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COBOL Conditional Statements

Conditional statements control program flow based on conditions. Learn IF-THEN-ELSE, EVALUATE-WHEN, nested conditions, and other decision-making constructs for effective program control.

Basic IF Statement

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       IF CUSTOMER-BALANCE > 0
           DISPLAY 'Customer has positive balance'
           PERFORM PROCESS-POSITIVE-BALANCE
       END-IF.

The basic IF statement executes code when a condition is true. The condition is evaluated, and if true, the statements between IF and END-IF execute. If false, execution continues after END-IF.

IF-THEN-ELSE Statement

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       IF TRANSACTION-TYPE = 'DEPOSIT'
           ADD TRANSACTION-AMOUNT TO ACCOUNT-BALANCE
           DISPLAY 'Deposit processed'
       ELSE
           SUBTRACT TRANSACTION-AMOUNT FROM ACCOUNT-BALANCE
           DISPLAY 'Withdrawal processed'
       END-IF.

IF-THEN-ELSE provides alternative execution paths. If the condition is true, the THEN clause executes. If false, the ELSE clause executes. Always use END-IF to close the statement.

Nested IF Statements

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       IF CUSTOMER-TYPE = 'INDIVIDUAL'
           IF CUSTOMER-AGE >= 18
               DISPLAY 'Adult individual customer'
               PERFORM PROCESS-ADULT-INDIVIDUAL
           ELSE
               DISPLAY 'Minor individual customer'
               PERFORM PROCESS-MINOR-INDIVIDUAL
           END-IF
       ELSE IF CUSTOMER-TYPE = 'CORPORATE'
           DISPLAY 'Corporate customer'
           PERFORM PROCESS-CORPORATE
       ELSE
           DISPLAY 'Unknown customer type'
           PERFORM PROCESS-UNKNOWN
       END-IF.

Nest IF statements for complex decision logic. Each IF must have a corresponding END-IF. Use proper indentation to show the nesting level. ELSE IF provides multiple condition checks.

EVALUATE Statement

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       EVALUATE CUSTOMER-STATUS
           WHEN 'ACTIVE'
               DISPLAY 'Active customer'
               PERFORM PROCESS-ACTIVE-CUSTOMER
           WHEN 'INACTIVE'
               DISPLAY 'Inactive customer'
               PERFORM PROCESS-INACTIVE-CUSTOMER
           WHEN 'SUSPENDED'
               DISPLAY 'Suspended customer'
               PERFORM PROCESS-SUSPENDED-CUSTOMER
           WHEN OTHER
               DISPLAY 'Unknown customer status'
               PERFORM PROCESS-UNKNOWN-STATUS
       END-EVALUATE.

EVALUATE is ideal for multi-way decisions based on a single variable. Each WHEN clause handles a specific value. WHEN OTHER catches any values not explicitly handled. Use END-EVALUATE to close.

EVALUATE with Multiple Conditions

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       EVALUATE TRUE
           WHEN CUSTOMER-TYPE = 'INDIVIDUAL' AND CUSTOMER-AGE >= 65
               PERFORM PROCESS-SENIOR-INDIVIDUAL
           WHEN CUSTOMER-TYPE = 'INDIVIDUAL' AND CUSTOMER-AGE < 65
               PERFORM PROCESS-NON-SENIOR-INDIVIDUAL
           WHEN CUSTOMER-TYPE = 'CORPORATE' AND COMPANY-SIZE = 'LARGE'
               PERFORM PROCESS-LARGE-CORPORATE
           WHEN CUSTOMER-TYPE = 'CORPORATE' AND COMPANY-SIZE = 'SMALL'
               PERFORM PROCESS-SMALL-CORPORATE
           WHEN OTHER
               PERFORM PROCESS-UNKNOWN-CUSTOMER
       END-EVALUATE.

Use EVALUATE TRUE with WHEN clauses for complex multi-condition logic. This pattern makes all conditions visible at the same level and is easier to maintain than deeply nested IF statements.

Conditional Expressions

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       *> Relational conditions
       IF CUSTOMER-BALANCE > 1000
           DISPLAY 'High balance customer'
       END-IF.
       
       *> Class conditions
       IF CUSTOMER-ID IS NUMERIC
           DISPLAY 'Valid numeric ID'
       END-IF.
       
       *> Sign conditions
       IF TRANSACTION-AMOUNT IS POSITIVE
           DISPLAY 'Credit transaction'
       END-IF.
       
       *> Complex conditions
       IF (CUSTOMER-TYPE = 'PREMIUM' AND CUSTOMER-BALANCE > 5000)
           OR (CUSTOMER-TYPE = 'GOLD' AND CUSTOMER-BALANCE > 2000)
           DISPLAY 'Eligible for special service'
       END-IF.

Use various condition types: relational (>, <, =), class (NUMERIC, ALPHABETIC), sign (POSITIVE, NEGATIVE, ZERO), and complex conditions with AND, OR, NOT operators.

Conditional Loops

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       *> Conditional loop with IF
       PERFORM UNTIL EOF OR ERROR-FLAG = 'Y'
           READ INPUT-FILE AT END SET EOF TO TRUE
           NOT AT END
               IF RECORD-TYPE = 'VALID'
                   PERFORM PROCESS-VALID-RECORD
               ELSE
                   DISPLAY 'Invalid record: ' RECORD-TYPE
                   SET ERROR-FLAG TO TRUE
               END-IF
       END-PERFORM.

Use conditional statements within loops to control processing based on data conditions. Check for both normal termination (EOF) and error conditions to provide robust error handling.

Early Exit Patterns

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       *> Early exit with error handling
       IF CUSTOMER-ID = 0
           DISPLAY 'Error: Invalid customer ID'
           PERFORM ERROR-HANDLING
           EXIT PARAGRAPH
       END-IF.
       
       IF CUSTOMER-STATUS = 'CLOSED'
           DISPLAY 'Error: Customer account closed'
           PERFORM ERROR-HANDLING
           EXIT PARAGRAPH
       END-IF.
       
       *> Main processing continues here
       DISPLAY 'Processing valid customer'
       PERFORM MAIN-PROCESSING.

Use early exit patterns to handle error conditions first, reducing nesting levels. Check for invalid conditions early and exit using EXIT PARAGRAPH or EXIT SECTION.

Conditional Processing

Business rules and decision trees

Control Flow

Program flow control