z/VM Module 6: The REXX Programming Language

1. z/VMModule 6: The REXX Programming Language

2. 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.

3. 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

4. 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

5. 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

6. 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

7. Features of REXX • Ease of use • Free format • Interpreted • Built-in functions • Parsing capabilities • Powerful debugger • Relationship with z/VM

8. 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

9. 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 /* . . . */

10. 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.

11. REXX Clauses • REXX programs consist of these types of clauses: • Instruction • Assignment • Label • Null • Commands

12. REXX Syntax Errors

13. REXX Variables

14. 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

15. 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

16. 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

17. 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

18. 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 */

19. 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

20. 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” */

21. Control Statements: IF – THEN

22. 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?’

23. Control Statements: ELSE Keyword

24. REXX Loops

25. Repetitive DO Loops

26. 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.

27. 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.

28. The SELECT Instruction

29. 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

30. 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.

31. Text - Concatenation

32. 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" */

33. 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

34. 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.

35. 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.

36. 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.

37. 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)

38. Why Use a Compiler? • Advantages of compiling REXX EXECS • Source can be hidden from end users • Load modules are loaded into memory faster • Compile programs using this CMS command: • REXXD [source-file-identifier]

39. How to Use the Compiler

40. 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

41. Subroutines

42. 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

43. 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

44. 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.)

45. 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