Equivalence - Logical equality

Enhanced Definition

In the context of mainframe computing, **logical equality** refers to the evaluation of whether two data items, expressions, or conditions hold the same value or state. It is a fundamental concept used in programming logic and control flow to make decisions, validate data, and direct program execution based on comparisons. In the mainframe context, logical equality (or equivalence) refers to the state where two data items, expressions, or system conditions are determined to have the same value, state, or meaning based on a defined comparison criterion. It is a fundamental concept for decision-making, data validation, and control flow within z/OS applications and system utilities.

Key Characteristics

- Conditional Processing: Forms the basis for IF/THEN/ELSE constructs, EVALUATE statements in COBOL, IF statements in JCL (z/OS 2.1+), and SELECT statements in REXX.
- Comparison Operators: Typically implemented using comparison operators such as =, EQ, EQUAL TO, == (in REXX or C/C++ on z/OS), or specific machine instructions like CR (Compare Register) or CLC (Compare Logical Character) in Assembler.
- Data Type Sensitivity: Comparisons often depend on the data types involved (e.g., numeric, character, packed decimal), requiring careful consideration to avoid unexpected results due to implicit conversions or differing representations.
- Return Code Evaluation: Crucial for evaluating job step completion codes (RC) in JCL using the COND parameter or IF statements, determining success or failure of utility programs or batch steps.
- Boolean Outcome: The result of a logical equality comparison is always a Boolean value: true (equal) or false (not equal), which then dictates the subsequent program path.

Use Cases

- Data Validation: Checking if an input field matches an expected value, a valid code from a lookup table, or a specific range boundary within a COBOL program.
- Control Flow in JCL: Using the COND parameter to execute or bypass subsequent job steps based on the return codes of preceding steps (e.g., COND=(4,LT) to bypass if the previous step's RC was less than 4).
- Program Logic in COBOL: Directing program execution based on the state of a flag (IF WS-FLAG = 'Y' THEN...), the value of a counter, or the content of a record field.
- Database Query Filtering: Specifying criteria in SQL WHERE clauses to retrieve records where a column's value is equal to a specific constant or another column's value (e.g., SELECT * FROM EMP WHERE DEPT = 'SALES').
- CICS Transaction Processing: Evaluating EIBRESP or EIBRESP2 after a CICS command to determine if the command