Comparand

Enhanced Definition

A comparand, in the context of mainframe programming and operations, refers to a value, variable, or expression that is used as one of the operands in a comparison operation. It is the data point against which another value (the comparator) is evaluated to determine their relationship (e.g., equality, greater than, less than). This evaluation is fundamental for decision-making and control flow in z/OS applications and job streams. A comparand, in the context of mainframe computing and programming, refers to a value, variable, literal, or expression that is used as one side of a comparison operation. It is the specific data item whose value is being evaluated against another value to determine a relationship (e.g., equality, greater than, less than).

Key Characteristics

- Can be a literal value (e.g., 100, 'ABC'), the content of a program variable (e.g., WS-AMOUNT), or the content of a CPU register.
- Its data type (e.g., PIC 9(5) COMP-3, PIC X(10)) and length are critical, as comparisons typically require compatible types for accurate results.
- Used extensively in conditional statements (IF, EVALUATE, WHEN) and loop control (PERFORM UNTIL) to dictate program execution paths.
- In JCL, comparands are often implied or explicitly stated in the COND parameter to control job step execution based on return codes.
- Can represent various data formats, including packed-decimal, binary, zoned decimal, character, and hexadecimal values.

Use Cases

- COBOL Conditional Logic: Determining if a transaction amount exceeds a limit using IF WS-TRANSACTION-AMOUNT > 5000 THEN.... Here, WS-TRANSACTION-AMOUNT is a comparand evaluated against the literal 5000.
- JCL Job Step Control: Specifying COND=(4,LT,STEP01) where 4 is a comparand for the return code of STEP01, ensuring the current step runs only if STEP01 ended with a return code less than 4.
- Assembler Language Comparisons: Using instructions like CLC (Compare Logical Character), CP (Compare Packed), or CR (Compare Registers) to evaluate two operands and set condition codes.
- DB2 WHERE Clauses: Filtering rows in a database query, such as SELECT CUST_NAME FROM CUSTOMER WHERE CUST_STATUS = 'ACTIVE', where 'ACTIVE' is a comparand for the CUST_STATUS column.
- IMS Message Processing: Checking the value of a field in an incoming message segment to route it to a specific processing routine.

Related Concepts

The comparand is intrinsically linked to comparison operators (e.g., =, >, <, NOT EQUAL, GE, LE) and conditional statements. It works in conjunction with these to form expressions that evaluate to either true or false, thereby driving control flow within a program or job. Its effective use relies on understanding data types and data conversion, as incompatible types can lead to incorrect comparison results. In the broader context, comparands are fundamental to decision logic, validation routines, and error handling across all mainframe programming languages and utilities.

Best Practices:

Ensure Data Type Compatibility: Always verify that the data types and lengths of the comparands are compatible to prevent unexpected results or abends, especially when comparing different numeric formats.
Understand Collating Sequence: For character string comparisons, be aware of the EBCDIC collating sequence, as it dictates the order of characters and can affect > or < comparisons.
Test Boundary Conditions: Thoroughly test comparisons at their boundary values (e.g., X = 0, X = 1, X = -1) to ensure the logic behaves as expected.
Use Explicit Conversions When Necessary: If comparing different numeric types (e.g., packed-decimal to binary), consider explicit conversions to ensure the comparison is performed on consistent representations.
Avoid Floating-Point Comparisons for Equality: Due to potential precision issues, avoid direct equality comparisons (=) with floating-point numbers; instead, check if the absolute difference is within an acceptable tolerance.