PsL Monthly 1993 December/PSL Monthly Shareware CD-ROM (December 1993).iso/prgmming/dos/basic/tutorqb.com/TUTOR-02.BAS

BASIC Source File | 1986-10-07 | 14.3 KB | 314 lines

goto example1
'                       The QuickBASIC Tutorial Series

'                                   Part 2
'                         Introduction to Programming

'                        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 a 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 part of the QB2 tutorial, we are going to assume that you have never
'programmed before. The QuickBASIC manual is primarily a reference book, not
'a book to learn by. The value of this tutorial is that it is interactive
'You will be able to try things out as you go, which greatly speeds up the
'learning process. However, for maximum benefit, as points are covered here,
'you should also read about them in the QB2 manaul.

'Here are some basics you will need to start with:
' - When entering code, you can put each statement on a separate line, or you
'can combine several on the same line with a colon (:) in between statements.
' - The compile ignores anything following a single quote mark (').
' - In your programs, you will constantly be testing the relationship between
'things. Here are the relational symbols. "AND" and "OR" can be used to make
'multiple tests.          =        equal to
'                         <        less than
'                         >        greater than
'                        <>        not equal to
'                    =<  or  <=    less than or equal to
'                    =>  or  >=    greater than or equal to
'(Press PgDn)
'page 3
'                                 Contents
'                 page
'                  4      What's a Program
'                  4      What's a Variable
'                  5      Integer Variables
'                  6      Single Precision Variables
'                  6      Double Precision Variables
'                  6      String Variables
'                  7      Declaring Variable Types
'                  7      Review
'                  8      Examples of Variable Types
'                  9      Arrays
'                  9      Loops
'                 10      FOR-NEXT Loops
'                 11      WHILE-WEND Loops
'                 12      String Handling Commands
'                 13      String Operators
'                 14      Other String Commands
'
'                To search for specific text, use Ctrl-F.
'(Press PgDn)
'page 4                 What's a Program, a Variable?

'WHAT'S A PROGRAM?
'A program is a set of instructions to the computer. The compiler converts
'your English-like instructions to machine language the computer can under-
'stand. With any computer language, such as QB2, you must use the "vocabulary"
'and "sentence structure" of the language exactly as proscribed.

'In movies and TV shows, the hero sits at the computer and types something
'like "PRINT NAMES OF ALL PEOPLE WHO EAT QUICHE" and the computer scrolls a
'list across the screen, usually a letter at a time with an electronic beep
'as each letter is printed. Ah well, that's showbiz.

'In the real world of QB2, if you enter something like the line above, the
'computer will quickly and quietly display "0 0 0 0 0 0 0", because it thinks
'each of those words ("NAMES" through "QUICHE") is a numeric variable with a
'value of 0 which you have asked it to print.

'WHAT'S A VARIABLE?
'A variable is used to store data while a program is running. A variable can
'be almost any combination of letters and numbers, but if you want to be able
'   (Press PgDn)
'page 5                         Variable Types

'to remember when you look back at your program later what a variable is for,
'it's a good idea to use a variable name that will remind you.

Example1:
INPUT "What were total sales"; SALES
INPUT "What is the sales tax rate"; TAX.RATE
PRINT "Sales tax on sales of" SALES "is" SALES * TAX.RATE
END'                                       variables 

'To run this example, just press Ctrl-R. The program should compile and then
'when you press Enter, it should run. If it does not, press Alt-F, L, Enter,
'and reload the program from the disk. If it still does not, go back to DOS
'and recopy the program from the distribution disk onto your work disk again.

'INTEGER VARIABLES...
'   can hold numbers from -32,768 to +32,767 with no decimal places. Integers
'take up the least amount of space in memory, which makes them faster to
'manipulate as well. The sign for an integer is "%".  Eg:  X%=3

'(PgDn)
'page 6                   (Variable Types, Continued)

'SINGLE PRECISION VARIABLES...
'   are precise to 7 digits total, on both sides of the decimal place. For
'example, 1234567 and 1234.567 are both single precision numbers.  Beyond
'seven digits you start getting rounding errors. Single precision numbers take
'twice as much space as integers, but only half as much as double precision
'numbers. You always want to use the smallest varible type that will hold the
'number accurately. A single precision variable is denoted with a "!":  X!=6.2

'DOUBLE PRECISION VARIABLES...
'   are accurate to 16 digits, which is the most accuracy you can squeeze
'out of the system in BASIC. Like single precision, the number of digits is
'the total on both sides of the decimal place. A double precision variable is
'indicated with a "#".  Eg:  X#=123456.789

'STRING VARIABLES
'A string variable holds text. In the interpreter a string variable is limited
'to 255 characters. In the compiler, a string variable may hold up to 32,767
'characters. The symbol for a string variable is a "$".  Eg:  X$="Hello"

'(PgDn)
'page 7                    Declaring Variable Types

'DECLARING VARIABLE TYPES
'It is a nuisance to have to keep typing the symbol for the variable type
'every time you enter a variable name. The DEFINT, DEFSNG, DEFDBL and DEFSTR
'commands let you establish that any variable name starting with the specified
'letter is a particular type of variable. For example, DEFDBL S means that any
'variable starting with the letter S is a double precision variable. This
'declaration can be overridden by using the symbol. For example, SALES would
'be a double-precision variable and SONG$ would be a string variable.

'REVIEW
' X% is an integer that can hold numbers from -32,768 to +32,767, no decimals.
' X! is single precision and can hold numbers accurately to 7 digits.
' X# is double precision and accurate to 16 digits.
' X$ is a string variable that can hold up to 32,767 characters.

'PROGRAMMING TIP
'At the start of a program, you can declare all variables as integers and use
'the appropriate symbols to override this declaration for other variable types
'only when needed. This insures that you will use the faster, smaller integers
'whenever possible.                   (PgDn)
'page 8                   Examples of Variable Types

Example2:   'Remove the quote from the start of lines 3 and 4 before running.
DEFINT A-Z               'To run this example, start by pressing Ctrl-PgUp to
A=123: PRINT A           'get to the first line of this file. Edit that line
'B=123456: PRINT B       'to say "goto example2" and press Ctrl-R to run.
'C=12345675: PRINT C     'You will get a compile-time error message saying
END                      '"Math overflow" for the preceeding line. Change the
'C's in that line to C# and press Ctrl-R. (You don't have to change the first
'line in the file again.) You still get the same error message. What's going
'on? Well, QB2 does not always point to the right line. The line above is also
'wrong. Change the B's in that line to B! and try again.

'Now let's try some cutting and pasting. Move to the first D in the next line:
'D="123456abc": PRINT D '(Press Alt-V, E, Enter to remove error message box.)
'With the NumLock off, press Shift-End. The line above should be highlighted.
'Press the Del key.  Move the cursor to the "E" in the "END" statement in
'Example2 above and press Enter. Move up a row to the blank line and press
'the Ins key and the text you "cut" out should reappear. Press Ctrl-R to run
'this modified code. You will get a "Data type conflict" for trying to assign
'a text string to a non-string variable. Add the "$" symbols and try again.
'(PgDn)
'page 9                         Arrays, Loops

'ARRAYS...
'  are what makes variables and programs really powerful. Thanks to spread-
'sheet programs, we now have a good analogy for arrays: rows and columns in
'the spreadsheet. A single array is like a single column of data in a spread-
'sheet. A two-element array is like multiple columns and rows.  An example of
'a variable array would be SALES#(12,50) where the 12 would be the number of
'columns in a spreadsheet, perhaps a column for each month, and 50 would be
'the number of rows, perhaps a product number on each row.

'LOOPS...
'   are what give arrays their power (and vice-versa).  Let's say you wanted
'to increase sales for each item in a column by 20%. In a spreadsheet, you
'would have to go down the column multiplying each of the 50 amounts by 120%.
'With a FOR-NEXT loop in BASIC, you could just say
'FOR ROW=1 TO 50
'  SALES#(COLUMN,ROW)=SALES#(COLUMN,ROW) * 1.2    'Some people get confused
'NEXT    ' by the structure "x=x+3" because it is algebraically impossible.
     ' This is not algebra. This is shorthand for a language which says
     '         "set  x  equal to the old value of  x  plus  3"
'(PgDn)
'page 10                        FOR-NEXT Loops

Example3:
FOR I=6 TO 9    ' This example tells the computer to set the value of I to
  PRINT "I="I   ' the value of the first number (6), then perform the action
NEXT            ' if I is less than or equal to the second number (9).  I is
'incremented by 1 when NEXT is reached and tested again to see if it is still
'less than or equal to 9.

FOR J=1 TO 8 STEP 3   ' You can also make the counter increment more than 1
  PRINT "J step" J    ' by using the STEP command. In this example, J will
NEXT                  ' increment by 3.

FOR K=5 TO 3 STEP -1  ' You can also make the counter decrease instead of
  PRINT "K loop" K    ' counting up by using a negative STEP amount. Here,
NEXT                  ' K will start at 5 and decrement by 1.
END

'To run these examples, press Ctrl-PgUp to get to the title screen, change the
'first line to "goto example3" and press Ctrl-R. Then return to this page.

' (PgDn)
'page 11                       WHILE-WEND Loops

'WHILE-WEND LOOPS...
'  repeat an action as long as the testing statment is true. In simplest form,
'a WHILE-WEND loop can be directly substituted for a FOR-NEXT loop:

example4:
I=6              ' Press PgUp to compare this loop to the first loop on the
WHILE I <= 9     ' previous screen. It has exactly the same result, although
  PRINT "I=" I   ' this method is quite a bit more cumbersome. For one thing,
  I=I+1          ' a FOR-NEXT loop increments the counter automatically. In
WEND             ' the WHILE-WEND loop, if you want a counter, you must set
END              ' it up and increment it yourself.

'WHILE-WEND loops are usually more efficient then FOR-NEXT loops when dealing
'with string variables, but we need to talk more about string variables first.
'In a textbook, we might have to go through a lot to illustrate the string-
'handling capabilities of BASIC. In this tutorial, however, all you have to do
'is press Ctrl-PgUp, change the line to "goto example5" and press Ctrl-R.
'(Example 5 is on the next page.)

'(PgDn)
'page 12                   String-Handling Commands

Example5:
X$="QuickBASIC Compiler"
PRINT "                     " X$
PRINT "left 5 characters:   " LEFT$(X$, 5)
PRINT "left 10 characters:  " LEFT$(X$,10)
PRINT "6th thru 10th char.: " MID$(X$,6,5)  'start at char. 6, print 5 char.
PRINT "6th thru last char.: " MID$(X$,6)    'start at char. 6, print the rest
PRINT "right 5 characters:  " RIGHT$(X$,5)
END

'Now here is an example of how a WHILE-WEND loop can be used with a string.
'We are going to look for someone's last name in this next example:
Example6:
WHOLE.NAME$="Nelson Ford":I=1           'Ctrl-PgUp, "goto example6", Ctrl-R
WHILE MID$(WHOLE.NAME$,I,1) <> " "      'to run this example.
  I=I+1
WEND
PRINT "Last name: " MID$(WHOLE.NAME$,I+1)
END
'      (PgDn)
'page 13                       String Operators

'String Operators:
' Symbol        Meaning                Example
'   <    precedes alphabetically      "A" < "B"
'   >    follows alphabetically       "AZ > "AX"
'   =    equals                        X$ = "A"
'  <>    not equal                    "A" <> "a"
'   +    join two strings together     X$ = "A"+"B"

'You can change just part of string by using MID$:
example7:
A$="I think IBM PCs are great."
B$=" Clones"        'Try leaving out the space before the C and run it.
X=INSTR(A$, "IBM")  'Remember the INSTR function?
PRINT A$
MID$(A$,X)=B$       'Replace "IBM PCs" with " Clones"
PRINT A$
END
'Only MID$ can be used in this way, not LEFT$ or RIGHT$.
'See "MID$ Statement" in the manual for other restrictions.
'(PgDn)
'page 14                     Other String Commands

'INSTR and LEN

     example8:
     A$="12345abc345xyz": PRINT "A$=" A$
     B$="345": PRINT "B$=" B$
'INSTR is the character number in the first string where the second string
'  begins. If the second string is not found, INSTR is 0.
     X=INSTR(A$, B$)
     PRINT "Starting @ char. #" X, MID$(A$, X, 6)
'You can specify a point for the search for a match to start:
     Y=INSTR( X+1, A$, B$)
'              -the starting point is the start of the first "345" + one
     PRINT "Starting @ char. #" Y, MID$(A$, Y)

'LEN returns the length of a string:
     PRINT "A$ is" LEN(A$) "char. long"
     END


'                               end of lesson 2



'end of file.  Press PgUp.