PsL Monthly 1993 December/PSL Monthly Shareware CD-ROM (December 1993).iso/prgmming/dos/basic/tutorqb.com/TUTOR-10.BAS

BASIC Source File | 1986-10-07 | 13.5 KB | 271 lines

goto example1
'                       The QuickBASIC Tutorial Series

'                                 Lesson 10
'                         QB2 Programming Techniques

'                       Copyright 1986  Ford Software

'                            4845 Willowbend Blvd
'                              Houston, TX 77035
'                               [phone]
'                          CompuServe ppn: 71355,470

'This Tutorial may not be copied or distributed in any form - printed, on disk
'or electronically - without the express written permission of Ford Software.
'          Unlicensed distribution is a violation of copyright law
'           and may be subject to civil and criminal prosecution.

'QuickBASIC is the registered trademark of the Microsoft Corporation.
'Ford Software is not associated in any way with the Microsoft Corporation.

'(Make sure NumLock is off and press PgDn)
'page 2
'                           I N T R O D U C T I O N

'In this tutorial, we assume that you have some familiarity with BASIC and
'have used a BASIC interpreter before. If you have not, you should run the
'earlier lessons in this series before running this one. At the very least,
'you should first run TUTOR-01 which covers the functioning of the QB2 editor.

'The purpose of this lesson is to go over the features of QB2 that are
'different from the Microsoft (or IBM) BASIC or BASICA interpreter.

'To run examples in this lesson, first press Ctrl-PgUp to get to the first
'line of this file. Change the line to "goto example#" where "#" is the number
'of the example you wish to run. Then return to the page you were on and press
'Ctrl-R to compile and run the code.

'This tutorial is meant to be used in conjunction with the QB2 manaul. As each
'topic is introduced in this tutorial, look up that topic in the manual and
'read about it first. The purpose of this tutorial is to provide interactive
'reinforcement of the material in the manual and present in a logical order.

'(Press PgDn)
'page 3
'                                 Contents
'
'                 page
'                  4    Labels and Line Numbers
'                  5    Long Lines
'                  6    Code Readability
'                  7    Program Structure
'                  8    Getting Rid of GOTO's
'                  8    ..Interpretive BASIC example
'                  8    ..In QB2 format
'                 10    Replacing IF-THEN With WHILE-WEND
'                 11    Block IF-THEN-ELSEIF-ELSE-END_IF
'
'
'
'  To move directly to a particular page, press Ctrl-F and enter "page n"
'   where "n" is the page number to go to. Then use Ctrl-Down to realign
'   the page on the screen. To search for a topic, press Ctrl-F
'
'
'(Press PgDn)
'page 4                     Labels and Line Numbers

'QB2 does not require lines to be numbered as the BASIC interpreter does.
'Sometimes, however, you will need to be able to identify a line or section of
'code. You can use a line number just like when using the interpreter, or you
'can use a "label".

'A label can be up to 40 characters of alphanumeric characters. It must be the
'first non-blank character on the line and must be followed by a colon (:).
'You do NOT add the colon when using GOTO, GOSUB, etc., just when defining it.
'In a label, there is no difference between capital and non-capital letters.
'SORT: is the same as Sort:. Spaces are not allowed in a label. "SORT.DATA:"
'is a valid label. "SORT DATA:" is not.

example1:
TEXT$="Hello"                 'This routine will call the subroutine by the
GOSUB Print.text              'name of "Print.text" to print TEXT$.
END
Print.text:                   'Press Ctrl-R to compile and run this routine.
PRINT TEXT$
RETURN
'      (PgDn)
'page 5                           Long Lines

'The BASIC interpreter limits lines to 255 characters. Some commands in BASIC,
'such as FIELD statements or IF-THEN-ELSE constructs, could easily take more
'than 255 characters. Under the interpreter, you have to find some way to
'break these into smaller lines.

'QB2 also limits lines in the editor ("physical lines") to 255 characters:     80        90       100       110       120       130       140       150       160       170       180       190       200       210       220       230       240       250
'23456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789
'QB2 allows you to combine several physical lines into one long program line
'of up to 32,767 characters (a "logical line"). Ending a physical line with
'an underscore character ("_") indicates that the logical line is continuing.

'Why should you want such a long line? In the interpreter, a common practice
'is to test for a value and use GOTO to branch around code if the test fails:
'1000 IF X <> 1 THEN GOTO 1050 where the lines between 1000 and 1050 do a lot
'of stuff that you don't want done unless X=1. In QB2, you would say:
'  IF X=1 THEN_
'    A=B:_            'Usage of GOTO can make code very difficult to follow.
'    C=D:_            'QB2's formatting capabilities make it a lot easier.
'    E=F:_
'    (etc)                                                   (Press PgDn)
'page 6                        Code Readability

'In QB2, you can still enter code just like you did in the interpreter:

'  INPUT "Enter 1-10"; X:FOR I=1 TO 10:IF I=X THEN PRINT I*5 ELSE PRINT I*2
'  IF I=7 THEN PRINT X;:PRINT "I EQUALS 7"
'  NEXT:END

'which is difficult to read, debug and modify. Or you can enter:

example2:
INPUT "Enter 1-10"; X       ' Notice the use of "_" to continue a logical line
FOR I=1 TO 10               ' onto the next line. A common mistake is to leave
  IF I=X_                   ' off a "_" or to put one where you didn't really
    THEN PRINT "I*5" I*5_   ' want it. Run this example to see that it works.
    ELSE PRINT "I*2" I*2    ' Remove one of the "_" continuation lines and run
  IF I=7 THEN_              ' it, but before you do, notice what page number
    PRINT X;:_              ' this is so that you can find your way back here.
    PRINT "I EQUALS 7"      ' Notice that you must also have a ":" before the
NEXT                        ' "_" to separate statements on the same logical
END                         ' line. Please study these two examples carefully.
'      (PgDn)
'page 7                 (Code Readability, continued)

'There are no hard and fast rules for making code more readable. I prefer to
'spread code out as much as possible; but there is also something to be said
'for combining small pieces of code onto one line so that you can see more of
'the code at once. For example, putting "X=1:Y=2:Z=3" onto one line does not
'impair readability that much and lets you see a couple of more lines on one
'screen.

'PROGRAM STRUCTURE:
'A program should be laid out much like a book. It should start with the title
'and author's name and copyright notice. There should be a version number and
'a few lines (Introduction) explaining the purpose of the program and listing
'any associated files. Then you should list the label of each major part of
'the program with a short description, similar to a Table of Contents.

'The first lines of actual code should be DIM and DEF statements, followed by
'GOSUBs to different routines such as to intialize program variables, open
'files, draw the screen, etc. Then go into your main program loop, which calls
'the subroutines. Load QPRINT to see an example of QB2 program structure.

'      (PgDn)
'page 8                     Getting Rid of GOTO's

'One of the major changes in going from interpretive BASIC to QB2 is getting
'rid of the GOTO's. Here is a typically snarled piece of code in BASIC:
example3:
1000 INPUT "ENTER A SINGLE-DIGIT NUMBER"; X
1010 IF X=1 THEN GOTO 1050
1020 FOR I=1 TO 10: IF X=I THEN PRINT 1 X
1030 NEXT
1040 GOTO 1060
1050 PRINT 1 "X=1"
1060 ' lines 1010-1050 in QB2 format:
 IF X=1_
   THEN PRINT 2 "X=1"_
   ELSE FOR I=1 TO 10:_
      IF X=I THEN PRINT 2 X:_  'and here is where things break down.
    NEXT  'Although NEXT is on a seperate line, the "_" in the line above
          'makes these two lines equivalent to
'1020 FOR I=1 TO 10: IF X=I THEN PRINT X: NEXT  'Most people will recognize
'in this example that the NEXT will not be executed except when X=I, but the
'QB structure may actually make this more difficult to recognize because it
'is easy to overlook the "_" at the end of the previous line.      (PgDn)
'page 9               (Getting Rid of GOTO's, continued)

'So how do we get around this? One way would be to branch around the rest of
IF X=1 THEN_             ' the code if x=1, just as we did in the original
  PRINT 3 "X=1":_        ' code with line numbers. (But we are getting back to
  GOTO Branch.around     ' the old "spagetti code" again when we do that.)
FOR I=1 TO 10            ' Note that the lines on the left no longer have a _
  IF X=I THEN PRINT 3 X  ' at the end of each one. That means that the NEXT no
NEXT                     ' longer is on the same logical line as the IF X...
Branch.around:           ' Run "example3" (which starts on the prior page).
END                      ' Enter "1" when prompted and you will see that all
             ' three sections of code work.

'Run it again and enter "3" at the prompt and notice that the second section
'of code does not work.

'The reason that the code above works when the code on the previous screen did
'not is because the IF line in the code above does not end with a "_". These
'marks are very easy to overlook and can cause some strange error messages.


'      (PgDn)
'page 10              (Getting Rid of GOTO's, continued)

'To avoid having to branch around, you can use a WHILE-WEND test instead of a
'IF-THEN test. Here is the same code with a WHILE-WEND test:
example4:   INPUT "ENTER A SINGLE-DIGIT NUMBER"; X
IF X=1_
  THEN PRINT 4 "X=1"_         'Press Ctrl-PgUp, change the first line to
  ELSE FOR I=1 TO 10:_        '"goto example4", and press Ctrl-R.
     WHILE I=X:_          'Try entering 1 and 2.
       PRINT 4 X:_
       I=I+1:_        ' <- This line is added to keep the WHILE-WEND
     WEND:_               'loop from being endless. Incrementing "I" to
       NEXT                   'break out is a convenient method in this case.
END                           'You usually have to come up with something
'different each time. Sometimes, you have to create a new variable just for
'breaking out of a loop if you don't want to change the other variables:
'  LOOP=1:_
'  WHILE (I=X) AND (LOOP=1):_
'    PRINT X:_
'    LOOP=0:_                 'This example would have worked just as well
'  WEND:_                     'in the code above as incrementing "I" did.
'                (PgDn)
'page 11              (Getting Rid of GOTO's, continued)

'Yet another way to take conditional action in the middle of a long logical
'line is to GOSUB to a routine that will make the test and take the action if
'necessary. Using our example:
example5:  INPUT "ENTER A SINGLE-DIGIT NUMBER"; X
IF X=1_                    ' Run this example to see that it works.
  THEN PRINT 5 "X=1":_     ' While this makes the main routine cleaner, it
  ELSE FOR I=1 TO 10:_     ' also can make following the code more difficult.
     GOSUB Test.I:_    ' Having a lot of little subroutines can muck up
       NEXT                ' your code something terrible. You must use your
END                        ' own judgement as to whether you should kludge up
Test.I:                    ' your code with WHILE-WEND loops and extra varia-
IF I=X THEN PRINT 5 I      ' bles or branch to another subroutine as in this
RETURN                     ' example.

'There is one more way around the problem. Most of the code you write with QB2
'can also be compiled without change by the IBM BASIC Compiler Version 2. One
'exception is the IF-THEN-ELSEIF-THEN-ELSE structure. As a result, you may
'prefer to not use this structure in your programs. If you are not worried
'about BASCOM2 compatibility, here is how this structure is used:
'                                                                  (PgDn)
'page 12              (Getting Rid of GOTO's, continued)

example6:  INPUT "ENTER A SINGLE-DIGIT NUMBER"; X
IF X=1 THEN
       PRINT "X=1"
  ELSE FOR I=1 TO 10            ' Note the lack of "_" line continuation
     IF I=X THEN PRINT I    ' symbols in this code. That's because the
       NEXT                     ' END IF defines the end of the IF-THEN
END IF                          ' processing, so all the code does not have
END                             ' to be in the same logical line.

'In other examples, we have varied the form of the IF-THEN command to suit
'our own taste. To use an IF-END_IF block, you must use an exact structure:

'  IF xxx THEN            'The THEN must be the last word on the line.
'        <action>
'    ELSEIF yyy THEN      'ELSEIF must be the first word on the line, but
'        <action>         'ELSEIF is optional, as seen in the example above.
'    ELSE <action>
'  END IF

'                            end of lesson 10
'If you were running this program and ended up here at the end of the code
'because of some error, just press Alt-V, E, Enter to get rid of the error
'message box. If you think the program is badly messed up, enter Alt-F, L,
'Enter and reload the program name shown. Otherwise, just press Ctrl-F, enter
'the page number you were on and use Ctrl-Down to realign the page on the
'screen.