1. HNDIT23073- Rapid Application Development
Week 4- Procedures and Functions

2. Procedure
A set or block of code statements that is given a name so that it can be invoked by another part of the program
Building blocks of applications
Modular Programming - Break large problems down into smaller ones
Used for Repeated or Shared Tasks - Container for code to be called multiple times
Makes the code easier to understand

3. Procedure (cont.) General Form:
Declaration statement (Procedure Header)
Code statements
End statement (Procedure Close)
Other names used by different programming languages to refer to procedures
Method
Subroutine
Function

4. Procedure Types in Visual Basic
(Event) Sub Procedure
Performs a logical action
Does not return a value
Executes when event occurs - not “Called” from code
(Gen Purpose) Sub Procedure
Must be “Called” from other code
Function
Performs Calculation
Does return a Value

5. Sub Procedure An abbreviation of the older term subroutine
Sub procedures are written to perform specific tasks
General purpose Sub procedures are not triggered by events but called from statements in some other location
Event procedures are special Sub procedures that are not called from statements, but rather are executed when a corresponding event occurs

6. Sub Procedure Syntax Public Sub SubName (parameterName As DataType)
‘*procedure code*’
End Sub
Public (optional) - the AccessSpecifier - establishes accessibility to the program
Sub and End - keywords
SubName - name used to refer to Sub - rules for naming Sub Procedures are the same as for variables, except Sub procedure names begin with uppercase letters.
ParameterList - a list of variables or values being passed to the sub procedure

7. Procedure Call Used to invoke (cause to execute) a procedure
Syntax: Call ProcedureName (Arguments)
Call (optional) - keyword
ProcedureName - name of Sub (or Function) to invoke
Arguments (if any, included in parenthesis) - Data passed to the procedure
Example:
Call Power (5, 2) OR
Power (5, 2)

8. Arguments
Values, Variables or Expressions placed in parentheses in a Call statement
Contain data needed by the procedure
The data is passed to the procedure when it is called
We’ve already done this with the Val functions
intNumber = Val(txtInput.Text)
Calls Val function and passes txtInput.Text

9. Parameters
Declared in procedure header (in parentheses after ProcedureName) (can have 0 or more)
Must be declared (similar to a variable) in the procedure header in order to accept an argument
Created as a local variable for the procedure
Syntax: (parameterName As DataType)
parameterName - name used to refer to parameter in procedure code (same naming rules as variables)
As - keyword
Data Type - type of value the parameter will contain

10. Passing Arguments to Parameters
When the procedure is called, the values of the arguments (in the call statement) are passed into (copied to) the corresponding parameters (in the procedure header).
Data needed by the Sub (or Function) and sent by the Call statement
The number of arguments and parameters must match.
The data type of each argument must match its corresponding parameter.

11. Sub Procedure Example Private Sub btnResult_Click() ‘* event procedure
Dim lng As Single, wid As Single
lng = 5
wid = 10
Call ComputeArea(lng, wid) ‘* call to sub
End Sub
Sub ComputeArea(Length As Single, Width As Single)
Dim Area As Single
Area = Length * Width
MsgBox(“Area = “ & Area)

12. Pass by Value or Pass by Reference
Arguments are usually passed ByVal
A copy of the value of the argument is stored in the parameter l0cation
Any changes in that value made by the called procedure are made only to the parameter location – original argument from calling procedure will not change
Arguments (variables) can also be passed ByRef
The reference (memory location) of the variable used as the argument is stored in the Parameter l0cation
Parameter references (points to) the original variable’s (argument’s) memory location
Any changes made by the called procedure are made to the original variable (argument) from calling procedure

13. ByVal/ByRef Example At declaration intX  5 intY  5 At termination
Sub ChangeValue(ByVal intY As Integer)
intY = 10
End Sub
ChangeValue(dim intX as integer = 5)
Result: intY = 10 intX = 5
_______________________________________________________
Sub ChangeValue(ByRef intY As Integer)
ChangeValue(intX = 5)
Result: intY = intX = 10
At termination
intX  5
intY  10
At declaration
intX  5  intY
At termination
intX  10  intY

14. Sub Procedure Example
Write a Sub Procedure named Power with 2 integer parameters (originally from text boxes). The sub should calculate the first parameter raised to the second parameter and display the results in a message box.
Obtain input
Call Sub

15. Functions Procedures that return a Value
Same form as Sub except must return a value
Must have an associated data type to tell program what type of data it will return
One of the numeric types String
Boolean Char
Object Etc.
Must include a “Return” statement or Assign a value to the name of the function
This statement causes the function to end and return the appropriate value

16. Function Syntax Public (optional) - the AccessSpecifier
Public Function FunctionName (parameterName As DataType)
As returnType
‘* function code
Return Value ‘* OR FunctionName = Value
End Function
Public (optional) - the AccessSpecifier
Function and End – keywords
FunctionName - name used to refer to function - Function procedure names begin with uppercase letters.
ParameterList - list of values being passed to the function
returnType - DataType of the value returned by the function
Return Value – required statement to return data to caller

17. Using Functions Functions ALWAYS return a value
When calling a function, you MUST do something with the value that is returned
Right hand side of an assignment statement
Part of an Output Statement
Must be suitable to the particular return type
A Function can be used in any statement where a literal or variable of the particular return type could be used

18. Function Example
Private Sub btnResult_Click() ‘* event procedure Dim lng As Single, wid As Single, area As Single lng = 5 wid = 10 area = ComputeArea(lng, wid) ‘* call to Function MsgBox(“Area = “ & area) End Sub Function ComputeArea(L As Single, W As Single) As Single Dim A As Single A = L * W Return A

19. Function Example
Write a function named Quotient with 2 Integer parameters (originally from text boxes) and a Double return type. The function should calculate the first parameter divided by the second and return the result.
Obtain Input
Call the function,
Display results in a message box.

20. Intrinsic Functions Pre-coded Functions provided by VB
Used to improve developer productivity – less time spent coding
Broad Range of Built-In Functions
Math calculations
Formatting
Time/Date functions
To find a function open the Object Browser
F2 OR
Select View Menu then Object Browser option
String manipulation
Many more

21. Numeric Functions Rnd - Returns a number between 0 and 1 (excluding 1)
Int(6 * Rnd) + 1 ‘* Returns a random integer from 1 through 6
Sqr(n) - Returns the square root of the number n
IsNumeric(s) - Returns true if the item s can be converted to a number, false if not
IsNumeric (“23.5”) ‘* Returns True
IsNumeric(“hello”) ‘* Returns False
Round(n, r) - Returns the number n rounded to r decimal places
Round(6.3819, 3) ‘* Returns 6.382
Int(n) - Returns the integer part of the number n
Int ( ) ‘* Returns 123

22. Formatting Functions
FormatNumber(n, [r]) - returns number n formatted with commas and r decimal places (default is 2 decimal places)
FormatNumber( ) ‘* Returns 8,765.45
FormatNumber( , 3) ‘* Returns 8,
FormatCurrency(n, [r]) – returns number n formatted with dollar sign, commas, and r decimal places (default 2)
FormatCurrency(65) ‘* Returns $65.00
FormatCurrency( ) ‘* Returns $65,273.82
FormatPercent(n, [r]) - returns number n formatted as percent with r decimal places (default 2)
FormatPercent(.658) ‘* Returns 65.80%
FormatCurrency(8.20) ‘* Returns %

23. Conversion Functions
Val(s) - Returns the numbers contained in the string s (stops at 1st non numeric item)
Val(“123abc”) ‘* Returns 123
Str(n) - Converts number n to a String
Str(123) ‘* Returns “123”
Specific conversion functions for each data type
CBool ( expr )
CByte ( expr )
CChar ( expr )
CDate ( expr )
CDbl ( expr )
CDec ( expr )
CInt ( expr )
CLng ( expr )
CObj ( expr )
CShort ( expr )
CSng ( expr )
CStr ( expr )

24. Conversion Functions Cint(n) - converts number n to an integer
Rounding can be done with the CInt function
CInt(12.4) ‘* Returns 12
CInt(12.5) ‘* Returns 13
CStr(n) - converts number n to a string
CStr(26) ‘* Returns “26”
CDec(n) - converts number n to a decimal value
Dim decPay as Decimal = CDec(“$1,500”)
CDate(s) – converts string s to a date
Dim datHired as Date = CDate(“05/10/2005”)

25. Invalid Conversions Conversion functions can fail
String “xyz” can’t be converted to a number
Dim dblSalary as Double = CDbl(“xyz”)
There’s no day 35 in the month of May
Dim datHired as Date = CDate(“05/35/2005”)
These failed conversions cause a runtime error called an invalid cast exception

26. String Functions
Left(s, n) - Returns the number of characters specified by n, starting at the beginning of the string s
Left(“Penguin”, 4) ‘* Returns “Peng”
Right(s, n) - Returns the number of characters specified by n, starting from the end of the string s
Right(“Penguin”, 5) ‘* Returns “nguin”
Mid(s, n, r) - Returns the substring from string s, starting at the position indicated by n and continuing for the length specified by r
Mid(“Penguin Penguin”, 5, 6 ) ‘* Returns “in Pen”

27. String Functions
UCase(s) - Converts any lowercase letters in string s to uppercase
UCase(“Yes”) ‘* Returns “YES”
Lcase(s) - Converts any uppercase letters in string s to lowercase
UCase(“Yes”) ‘* Returns “yes”
InStr(s, t) - Searches for the first occurrence of string t in string s and returns the starting position in s at which t is found, -1 if not found
InStr(“John Smith”, “h”) ‘* Returns 2
Len(s) - Returns the number of characters in string s
Len(“Yes Yes”) ‘* Returns 7

28. Input Box InputBox Syntax: Example: Prompts the user for
keyboard input
Input is returned to
the program for processing
Syntax:
Result = InputBox (prompt, [title])
Example:
strInput = InputBox (“Enter Input”)
strName = InputBox (“What is your name”, “Name”)

29. Message Box MsgBox Syntax Example Displays a message in a dialog box
MsgBox (prompt, [title])
Example
MsgBox (“Hello World”, “Chapter 7_1”)
MsgBox (“The Result is “ & Result)

30. Example
Create an application that adds items to a sales receipt one at a time using an input text box and a button. Each time the button is pressed, the new item price should be added to a list box control which acts as a receipt. The program should also contain output labels for subtotal, sales tax and total that should be updated when an item is added to the receipt. (Ensure that the prices are numeric and that the output is formatted to currency)

31. TOE Chart TASK OBJECT EVENT Input Price Text Box, Label None
Display Receipt
List Box
Display Subtotal
Label, Label
Display Sales Tax
Display Total
Add Item to Receipt
Button
Click
Exit

32. Interface

33. Code
Variables
Subtotal
Tax
Total
What Data Types?
Where to Declare?

34. Add to Receipt Button Event
Read in Data from Text Box
Convert to Number
CDbl
Val
Add to ListBox
Formatted as Currency
Update Subtotal, Tax, and Total Variables
Update Output Displays

35. Convert to Number
Format to Currency
Format to Currency

36. Example
Write a VB application to have the user input a first name, middle name, and last name. Produce an output string stating the full name and the initials (with periods). Be sure that each name and initial is capitalized. Format and display the output in a label. Use 3 separate sub procedures to store input, build output string and display output. Use a function to capitalize a single name and a function to produce the formatted initials from all three names.

37. Reference