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QBasic

In this course we will be using QBasic 4.5. 

Q - Quick  B - Beginners  A - All-purpose  S - Symbolic  I - Instruction  C - Code

Operating Environment

Menu bar.  Activate by pressing the Alt key and the first letter of the name of the menu you want to open.  A pull-down menu appears.  Use the mouse to select a command.  Deactivate using the Esc key.  
View Window.  This is the largest window.  In the view window you can enter, modify and and display programs.  The title line displays the name of the current program.  
Immediate Window.  The narrow below the view window is the immediate window.  The immediate window is used to execute statements as they are entered.   Statements entered in this window are not a part of the current program.  
Status Line.  This line contains the most frequently used functions and the row and column numbers of the location of the cursor on the screen.  
Dialog boxes.  Use the tab key or the mouse to select an option.  Use the Enter key to select the highlighted button.  Use the Esc key to close the dialog box and return to the view window.  
Cursor movement.  Use the arrow keys to move around in the view window.  The Page Up and Page down keys move you through the program one screen at a time.  Use Ctrl + Home to move to the first line of the program and Ctrl + End to move to the last line of the program.  
Editing programs.  To select text use the following steps: (i) Use the arrow keys or the mouse to position the cursor.  (ii) Hold down the Shift key and use the arrow keys to select words (left or right arrow) or lines (up or down arrows).  Use the Editing menu to cut and paste.  
Executing programs.  To run the program press the F5 key.

Fundamentals

Each line may contain up to 255 characters.  Each line may have more than one command provided that they are separated by colons.

Keywords.  Keywords are words that have a special meaning in QBasic and indicate an action to be preformed.

BEEP, CALL, CHAIN, CHDIR, CIRCLE, CLEAR, CLOSE, CLS, COLOR, COM, COMMON, CONST, DATA, DATE, DECLARE, DEF, DIM, DO, DO UNTIL, DO WHILE, DRAW, END, ERASE, ERROR, EXIT, FIELD, FILES, FOR, FUNCTION, GET, GOTO, IF, ELSE, END IF, INPUT, KEY, KILL, LET, LINE, LOCATE, LOCK, LOOP, LPRINT, LSET, MID, MKDIR, NAME, NEXT, ON COM, ON, ON KEY, ON PEN, ON PEN GOSUB, ON PLAY GOSUB, ON STRIG GOSUB, ON TIMER GOSUB, ON EVENT GOSUB, OPEN, OPTION BASE, OUT, PAINT, PALETTE, PCOPY, PEN, PLAY, POKE, PRESET, PRINT, PSET, PUT, RANDOMIZE, READ, REDIM, RESET, RESTORE, RESUME, RETURN, RMDIR, RESET, RUN, SCREEN, SEEK, SELECT CASE, CASE, END CASE, SHARED, SHELL, SLEEP, SOUND, STATIC, STOP, STRIG, SUB, SWAP, SYSTEM, TIME, TROFF, TRON, TYPE, UEVENT, UNLOCK, VIEW, WEND, WHILE, WIDTH, WINDOW, WRITE

Variables.  A location in storage whose value can change as the program is executed.  Keywords may not be used as variable names.  QBasic is not case sensitive.  COUNT, Count and count are all the same variable.  Variables can be numeric or string.  Variables can be declared to be of a specific type by appending with a special character ($ - String
% - Short Integer  &  - Long Integer !   - Single Precision  #   - Double Precision).  Some programmers like to use periods to separate words.  For example Employee.Name$, Employee.Address$ and Employee.Salary.  
Constants.  A location in storage whose value does not change as the program is executed.  The numeric constants in QBasic can be integer, fixed point or in exponential form (174, -35.1, 62E-23).  String constants are enclosed in double quotes.  
Assignment statements.  The equals sign in a line of computer code has a different meaning than the equals sign in mathematics.  In computer science the equals sign means that the information on the right is being assigned to a storage location on the left.
Arithmetic operators  + addition  - subtraction * multiplication  / division ^ exponentiation
Comments.  Documentation makes programs easier to debug and maintain.  Internal comments may be included using  the following syntax (i) REM comments or (ii) ' comments.  These comments are ignored by the interpreter/compiler.

Compilers/Interpreters

Computers cannot directly execute programs written in a high level language such as QBasic.  Computers must first translate the QBasic statements into machine language instructions that are understood by the computer.  Compilers and interpreters are two types of software programs that can perform this translation.  Compilers translate the entire program into machine language and create an executable file.  Interpreters translate each statement into equivalent machine language instructions.  The machine level instructions are executed and the results are obtained without producing an intermediate machine language program.

Input/Output

The INPUT statement is used for entering small amounts of data or to process data as it occurs.  The READ,  DATA and RESTORE statements are for processing large amounts of data especially when the data is internal to the operation of the program.  DATA statements can only appear in the main program, they cannot be placed in subprograms.  In QBasic DATA statements are non-executable and are frequently placed at the end of the program.  The READ statement must be place before any assignment statements requiring the use of that data.  The PRINT statement is used to print information to the screen.  Commas used in PRINT statements have the effect of moving you from one print zone to the next and are convenient for placing output in a tabular format.  Semicolons used in PRINT statements allow you to display information in a packed or compressed format.  The PRINT USING statement may be used to format the output. The LOCATE statement allows you to control the position of the cursor on the screen.  The CLS statement can be used to clear the screen.  If this statement is not used, the screen will show the results of previous program runs. 

INPUT INPUT "Enter an ordered pair x,y" , x, y
READ
DATA
RESTORE		 READ x1, x2, x3
sum = x1 + x2 + x3
RESTORE 'Reset pointer to first data item
READ x1, x2, x3
product = x1*x2*x3
PRINT x1, x2, x3, sum
PRINT x1, x2, x3 product
DATA 1, 3, 9
PRINT PRINT "Distance = ", rate*time
PRINT "Income "; TAB(10); "Tax rate"; SPC(5); "Tax"
PRINT USING PRINT USING "$$######.##"; income
LOCATE  LOCATE 10, 20: INPUT "Enter x", x

Decisions

In structured programming all program logic is a combination of the following basic three control structures.  In this section we add the decision making commands.

  1. Sequence - actions performed in the order they appear in the program.
  2. Repetition - repeated execution of a particular block of code.
  3. Decisions - choice between several blocks of code.

IF

SELECT CASE
IF tax >= 0 THEN
     switch$ = "y"
     PRINT gross.pay
ELSE
     switch$ = "n"
END IF 		SELECT CASE gpa
    CASE IS >= 90
         Letter.Grade$ = "A"
   CASE IS >= 80
         Letter.Grade$ = "B"
  CASE IS >= 70
         Letter.Grade$ = "C"
       CASE IS >= 60
         Letter.Grade$ = "D"
   CASE IS >= 0, IS < 60
         Letter.Grade$ = "F"   
   CASE ELSE
         PRINT "Negative letter grade."
END SELECT

Loops

Loops are used for repetitive calculations such as counters and accumulators and for inputting or outputting large amounts of data.  A do loop begins with a DO statement and ends with a LOOP statement.  There are four types of do loops available.  

DO LOOPS FOR loops
  Execute while a
condition is true		 Execute until a
condition is true		  
Terminate
at the top		 i=1
DO WHILE i < 5
      PRINT i
      i = i + 1
LOOP 		i=1
DO UNTIL i > 5
      PRINT i
      i = i + 1
LOOP
sum = 0
FOR count = 1 TO 100 STEP 1
     sum = sum + count
NEXT count
PRINT "The sum is "; sum
Terminate at
the bottom		 i=1
DO 
      PRINT i
      i = i + 1
LOOP WHILE i < 5 		i=1
DO 
      PRINT i
      i = i + 1
LOOP UNTIL i > 5 		FOR i = 1 TO 4
     PRINT "Outer loop ", i
     FOR j = 1 TO 3
          PRINT "Inner loop ", i,j
    NEXT j
    PRINT
NEXT i

Functions

The table below contains a partial list of the functions built into QBasic.

ABS, COS, DATE$, EOF, EXP, INT, LCASE$, LEFT$, LEN$, LOG, POINT, POS, RND, SGN,  SIN, SPC, SQR, TAB, TAN, TIMER, TIME$, UCASE$

Arrays

An array is a string or numeric storage variable that allocated a specific number of storage locations.  In other words a programmer can store more than one value under the same variable name.  This is much more convenient when the data is related.  Arrays are commonly used for sorting and other table processing applications.  Some examples of array declarations and manipulation are shown below.

1 
dimensional
arrays

 DIM Month(1 TO 12)
FOR i = 1 TO 12
     READ Month(i)
NEXT i
FOR i = 1 TO 12
     PRINT Month(i)
NEXT i
DATA 12, 23, 34, 45, 43, 34
DATA 34, 56, 76, 87, 34, 56

2 
dimensional
arrays

 DIM Table(1 TO 5, 1 TO 5)
FOR i = 1 TO 5
     FOR j = 1 TO 5
          Table(i,j) = 0
    NEXT j
NEXT i

Subprograms 

Subroutines are a convenient method for dividing a large program into smaller sections that each carry out a specific task.  If you want the routines to be more independent use subprograms instead.  If the specific task requires several numbers and produces only one number, a function may be more appropriate.

Subroutines Functions Subprograms
GOSUB Accept.Data
GOSUB Add.Data
GOSUB Display
END

Accept.Data:
     INPUT "Number 1", x1
     INPUT "Number 2", x2
RETURN

Add.Data:
     sum = x1 + x2
RETURN

Display:
     PRINT "Sum is = ", sum
RETURN

 p = 500
r = 0.08/12
c = 2
PRINT "FV is"; FV(p,r,c)
END

FUNCTION FV(pp, rr, nn)
     FV = pp*(1 + rr/cc)^cc
END FUNCTION

 

 CALL Ps(4.5, 5*6, "Fun")
END

SUB Ps(x, y, s$)
     PRINT x, y, s$
END SUB

 

Files 

When using large amount of data, it is preferable to have the data stored in files to reduce the amount of data entry needed.  QBasic provides two types of file organization - sequential and random.  A sequential file is limited to sequential processing.  The records can be processed only in the order they are placed in the file.  In a random the processing sequence is not related to the sequence in which the records are stored.  If the tenth record is needed, it can be accessed directly without processing the previous nine.  

Sequential file processing
Read from file OPEN "INVENT.DAT" FOR INPUT AS #1
DO WHILE NOT EOF(1)
     INPUT #1, Stock$, Location$, Cost, Price, Quantity
LOOP
Write to a file OPEN "INVENT.DAT" FOR OUTPUT AS #1
     WRITE #1, Stock$, Location$, Cost, Price, Quantity
CLOSE #1

Graphics

The SCREEN command can be used to select a graphics mode.

SCREEN 0  Text Mode
(25 rows and 80 columns)
SCREEN 1 Medium Resolution Graphics
(200 rows and 320 columns)
SCREEN 2 High Resolution Graphics
(200 rows and 640 columns) 

Text-Mode Graphics

You can use the usual letters and numbers on the keyboard or you can extend the character set using the CHR$(code) command where code is an integer between 128 and 255.  The string command STRING$(n, code) is useful for repeating characters.  The COLOR foreground, background command may be used to set foreground and background colors for the screen (0-black, 1-blue, 2-green, 3-cyan, 4-red, 5-magenta, 6-brown, 7-white, 8-grey).  As an example consider the following program which draws a border.

Program for displaying a border in text-mode.
SCREEN 0
COLOR 7, 4
LOCATE 8,9: PRINT CHR$(201); STRING$(20,205); CHR$(187)
FOR I = 9 TO 17
LOCATE I, 9:   PRINT CHR$(186)
LOCATE I, 30: PRINT CHR$(186)
NEXT I
LOCATE 18, 9: PRINT CHR$(200); STRING$(20,205); CHR$(188)
END

Medium and High Resolution Graphics Modes

COLOR background, palette

Choose a background color using the numbering scheme presented in text graphics mode above.  The choices of palettes for the foreground are shown in the table below.  Select palette number 0 or 1.  Graphics statements following the color statement may then select a color from the chosen palette.

Palette 0 Palette 1 Number
Background color Background color 0
Green Cyan 1
Red Magenta 2
Brown White 3

PSET, PRESET

The point set and point reset statements may be used to set a point on the screen to one of the four colors from the active palette.  

SCREEN 1
COLOR 5, 0
FOR J = 1 TO 100
PSET (50, J)
NEXT J
END

LINE

LINE (x1,y1) - (x2, y2) , color, box Draws a line connecting the two points.  Color selects a color from the active palette.  Box is B for box or BF for a box filled with color.  
LINE -(x,y)  Draws a line from the last point referenced.

CIRCLE

CIRCLE (x,y), radius, color, start, end, shape (x,y) is the radius.  color selects the color from the active palette.  start and end are the angles as measured in radians (negative values cause a wedge to be drawn).  Shape is the ratio of the radius in the y-direction to the radius in the x-direction and can be used to draw ellipses. 

PAINT 

PAINT (x,y), paint, boundary (x,y) are the coordinates of a point within the region to be filled with color.  Paint is a numeric or string expression.  The numeric expression determines the color.  If a string expression is used, it determines the tiling pattern. Pi=3.1415926
SCREEN 1: CLS
COLOR 1, 0
CIRCLE (40,40), 20, 1
PAINT (40, 40), 3, 1
END

DRAW

The DRAW command instructs the computer to draw an object an object defined by a string expression.  The string expression contains codes for drawing lines, points, setting colors, and performing other operations all within a single command.  

' Draw a sailboat
SCREEN 1: CLS
COLOR 9, 1
DRAW "C2 L10 F10 R20 E10 
              L30 BU1 C3 U40 F30 L30"
PAINT (160, 105), 2, 2
PAINT (165, 85), 3, 2
END		 (1) C2 selects the color magenta for the boat (2) L10 moves and draws to the left 10 (3) F10 move and draw diagonally down and right 10 (4) R20 move and draw to the right 20 (5) E10 move and draw diagonally up and right 10 (6) L30 move and draw to the left 30 (7) BU1 move up one row (8) C3 select the color white for the sail (9) U40 move and draw up 40 (10) F30 move and draw diagonally down and right 30 (11) L30 move and draw to the left 30.

WINDOW

The WINDOW statement redefines the coordinates of the screen.  In medium and high resolution the upper left hand corner is the origin (0,0) and the coordinate extends down and to the right.   After using the window statement the coordinate system is the same as the one used in mathematics.  When using the command WINDOW (x1,y1) - (x2,y2), the first point is the upper left hand corner and the second point is the lower right hand corner. 

'Plot y=x^2
SCREEN 1: CLS
COLOR 9, 7
WINDOW (-10,10)-(10,-10)
FOR I = -10 TO 10 STEP 0.1
     PSET (I,I*I)
NEXT I
END

GET and PUT

The GET and PUT commands are used for creating animations.  Use the GET statement to store a figure and use PUT statement to move the figure from one location to another.

Activities

  1. Write programs to compute area of rectangles, triangles and circles.

  2. Write a program that inputs a name, converts to uppercase letters and then outputs.  This program should concatenate the first and last names.

  3. Write a program that either computes the future value of an annuity or compute payment size needed to amortize a loan.

  4. Statistics functions such as mean, standard deviation, skewness and kurtosis.

  5. Discrete logistic equation (time series and final state diagram).

  6. Julia sets and other fractals.

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