1. Chapter 2 Modular Programs with Calculations and Strings
QBASIC
Chapter 2
Modular Programs with Calculations and Strings

2. QBASIC Character Set Letters: a-z and A-Z Digits: 0-9
Blank: the space character ( )
Special characters: + - * / \ = < > . , ’ ” ( ) : ; ^ _ $ # ? ! % &

3. Qbasic Keywords A keyword has a predefined meaning within Qbasic.
Examples: LET END REM PRINT

4. Constants & Variables Constants Variables
“literal” values that cannot be changed
May or may not have a label
Variables
Value can be changed
Must be referenced by a label

5. QBASIC Constants Numeric Constants
Can only contain these 13 characters:
If not identified with a label they are called a “literal”
Can not have + - internal or trailing
Can only have one decimal

6. Numeric Constants
amount =

7. Labels A name assigned to represent a variable. Should be meaningful.
Must start with a letter
Should be meaningful.
Can have periods imbedded.
Should carry the data type.

8. The LET statement
Assigns a value to a variable. Can be Explicit or Implicit
LET variable.Name = value
LET my.nbr! = 0
LET my.str$ = “This is my string”
LET tot! = tot! + purchases! + taxes!

9. QBASIC Data Types All data in QBASIC is identified by a data type
Numbers
% – Integer -32,768 to 32,767
& – Long integer -2,147,483,648 to 2,147,483,647
! – Single precision 7 digit (1038 ) (Default data type)
# – Double precision 15 digit (10308 )

10. QBASIC Data Types Strings: $ - data type identifier
Any set of characters enclosed in double quotation marks.
“ ”
Maximum of 14,656 Characters

11. Arithmetic Operators * – Multiplication ^ – Exponentiation
/ – Division
\ – Integer Division
MOD – Modula (remainder)
+ – Addition
- – Subtraction

12. Calculations Order of operations (precedence) ^ (Power) +, - (Unary)
*, /, \, MOD, +, - (Arithmetic)
<, <=, >, >=, <>, = (Relational)
Boolean operators
NOT, AND, OR, XOR, EQV, IMP

13. Some math Examples LET A% = 100 LET B% = 4 PRINT A% * B%
PRINT A% + B% - B% * 4 MOD 3
400
PRINT A% + B% - B% * (4 MOD 3)
103
100

14. Functions
A function is a set of instructions that perform a specific task.
FunctionName (argument1, …)
Built-in & User defined
X$ = INT(X)

15. Built-in Functions Conversions System MKI$ - CVI, MKD$ - CVD
CHR$ - ASC
CINT(), CSGN(), CDBL()
System
DATE$ - TIME$

16. The INPUT statement
INPUT variable-list INPUT “prompt ” ; or , variable-list
prompt – any valid string
; – Question mark generated
, – No Question mark generated
variable-list – mix and match separate with commas

17. Using INPUT to prompt users
INPUT “Want a date”; date.in
Want a date?__
INPUT “Enter date”, date.in Enter date __

18. Modular Programming As we have seen previously:
Heirarchy Charts allow us to break large problems into smaller more manageable blocks that perform a specific unit of work
Each of these “blocks” can be represented as a “subroutine”. A fixed combination of instructions that can be re-used.

19. Subroutines
A subroutine is identified with a label just like variables.
Subroutines are “called” from someplace in the program using GOSUB and return to the statement following the call using the RETURN statement.

20. GOSUB label.name GOSUB my.summation PRINT “The sum is “; x% END
my.summation: INPUT “Number Please:”, y% x%=(y-1)+(y-2)+(y-3)… RETURN

21. RETURN [ label.name] Returns to caller by default
Command following the GOSUB gets control
Returns to label if specified
Should only be used under special circumstances.

22. Hierarchy Chart Score Average Program Input Name & Scores
Calculate Average
Output Name & Average

23. Flowchart Start End Return Start Return Return Return End
Input name & scores
Calculate Average
Output Name & Average
End
Input Scores
Write Output
Input Name
Input 3 Scores
Return
Avg1 = (3 Scores)/3
Average = Avg1 Round
Print Name & Average
Start
Input Scores
Calculate Average
Write Output
Input name & scores
Input Name
Avg1 = (3 Scores)/3
Print Name & Average
Input 3 Scores
Return
Calculate Average
Average = Avg1 Round
Return
Output Name & Average
Return
End

24. Code
MAIN: GOSUB Input.Name.Scores GOSUB Calculate.Average GOSUB Write.Output
END.MAIN: END
Input.Name.Scores: INPUT “Enter Name: “ , Name$ INPUT “Enter Score 1: “ , Score1% INPUT “Enter Score 2: “ , Score2% INPUT “Enter Score 3: “ , Score3% RETURN
Calculate.Average: Avg=INT((Score1%+ Score2%+ Score3%)/3)):RETURN
Write.Output: PRINT Name$; ” – “; Avg: RETURN