Constant

Enhanced Definition

In the context of IBM mainframe systems, a constant is a fixed, unchangeable data value that is defined within a program, JCL, or script and remains unaltered throughout its execution. It represents a piece of information whose value is known at compile or assembly time and does not vary during runtime. In mainframe programming, particularly within languages like COBOL, Assembler, or PL/I, a constant is a fixed data value that is defined at compile time and remains unchanged throughout the execution of a program. It provides a reliable, immutable reference for specific values, ensuring data integrity and consistency.

Key Characteristics

- Immutability: Once defined, the value of a constant cannot be modified by the program or system during its execution.
- Definition Time: Constants are typically defined at the time the program is written or assembled/compiled, or specified directly as literals.
- Data Types: They can represent various data types, including numeric (e.g., PIC 9(5)), alphanumeric (e.g., PIC X(20)), hexadecimal, or packed decimal values.
- Storage: In compiled languages like COBOL, constants are often stored in the WORKING-STORAGE SECTION or LINKAGE SECTION with a VALUE clause, or directly as literals in the PROCEDURE DIVISION. In Assembler, they are defined using DC (Define Constant) statements.
- Readability and Maintainability: Using named constants instead of "magic numbers" or hardcoded strings significantly improves code readability and simplifies maintenance.

Use Cases

- COBOL Program Parameters: Defining fixed sizes for arrays, maximum record counts, standard error codes, or tax rates that are stable over time.
- JCL Symbolic Parameters: Providing fixed values for dataset qualifiers, utility options, or system parameters that are consistent across job executions.
- Assembler Data Definition: Storing fixed addresses, bit masks, system messages, or table entries that are integral to the program's logic.
- REXX Scripts: Defining fixed string literals, numeric thresholds, or command prefixes used repeatedly within a script.
- Database Schema Definitions: Specifying default values for columns in DB2 tables or fixed lengths for fields in IMS segments.

Related Concepts

Constants are fundamentally different from variables, which are named storage locations whose values can change during program execution. They are often used in conjunction with literals, which are direct representations of a constant value (e.g., MOVE 100 TO WS-COUNT where 100 is a numeric literal). Constants adhere to specific data types (e.g., PIC clauses in COBOL) which dictate their storage format and valid operations. In JCL, symbolic parameters can effectively act as constants, allowing a fixed value to be substituted throughout a job stream.

Best Practices:

Use Named Constants: Always define constants with meaningful names (e.g., 01 WS-MAX-EMPLOYEES PIC 9(5) VALUE 1000.) rather than embedding "magic numbers" directly into logic, enhancing clarity and ease of modification.
Centralize Definitions: For constants used across multiple programs or modules, define them in copybooks (COBOL), include files (Assembler), or parameter libraries (JCL) to ensure consistency and simplify updates.
Choose Appropriate Data Types: Select the correct PIC clause or data type definition to match the constant's intended use and prevent data truncation or conversion errors.
Document Purpose: Clearly document the purpose and expected value range of each constant, especially when its meaning isn't immediately obvious from its name.
Avoid Redundancy: Define a constant once and reference it multiple times, rather than repeating the same literal value throughout the code.