Conditional

• Condition Evaluation: Involves evaluating a logical expression (e.g., comparing values, checking return codes, testing flags) that results in a true or false outcome.
• Flow Control: Directs the execution path of a program or job, allowing different code segments or job steps to run based on the evaluated condition.
• Ubiquitous in Mainframe Languages: Implemented through constructs like IF/THEN/ELSE in COBOL, REXX, and JCL, EVALUATE in COBOL, SELECT in REXX, and conditional branch instructions in Assembler.
• Return Code Dependency: Often relies on return codes (RCs) from previous programs or job steps to determine subsequent actions, particularly in JCL and batch processing.
• Error Handling: Essential for robust error handling, allowing programs or jobs to react differently to successful completion, warnings, or critical errors.
• Dynamic Behavior: Enables applications and batch jobs to exhibit dynamic behavior, adapting their processing based on input data, system status, or previous processing results.

• JCL Job Step Execution: Using the COND parameter on an EXEC statement or IF/THEN/ELSE/ENDIF constructs to skip or execute job steps based on return codes from prior steps.

jcl
//STEP01 EXEC PGM=PROGA
//STEP02 EXEC PGM=PROGB,COND=(0,LT,STEP01)  // Execute if STEP01 RC < 0 (error)
//IF (RC EQ 0) THEN
//STEP03 EXEC PGM=PROGC
//ENDIF

• COBOL Program Logic: Implementing decision-making within a COBOL program using IF statements to process data differently based on its value or status.

cobol
IF WS-TRANSACTION-TYPE = 'ADD'
PERFORM 1000-ADD-RECORD
ELSE IF WS-TRANSACTION-TYPE = 'DEL'
PERFORM 2000-DELETE-RECORD
ELSE
DISPLAY 'INVALID TRANSACTION TYPE'
END-IF.

• REXX Scripting: Controlling the flow of a REXX EXEC based on user input, file existence, or command output, often used for