Pick/BASIC: A Programmer's Guide
WWW Edition January, 2000
Chapter 18
Additional PICK/BASIC Concepts
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Jonathan E. Sisk's Pick/BASIC: A Programmer's Guide WWW Edition January, 2000
Chapter 18 |
This chapter covers a variety of topics which were touched on lightly, or not at all, during the tutorial portion of the book. These include structured programming and programming standards, common PICK/BASIC instructions not used in the tutorial, and dealing with the Dictionary level of your BP (Basic Programs) file.
The phrase structured programming has been used to mean a collection of programming techniques designed to make programs better. Programmers disagree on what constitutes "better." Does it mean faster? Easier to analyze? Or something else? Some programmers prefer the term "programming standards," which doesn't limit the concept to control structures. Some common programming standards include the following:
Whether or not you are a newcomer to programming, you would do well to adopt some programming standards when you write programs.
Some instructions were not discussed during the course of this book. Those instructions follow in this section, along with a short, summary explanation of how they are used. The syntax for these instructions, as well as all of those covered in the text is found in Appendix A.
The CHAIN statement is somewhat similar to the EXECUTE statement, in that it allows any TCL expression to be executed. The one major difference between CHAIN and EXECUTE is that the EXECUTE statement is capable of returning to the program that issued the statement, and retaining all of the program variables. The CHAIN statement does not return to the program that executes it.
The COM or COMMON statement is used to declare variables that are to be "shared" among external subroutines. This is the alternative to passing variables into an external subroutine as arguments (immediately following the program name on the line that executes the CALL statement).
The common opinion about COMMON is: Don't use it. Using it makes program maintenance much more tedious, because each time a new variable is added to one program, it must be added to all the other programs that also will use it.
The concept of group locks was discussed earlier in the text. The statement for reading a dynamic array is, of course, the READ statement, and the dimensioned array equivalent is the MATREAD statement.
The READU and MATREADU forms of these statements cause the group in which the requested item is found to be "group locked." These group locks remain in effect until:
Note that many of the implementations are now supporting item locks instead of just group locks.
The WRITEU and MATWRITEU statements differ from their "normal" counterparts in that they keep the group lock set even after the WRITE statement.
This statement is exactly like the ON-GOSUB statement discussed earlier, but with the difference that execution does not automatically return to the next line after completion.
The PROCWRITE statement provides PICK/BASIC the ability to "write" a string of characters, each of which is delimited by a space (except on Ultimate and McDonnell Douglas systems, where it is treated as a dynamic array), over the previous contents of the Primary Input Buffer.
The READV statement is typically used by those who don't understand it. Rather than reading an entire item in one trip to disk, like the READ or MATREAD statement did, this reads items one attribute at a time. It has the general form:
READV variable.name FROM file.variable, id.expression, ...
... amc.expression {THEN statement(s)} ELSE statement(s)
Note the amc.expression following the id.expression. This tells the statement which attribute from the array to read and store in the specified variable. This tends to be extremely wasteful. The only time it is allowed is when you only need one attribute from an item.
The RELEASE statement is used to release one or more group locks set by the current process.
It has two general forms:
RELEASE
releases all group locks set by the current process.
RELEASE file.variable,id.expression
releases the group lock set on the group that the specified item resides in.
This intrinsic function has quite a few powerful features that come in handy from time to time. Unfortunately, it is not consistent across all Pick systems, so consult your PICK/BASIC manual for more information.
To illustrate some of its features, as found in "generic" Pick, here are two of the functions of SYSTEM:
SYSTEM(2) Returns the current page width as defined by the TERM statement:
IF SYSTEM(2) > 80 THEN ...SYSTEM(14) Returns the number of characters awaiting input in the input buffer.
IF SYSTEM(14) THEN ...
The NULL statement is used almost exclusively as a mate for the THEN clause in an IF-THEN-ELSE statement. The NULL statement does absolutely nothing (this is what technical types call a "no-op"). It can be used as follows:
IF conditional. expression THEN NULL ELSE statement
There are cases where "nested" IF statements are needed (Fig. 18-1 ). Special caution must be applied in terminating all of the initiators. Inevitably, you will run into something that appears even worse than this example. This is why there is the CASE statement, which simplifies these situations.
This brings up an important point. With a syntactical structure that is so flexible, some problems may crop up elsewhere; these problems typically occur by having one too many, or one too few END statements. For example, consider this case:
IF conditional.expression THEN <cr> (starts level 1 )
statement...
IF conditional.expression THEN <cr> (starts level 2)
statement...
IF conditional.expression THEN <cr> (starts level 3)
statement...
END ELSE (ends level-3 THEN)
statement... (level-3 ELSE)
END
statement(s) (more code may go here)
END ELSE (ends level-2 THEN)
statement... (level-2 ELSE)
statement(s) (more code may go here)
END ELSE (ends level-1 THEN)
statement (level-1 ELSE)
END (ends level-1 ELSE)
Fig. 18-1, Nesting IF-THEN-ELSE structures.
001 IF conditional.expression THEN statement
002 IF conditional.expression THEN statement
003 IF conditional.expression ELSE statement
004 statement
005 statement
006 END
007 END
008 more code
009 more code
010 END
The programmer may have forgotten one of the critical END statements in the nested IF-THEN. The effect is that the compiler erroneously thinks that the first END statement (at line 6) belongs to the third level, that the second END statement (at line 7) belongs to the second level, and that the third END statement (in this case, at line 10) belongs to the first level. This sneaks through the compiler without any problem, other than the effect of not working properly.
The PRECISION statement declares the number of positions to be carried in mathematical calculations. It needs to be declared only once. The normal default setting on most systems is 4 (four), if left undeclared. The maximum varies from system to system. Ultimate now allows a maximum of 9, McDonnell Douglas allows 6, and the rest of the generic machines (unless they have been changed) allow 4.
In Chapter 2, comment sections were introduced. Many other useful pieces of information could have been included in the comment section, but only the first four lines were used throughout the examples. Among the other kinds of information which could be included are:
Now that you have diligently followed the convention of filling out the comments section in each of your programs, they may be put to some practical use. This involves using the Editor to build attribute definition items in the DICT (dictionary) level of the BP file, as shown in Fig. 18-2.
Since these three attributes are the only ones that are of interest for the moment, these attribute definition items will suffice. Naturally, if you decide to use any of the other suggested items in your templates, attribute definition items should be placed in the dictionary to correspond with them.
Now build the "implicit" attribute defining items, which show themselves automatically.
>COPY DICT BP DESCRIPTION DATE AUTHOR<cr>
TO:1 2 3<cr>
ACCESS sentences may now be used on the BP file. For example:
>SORT BP BY DATE DATE DESCRIPTION AUTHOR<CR>
or, for the report in order by program name:
>SORT BP<cr>
or, to direct the output to the printer, simply add a (P) option:
>SORT BP (P)<cr>
Item-id: DESCRIPTION 001 A 002 2 003 DESCRIPTION 004 005 006 007 008 T3,80 009 T 010 30 Item id: DATE 001 A 002 3 003 DATE 004 005 006 007 D2/ 008 T3,8]DI (The "]" character is a value mark. 009 R 010 8 Item-id: AUTHOR 001 A 002 4 003 AUTHOR 004 005 006 007 008 T3,3 009 L 010 3
Fig. 18-2. Attribute definitions in the DICT portion of the BP file.
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