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z/VM
Module 6: The REXX Programming Language
© 2004 IBM Corporation
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Module Objectives
 Data structures

IF-THEN-ELSE

SELECT

LOOPS
 Data formats

An example of formatting numbers and strings are:
– FORMAT( )  numerical
– SUBSTR( )  string manipulation
 Input/Output (I/O) functions

STREAM( ) function

CHARIN, LINEIN, CHAROUT, LINEOUT instructions
 Parameters

To retrieve parameters use:
– ARG, PULL, etc.
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Objectives
 Describe REXX and how it works with z/VM
 Describe how to write REXX programs using:

Comments

Keywords and literal strings

Clauses

Syntax error messages
 Explain the use of REXX variables with names, values, and
assignments
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Objectives continued
 Understand which expressions can be used within a REXX clause:

Operators and terms

Comparisons (equal, and, or)

Functions
 Learn the control statements for manipulating data flow:

IF – THEN

ELSE keyword

DO LOOPS (repetitive and conditional)

Selection
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Objectives continued
 Explain arithmetic, text, and conversational expressions for
manipulating and gathering data
 Show how to issue CMS and CP commands within a REXX
EXEC
 Explain the subcommands and macros used in REXX EXECs
 Introduce REXX subroutines
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What is REXX?
 REstructured eXtended eXecutor language
 A versatile, easy to use, structured programming language
 A programming language that is easy for both computer
professionals and general users to learn and use
 A compiler can be used to translate REXX source programs
into compiled programs
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Features of REXX







Ease of use
Free format
Interpreted
Built-in functions
Parsing capabilities
Powerful debugger
Relationship with z/VM
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How a Program Works
 A REXX program is a list of instructions, something like a recipe
 A computer communicates with users through questions displayed
and answers typed in
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Comments in REXX Programs
 Comments in programs:
•/* . . . */, this is used for descriptions and explanations
 Comments with special meaning to CMS
•To determine you are writing a REXX program the first line must contain /*
. . . */
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Keywords and Literal Strings
 Keywords are instructions that describe an action, such as
PULL, IF, and SAY.
 REXX reads each individual clause, then processes it before
going on to the next (interpreted language).
 A literal string is a set of characters bounded by quotation
marks.
 REXX processes a clause containing a variable by substituting
the variable name with the stored data.
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REXX Clauses
 REXX programs consist of these types of clauses:
•Instruction
•Assignment
•Label
•Null
•Commands
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REXX Syntax Errors
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REXX Variables
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Names, Values, and Assignments
 Information stored in a variable is called its value.
 It is possible to make variable names anything, but a good idea
to create meaningful names.
 An instruction that stores a value in a variable or changes its
value is called an assignment.
• In formal terms, the syntax might look like this:
• symbol = expression
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Other Assignments
 The PULL instruction:

Pauses the running program to allow the user to enter data

Can be used to pull in each piece of data or allow the user to
enter multiple amounts of data separated by spaces
 The ARG instruction:

Like PULL, but data items are entered at the command prompt
with the program name
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Assignments and Instructions
 One way to write this EXEC is:
/* SUBMUL1 EXEC */
ARG first second
say first “-” second “=” first-second
say first “*” second “=” first*second
 Another way to write this EXEC is:
/* SUBMUL2 EXEC */
say “Enter two numbers to multiply and subtract.”
pull first second
say first “-” second “=” first-second
say first “*” second “=” first*second
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REXX Expressions
 Operators and terms:

Operators include +, -, /, %, *, ||

Operators manipulate numbers, strings in quotes, variables,
results from function calls and evaluated expressions
 Parentheses:

The language processor evaluates the expression inside the
parentheses first
• The value of 10 * ( 3 || 4 ) is: 340
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REXX Expressions (Comparison, True, and False)
 Comparisons:
– > Greater than
– = Equal
– < Less than
 TRUE, the computed result is 1
– say 4 < 7
• /* represents a “1”, which means TRUE */
 FALSE, the computed result is 0
– say “Chalk” = “Cheese”
• /* represents a “0”, which meaning FALSE */
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REXX Expressions ( =, &, | )
 The equal sign (=) can have two meanings
– Can be an assignment if found at the beginning after the symbol
– An equal sign anywhere else stands for the comparison operator
 Use the AND (&) operator to write an expression that is true
when everything else is also true
 Use the OR (|) operator when any part of an expression can be
true
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REXX Functions
 Function calls can be written anywhere in an expression.
 The function performs the computation named by the function
and returns the result.
 When the value of the function has been calculated, the result
is put back into the expression in place of the function call.
– An example is:
• say 7 + HALF(6)
/* becomes 7 + 3 which says “10” */
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Control Statements: IF – THEN
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Examples and Notes: IF - THEN
 The THEN instruction may be an assignment, command, or keyword.
 The NOP instruction can be used when no operations are necessary.
 An important property of the THEN keyword is that is does not need to
start a clause, therefore a semicolon is not needed.
 Another example is:

If answer=‘YES’ then say ‘OK!’; else say ‘Why not?’
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Control Statements: ELSE Keyword
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REXX Loops
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Repetitive DO Loops
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Conditional DO Loops
 Conditional loops continue to be
executed as long as some
condition is satisfied.
 The simplest way to code these
loops is to use DO FOREVER
and LEAVE instructions.
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Conditional Loops: The Choice
 There are three kinds of Conditional Loops

The decision is made before processing starts
– Checking occurs before entering the loop and continues after each
iteration.

The decision is made after the first pass through the loop and
again after every subsequent pass.
– Data is requested for the user.

The decision is made during each pass.
– The decision to leave might depend on information obtained during
the loop.
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The SELECT Instruction
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A SELECT Instruction
SELECT
WHEN morning THEN DO
say “Take shower”
say “Eat breakfast.”
say “Get ready for work.”
end
WHEN afternoon THEN DO until ans=Y
say “Did you eat lunch? (Y/N)
PARSE UPPER PULL ans
end
otherwise say “It is in the evening -- get ready for bed!!”
end
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Arithmetic
 The addition, subtraction and multiplication operations are performed
in the usual way.
–
–
–
–
+  Addition
-  Subtraction
*  Multiplication
**  Power function
 The result of a % operation is the whole number portion. The
remainder is dropped.
 The result of a // operation is the remainder portion. The whole
number is dropped.
 The result of a / operator is a combination of both operations above.
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Text - Concatenation
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Text – String Manipulation
 SUBSTR() Function:

To select a part of a string to use:
– WORD = “reveal”
– say substr(WORD, 2, 3) /* says “eve” */
 LENGTH() Function:

To find out the length of a REXX variable:
– WORD = "reveal"
– say length(WORD) /* says "6" */
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Text – String Manipulation continued
 COPIES():

Produces a number of copies of the string. The arguments are:
• The string to be copied
• The number of copies required
 LEFT():

Obtains a string that is padded or truncated on the right
 RIGHT():

Obtains a string that is padded or truncated on the left
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Conversations – SAY and PULL
 The SAY instruction and its expression are computed and the result
is displayed as a new line on the screen.
 The PULL instruction is able to collect an answer that has been
displayed by the SAY instruction.
 The PARSE PULL instruction brings in the data just as it is, without
converting the lowercase letters to uppercase.
 The UPPER instruction translates the value of one or more variables
to uppercase.
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Conversation – Parsing Words
 PULL can also fetch each word into a different variable
 Using the period as a place holder in this statement (PULL . .
lastname .) means to discard the first two words and assign the third
word to lastname.
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Issuing Commands to CMS and CP
 The language processor can operate in a number of environments.
 Use quotes to avoid errors when writing CMS and CP commands
within REXX.
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Issuing Commands – Return Codes
 More examples:
1) access 591 591
DMSACC113S B(591) not attached or invalid device
address
Ready (00100);
2) copyfile profile exec a = = b (for luck
Invalid parameter LUCK in the option FOR field
Ready (00024);
3) erase junk exec
File JUNK EXEC A not found
Ready (00028)
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Why Use a Compiler?
 Advantages of compiling REXX EXECS
Source
Load
can be hidden from end users
modules are loaded into memory faster
 Compile programs using this CMS command:
REXXD
[source-file-identifier]
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How to Use the Compiler
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XEDIT Subcommands and Macros
 The first word on the command line is assumed to be a subcommand
 Words that are not subcommands are interpreted as macros
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Subroutines
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Conclusion
 REXX was created as a procedural language that allows programs and
algorithms to be written in a clear and structured way.
 Topics in this module:

Comments

Clauses

Variables

Expressions

Control statements:
–
–
–
–
IF THEN
ELSE
Loops
Selection
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Glossary
Clause – a line of code or a statement within a REXX program
Parsing – manipulates character strings to let your program read
and separate characters, number, and mixed inputs
PL/I – was developed as the universal programming language,
where definitions were not needed
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Glossary
 REXX – REstrutured eXtneded eXecutor language, a versatile,
easy to use structured programming language that is an
integral part of z/VM.
 REXXCompiler – translates REXX source programs into
compiled programs. (Compiled programs run much faster than
interpreted programs.)
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References
z/VM: REXX/VM User’s Guide –Version 3 Release 1.0  SC245962-00
The REXX Language: A Practical Approach to Programming –by
Michael Cowlishaw
Website: Rexx Language Association
© 2004 IBM Corporation