EVALUATE
In the context of mainframe programming, particularly COBOL, `EVALUATE` is a powerful multi-way conditional statement used to select one of several possible sets of imperative statements for execution. It provides a structured and often more readable alternative to a series of nested `IF` statements for determining actions based on the value of one or more identifiers, expressions, or conditions.
Key Characteristics
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- COBOL Specific: Primarily found in COBOL programs, it's a core part of the language's procedural control flow within the
PROCEDURE DIVISION. - Multi-way Selection: Allows a single subject (an identifier, expression, or condition) to be compared against multiple possible values, ranges, or conditions.
WHENClauses: Each possible value or condition is specified in aWHENclause, followed by the imperative statements to be executed if thatWHENcondition is met.WHEN OTHERClause: An optional but highly recommended clause that specifies the actions to be taken if none of the precedingWHENconditions are satisfied, acting as a default or catch-all.TRUE/FALSE/ANY: Can evaluate conditions (EVALUATE TRUE) or specific values.ANYcan be used in aWHENclause to match any value for a specific subject, effectively acting as a wildcard.THROUGH/THRU: Allows specifying a range of values (e.g.,WHEN 1 THROUGH 5) for a more concise representation of multiple conditions.
- COBOL Specific: Primarily found in COBOL programs, it's a core part of the language's procedural control flow within the
Use Cases
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- Transaction Code Processing: Dispatching control to different routines based on a transaction code read from an input record, a CICS
COMMAREA, or user input (e.g.,ADD,UPDATE,DELETE,INQUIRY). - Menu Selection: Handling user input from a terminal menu, where different numeric or character choices lead to distinct program paths or function calls.
- Error Code Handling: Processing various error or status codes returned by a system call, utility, or database operation (e.g., DB2 SQLCODEs, VSAM status codes), executing specific error routines for each code.
- Complex Business Rule Evaluation: Implementing intricate business logic where actions depend on multiple criteria or ranges of values for one or more data items, providing a clear structure for complex decision trees.
- Data Validation: Checking a field against a set of valid values or ranges defined in the
DATA DIVISIONand taking appropriate action for valid or invalid entries.
- Transaction Code Processing: Dispatching control to different routines based on a transaction code read from an input record, a CICS
Related Concepts
The EVALUATE statement is a structured alternative to the IF statement, particularly for scenarios involving multiple conditions or values for a single subject. While IF statements are suitable for binary (true/false) decisions, EVALUATE excels in multi-way branching, improving code readability and maintainability over deeply nested IF structures. It often works in conjunction with PERFORM statements, where WHEN clauses might PERFORM specific paragraphs or sections, encapsulating reusable logic. It operates on data items, which are defined in the DATA DIVISION, and directly influences the program's control flow, similar to GO TO (though EVALUATE is a