement
  • PERFORM statement
  • READ statement

3. What is the relationship between WHEN and EVALUATE?

  • They are unrelated
  • WHEN is used within EVALUATE to define conditions
  • EVALUATE is used within WHEN
  • They are the same thing

4. What are the main components of a WHEN clause?

  • Only the condition
  • Condition and the code to execute
  • Only the code to execute
  • Only the WHEN keyword

5. When should you use the WHEN clause?

  • Only for simple conditions
  • For multiple conditional branches in EVALUATE statements
  • Only for error handling
  • Only for file operations

Frequently Asked Questions

Related Concepts

EVALUATE Statement

Understanding the EVALUATE statement structure

IF Statement

Alternative conditional execution with IF

Conditional Execution

Overview of conditional execution in COBOL

Program Control

Program control and branching techniques

Error Handling

Error handling and exception management

UNSTRING and END-UNSTRINGWRITE and END-WRITE