QBasic Programming
Lecture [6]
QBasic stands for Beginner‘s All-Purpose Symbolic Instruction. It is a programming language
written for computers back in 1975, by Bill Gates & Paul Allen. It is ease of use , its English-like
commands and its power.
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Name of current program
This is the Program Area
This is the Immediate Mode Area
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Lecturer: Dr. Laith Abdullah Mohammed
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•The Character set:
1.
2.
3.
The Letters: its from A to Z (small or capital).
The Digits: its from 0 to 9
Special symbols: like (+,-,^,?,!,#, etc.)
•The Constants:
1.
2.
Numeric constants: include all numbers (real, not real, integer, …..).
Example: 25, -230, 0, 16.44, 0.88
Character constants: include all characters sets (letters, digits, symbols) between two “ “. Example: “ BASIC
“ , “ The width is = 83 “, “ Telephone Number 07901 “.
•The Variables:
A variable is a name which can contain a value. The variables must include the conditions below:
1. From A to Z (include all letters).
2. Not contained symbols except dot (.).
3. Maximum length of variable is 40.
4. Must not represent any word which is defined as a special word in QBASIC.
5. Must be start by letters.
A. Numeric Variables:
Like: M, A2, WE,…….etc
In Numeric variables the symbol (%) (Integer) mean make the numeric variable as real number.
Like: A%=6.2
its mean A%=6
A%=6.6
its mean A%=7
A%=6.5
its mean A%=7
A%=-6.5
its mean A%=-6
Using symbol (&) (Long) with numeric variable make it long variable.
Using symbol (!) (Single) with numeric variable mean the length of variable equal to 7 digits or less.
Using symbol (#) (Double) with numeric variable mean the length of variable more than 7 digits.
B. Character (String) Variables: If the variable holds symbols or text, it may be a character variable or a string
variable. like: M$, A2$, WE$,…….etc
Numeric variables:
Single-precision variables (floating-point variables): these types of variables are used
to store numbers that contain a decimal value such as 1.89 or 3.141593.
INTEGER: A non-floating point variable (no decimal value) that can store integers
between -32,768 and 32,767
LONG: same as INTEGER, but can contain numbers between -2,147,483,648 and
2,147,483,647
DOUBLE: same as SINGLE, but can have twice as many digits. (like: 983288.18)
SINGLE: single precision variables. (like: 39.2932)
To define a variable‘s type, use DIM with the AS attribute.
DIM var1 AS INTEGER
DIM var2 AS LONG
DIM var3 AS DOUBLE
var1=15.28
var2=-2000000000
var3=12345678.12345678
PRINT var1
PRINT var2
PRINT var3
Output:
15
-2000000000
12345678.12345678
Example:
B=26
A$=“ I Like to Learn QBASIC ”
PRINT B
PRINT A$
Output:
26
I Like to Learn QBASIC
Strings variables:
String variables are ones that can hold all ASCII characters (Letters, Numbers, Symbols). They
can not be used in math problems. When asking questions about
them, or changing their content, the expressions must be in quotes.
Example:
String variables are letters and numbers followed by a dollar sign ($).
X$= "Hello World ! "
String names must have a letter as the first character,
D$= "Hi “+X$
but everything else is up to you.
PRINT X$
Examples:
PRINT D$
" 0123456789 "
" abc123 "
" 1+1=2 "
Output:
"!@&%$§?><°^ "
Hello World !
" Hi "
Hi Hello World !
Expressions:
Operator
Function
Example
Result
+
Add
8+2
10
*
Multiply
8*2
16
/
Divide
8/2
4
-
Subtract
8-2
6
^
Exponentiation
8^2
64
Priority of operations:
1. The parenthesis (from left to right & from inside to outside).
2. The exponentiation (from left to right).
3. Division & Multiply (from left to right).
4. Add & Subtract (from left to right).
Example:
Z= 500+(10*7)
rate=50
time=2
distance=rate*time
PRINT Z
PRINT distance
Output:
570
100
Example: Write the following expression using BASIC format. And define the priority of the
calculation for the expression
Solution:
8
X = (A^3
-
1
7*
3
B^2) / (3*B + D^(1/2))
2
5
4
6
7
Example: Based on the priority of the calculation calculate the following:
2+3/4*5
2+3^2
3*4/5
3+4-5
⇒
⇒
⇒
⇒
= 5.75
2+9 = 11
12/5 = 2.4
7-5 = 2
Comparison Operators:
Operator
Meaning
Example
=
Equal to
IF a=15 THEN...
<>
Not equal to
IF a<>15 THEN...
<
Less than
IF a<15 THEN...
<=
Less or equal to
IF a<=15 THEN...
>
More than
IF a>15 THEN...
>=
More or equal to
IF a>=15 THEN...
Logical Operators:
1. OR operator: the result of this operator is true if one of the statements is true.
A
B
A
OR
B
Example
True
True
True
IF 10<14 OR 3^2=9 THEN...
True
False
True
IF 10<14 OR 3^2=5 THEN...
False
True
True
IF 10<2 OR 3^2=9 THEN...
False
False
False
IF 10<2 OR 3^2=5 THEN...
2. AND operator: the result of this operator is true when both statements
are true.
A
B
A
Example
AND B
True
True
True
IF 10<14 AND 3^2=9 THEN...
True
False
False
IF 10<14 AND 3^2=5 THEN...
False
True
False
IF 10<2
AND 3^2=9 THEN...
False
False
False
IF 10<2
AND 3^2=5 THEN...
3. NOT operator:
Example:
NOT 15 > 10
NOT 15 = 8
NOT (5 >= 5 AND 6/2= 4)
False
True
True
Note: ♦ The priority in Logical operations are:
1. The parenthesis ( ).
2. NOT operator.
3. AND operator.
4. OR operator.
♦The priority between Mathematical Expression and Logical operations are:
1. Mathematical Expressions.
2. Logical Operations.
Example: Decide which statement is True and which is False.
According to Priority Law,
A=3 , B=10 , C=100
C=100
True
C<100
False
2
1
A=3 OR C>100 AND B<10
1
True
2
(A=3 OR C>100) AND B<10
1
2
False
3
B<=10 AND C=10 AND A*B=6 OR A<>13
2
3
True
1
B<=10 AND C=10 AND (A*B=6 OR A<>13)
False
Adding Documentation to the program:
Documenting your program allows you to remind yourself about something in your program.
Plus, if your program is seen by other pepole, documenting can help them understand your
code. The REM (remark) command enables you to add comments to your program without
the text being treated like an instruction.
General Form:
REM or ‘ followed by comments.
Example:
CLS
‘ This command clears the screen
REM Program to compute area
LET command:
General Form:
LET variable=expression [Note: the word LET is optional].
Purpose:
Algebraic or string depending upon the variable – uses + - ^ \ MOD.
Example:
LET X= A+B*C
LET J=J+1
LET C=SQRT (A*A+B^2)
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