IBM TSO/E REXX User Manual

IF expression THEN
instruction
ELSE NOP
If you have more than one instruction for a condition, begin the set of instructions
with a DO and end them with an END.
IF weather = rainy THEN
SAY 'Find a good book.'
ELSE
DO
SAY 'Would you like to play tennis or golf?'
PULL answer
END
Without the enclosing DO and END, the language processor assumes only one
instruction for the ELSE clause.
Nested IF/THEN/ELSE Instructions
Sometimes it is necessary to have one or more IF/THEN/ELSE instructions within
other IF/THEN/ELSE instructions. Having one type of instruction within another is
called nesting. With nested IF instructions, it is important to match each IF with an
ELSE and each DO with an END.
IF weather = fine THEN
DO
SAY 'What a lovely day!'
IF tenniscourt = free THEN
SAY 'Shall we play tennis?'
ELSE NOP
END
ELSE
SAY 'Shall we take our raincoats?'
Not matching nested IFs to ELSEs and DOs to ENDs can have some surprising
results. If you eliminate the DOs and ENDs and the ELSE NOP, as in the following
example, what is the outcome?
Example of Missing Instructions
/******************************** REXX *****************************/
/* This exec demonstrates what can happen when you do not include */
/* DOs, ENDs, and ELSEs in nested IF/THEN/ELSE instructions. */
/*******************************************************************/
weather = 'fine'
tenniscourt = 'occupied'
IF weather = 'fine' THEN
SAY 'What a lovely day!'
IF tenniscourt = 'free' THEN
SAY 'Shall we play tennis?'
ELSE
SAY 'Shall we take our raincoats?'
By looking at the exec you might assume the ELSE belongs to the first IF. However,
the language processor associates an ELSE with the nearest unpaired IF. The
outcome is as follows:
What a lovely day!
Shall we take our raincoats?
Using Conditional Instructions
Chapter 4. Controlling the Flow Within an Exec 43

IBM TSO/E REXX Specifications

General

Brand	IBM
Model	TSO/E REXX
Category	Computer Hardware
Language	English

Summary

Part 1. Learning the REXX Language

Chapter 1. Introduction

Explains the REXX programming language, its features, and suitability for various users.

Chapter 2. Writing and Running a REXX Exec

Details the process of writing, running, and understanding REXX execs, including syntax rules and common issues.

Chapter 3. Using Variables and Expressions

Covers REXX variables, expressions, operators, and tracing techniques for data manipulation and calculations.

Chapter 4. Controlling the Flow Within an Exec

Explains how to alter REXX exec execution using conditional, looping, and interrupt instructions.

Chapter 5. Using Functions

Describes REXX functions, including built-in, user-written, and packages, and how to use them.

Chapter 6. Writing Subroutines and Functions

Guides on writing, calling, and passing information to REXX subroutines and functions.

Chapter 7. Manipulating Data

Covers REXX data manipulation using compound variables, stems, and various parsing techniques.

Part 2. Using REXX

Chapter 8. Entering Commands from an Exec

Explains how REXX execs can interact with the operating system by issuing TSO/E and host commands.

Chapter 9. Diagnosing Problems Within an Exec

Details techniques for diagnosing and debugging REXX execs using tracing and special variables.

Chapter 10. Using TSO/E External Functions

Describes TSO/E external functions available for interacting with the system and performing specific tasks.

Chapter 11. Storing Information in the Data Stack

Explains how to use the REXX data stack for temporary storage, passing information, and managing buffers.

Chapter 12. Processing Data and Input/Output Processing

Covers dynamic REXX expression modification and processing data to/from data sets using EXECIO.

Chapter 13. Using REXX in TSO/E and Other MVS Address Spaces

Explains how to run REXX execs in TSO/E and non-TSO/E MVS address spaces and available services.

Part 3. Appendixes

Appendix A. Allocating Data Sets

Guides on creating and allocating data sets, including checklists for ISPF/PDF and ALLOCATE commands.

Appendix B. Specifying Alternate Libraries with the ALTLIB Command

Details using the ALTLIB command to specify and manage alternative libraries for implicit execution.

Appendix C. Comparisons Between CLIST and REXX

Compares REXX and CLIST languages, highlighting similarities and differences in features and capabilities.

Appendix D. Notices

Provides legal notices, warranty disclaimers, copyright, and trademark information.

Bibliography

TSO/E Publications

Lists IBM publications related to TSO/E, including administration, CLISTs, and general information.

Related Publications

Lists IBM publications related to z/OS MVS and ISPF, covering system programming and administration.

Index