Lesson 1 Understanding Variables

Introduction

Lesson 1 covers the following topics dealing with variables and how to define them:

As you become more experienced, you will probably find yourself creating more and more specialized user commands. In doing so, you will probably find the use of variables invaluable. Variables are similar to the parameters that you have already used in the past lessons. Throughout the lessons in this module, you will learn how to use both system-defined and user-defined variables in your user commands.

Command files in this lesson

In this lesson, you will execute command files that use variables. The tasks performed by the command files could also be performed by UDCs; however, this would involve constantly uncataloging and recataloging UDC files to add or modify UDCs. The LF and the STATS command files will be used to illustrate various aspects of variables and expressions. Also, the contents of each file serve as good review for system-defined formal files, file equations, and backreferencing.

	NOTE: Each of these command files could become a UDC in a UDC file if you wished.

Don't expect to understand everything about the LF and STATS command files immediately. You'll be looking at the pieces that make up the files as you progress through the lesson.

  ECHO *** CURRENT JOBS = !HPJOBCOUNT

  ECHO *** CURRENT JOB LIMIT = !HPJOBLIMIT

  IF HPJOBCOUNT<=HPJOBLIMIT-3

     ECHO *** NUMBER OF JOBS IS STILL REASONABLE --- STREAM JOB NOW ***

  ELSE

     ECHO *** APPROACHING JOB LIMIT --- STREAM JOB LATER ***

  ENDIF

  SETVAR FILENAME,"//"

  SETVAR RESPONSE,"N"

  SETVAR OPTION,"1"

  INPUT RESPONSE;PROMPT="DO YOU WISH TO LIST ANY FILE(S)? (Y OR N): "

  WHILE RESPONSE='Y' DO

     INPUT FILENAME;PROMPT="ENTER FILE.GROUP.ACCT (@ ALLOWED,// TO END): "

     IF FILENAME <> '//' THEN

        INPUT OPTION;PROMPT="ENTER LISTFILE OPTION (1,2,3): "

        LISTFILE !FILENAME,!OPTION

     ELSE

        SETVAR RESPONSE,'N'

     ENDIF

  ENDWHILE

  DELETEVAR FILENAME,RESPONSE,OPTION

Assigning and displaying variable values

Now that you have had a chance to look at the command files, you'll be examining the variables within them. An MPE/iX variable is just like a programming variable. It is a placeholder that appears in command syntax. When the actual command is entered, a value is specified to replace the placeholder.

To define (create) a variable, only a SETVAR statement is required:

   SETVAR var_name,var_value

To display the variable value, all that is required is a SHOWVAR statement:

   SHOWVAR var_name

	NOTE: There are other ways to display a variable's value, but the SHOWVAR command suffices for this lesson.For Programmers Unlike a programming variable, an MPE/iX variable is declared and assigned a value in one step. No type statement is required, since the variable type is automatically defined when the value is assigned. An exception to this rule is that any variable used with the INPUT command is considered type string, regardless of the value that it contains.

To demonstrate the ease of assigning and displaying values, enter the following statements:

x = your user number

   SETVAR USER,"USERx"

   SETVAR GROUP,"CLASS"

   SETVAR ACCT,"ACCTx"

   SHOWVAR USER,GROUP,ACCT

You should see the following:

   USER = USERx

   GROUP = CLASS

   ACCT = ACCTx

	NOTE: Either single or double quotation marks may be used. In special cases, you must use double quotes. For example, you may wish to use a single quotation mark within a character string: SETVAR WHEN,"The meeting's at 8:00" Also, the variable name and value hmay be separated by a space or a comma: SETVAR WHEN,"The meeting's at 8:00" or SETVAR WHEN "The meeting's at 8:00"

Here are some simple examples of assigning variable values in the command files:

The STATS command file makes reference to a variable called HPJOBLIMIT.

The value of this variable is the number of jobs (job limit) that can run at one time. This is the same value that you see as JLIMIT when you do a SHOWJOB. This is a system-defined variable which happens to be read only. Its initial value is set by the LIMIT command.

This value can be changed only by the operator or system manager at the system console.

The LF command file creates and references several variables, one of which is RESPONSE. It is a user-defined variable that refers to an answer that the user must make to a specific prompt. Near the end of the file, it is automatically set to N (No):

   SETVAR RESPONSE "N"

	NOTE: Notice also, that three of the variables are assigned initial values at the beginning of the file, just in case the variables already exist and have some other values. This is a good programming practice. Text values for variables require quotation marks around them; integer values require that you do not use quotation marks.

Variable names

Regardless of how it is defined, a variable must have a name that follows standard naming conventions:

Examples of valid names are:

Examples of invalid names are:

If you look in the two command files, you will see these variables:

STATS file: LF file:

   HPJOBLIMIT       RESPONSE 

   HPJOBCOUNT       FILENAME

                      OPTION

Variable types

The value assigned to the variable determines the variable type, as follows:

   HPJOBLIMIT (number of jobs allowed to run):   



   HPJOBCOUNT (number of jobs currently running):



   RESPONSE   (user response - either Y or N):   



   FILENAME   (user-supplied file name):         

There are two classes of variables:

The variables in both classes can be of any of the three types mentioned earlier.

System-defined variables

System-defined variables are variables whose names are predefined by the system, following the standard naming conventions. In general, system-defined variables are "read only," having values that are assigned and modified only by the system or system manager. Their initial values are set during system configuration and can be changed only at the system console, using special commands.

alues of a few system-defined variables can be modified by a user for that user's session.

Some of the more useful system-defined variables are shown below. A comprehensive list appears in appendix A of the MPE/iX Commands Reference Manual Volumes 1 and 2 (32650-90003 and 32650-90364), indicating whether such a variable is read only (not modifiable) or read/write (modifiable).

The same information is available in the online Help facility. Enter HELP VARIABLESReturn to see the information on system-defined variables.

    HPJOBLIMIT    =  



    HPSESLIMIT    =  



    HPPROMPT       =  



    HPPATH          =  



    HPVERSION      =  



    HPUSER          =  



    HPSESCOUNT    =

To illustrate the use of SETVAR with a modifiable, system-defined variable, please enter the commands as shown in this example:

Example:

Suppose that that you want to change the prompt to "Hi There: ". You can change the value of the system-defined variable, HPPROMPT, by entering:

   SETVAR HPPROMPT,"Hi There: "

Do this now. Press RETURN. You should see the new prompt. The new prompt remains in effect until you log off. You modify it later.

Use the SHOWVAR command to verify the value of the current prompt:

   SHOWVAR HPPROMPT

Use the SHOWVAR command again to determine your current capabilities:

   SHOWVAR HPUSERCAPF

Your capabilities should be AM, IA, BA, ND, SF, and PH.

User-defined variables

User-defined variables are variables with names and values that the user specifies. Once again, names must follow standard conventions. You can read, modify, or delete any user-defined variable. User-defined variables are session dependent. They are good only for the one user during that one session. They remain as long as the user who created them does not log off. They remain even if the user changes groups using the CHGROUP command.

Enter this command:

   CHGROUP PUB

What prompt do you get? Yes, it remains the same even when you change groups.

	NOTE: You can also use CG to perform a CHGROUP. It is a UDC works as long as MYUDC3 is one of your cataloged UDC files.

Return to the CLASS group:

   CG CLASS

Deleting variables

To delete any user-defined variable and its value, you need only issue the DELETEVAR command:

   DELETEVAR variable_name

You cannot, however, delete any system-defined variable. Its value may be changed, but its definition remains the same.

To illustrate this, delete the variables that you defined earlier:

   DELETEVAR USER,GROUP,ACCT

Now use SHOWVAR to verify that they are gone.

   SHOWVAR USER,GROUP,ACCT

You should get error messages indicating that the variables do not exist.

Now try to do the same with these system-defined variables:

   DELETEVAR HPPROMPT,HPJOBLIMIT,HPSESLIMIT

What message do you get?

   VARIABLE IS NOT DELETABLE