© 2004 IBM Corporation IBM ^ z/VM Module 6: The REXX Programming Language

© 2004 IBM Corporation

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z/VMModule 6: The REXX Programming Language


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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

SELECTWHEN morning THEN DO

say “Take shower”say “Eat breakfast.”say “Get ready for work.”end

WHEN afternoon THEN DO until ans=Ysay “Did you eat lunch? (Y/N)PARSE UPPER PULL ansend

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 fieldReady (00024);

3) erase junk execFile JUNK EXEC A not foundReady (00028)


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Why Use a Compiler?

Advantages of compiling REXX EXECSSource can be hidden from end usersLoad 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 programParsing – manipulates character strings to let your program read

and separate characters, number, and mixed inputsPL/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 SC24-5962-00

The REXX Language: A Practical Approach to Programming –by Michael Cowlishaw

Website: Rexx Language Association

Documents

© 2004 IBM Corporation IBM ^ z/VM Module 6: The REXX Programming Language