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CONTAINMENT AND EVEDRIVEN PROGRAMMI

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Containment/aggregation/composition 

• These are links/associations between objects that allow them communicate.

• For instance: a form on a screen is an object. On the object foare other objects, e.g. delete button, display button, exit butt

• These buttons (which are objects) communicate with the formobject).

• Thus the linkage between the form and the buttons is called thcontainment.

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Event Driven Programming 

• This is whereby the sequence of code execution is determined(triggered) by external events or by user actions, e.g. clicking on a form is an event that can form a program to execute certsections of the code.

• Thus clicking a button or menu item is an event.

• This is important because the programmer cannot predict the sequence a user can perform his/her tasks.

• Event handlers: small program codes which are invoked (calleresponse to external events.

• Dispatcher: small program codes that call event handlers so thcan be processed.

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Handling Strings

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What is a string?• A string is a series of alphanumeric characters enclosed in quota

• A string is data type that stores one or more characters, which calphabetic or alphanumeric.

• If a variable is declared as a string, it must not be used for dailymathematical calculations because strings are not numbers.

• A string is sometimes just referred to as „text‟. Any type of alph

numeric data can be stored as a string: “Birmingham City”, “3/“36.85” are all examples of strings. 

• Each character within a string will be stored in one byte using itmodern systems might store each character in two bytes using i

• The maximum length of a string is limited only by the available

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Declaration of strings

Dim Name As String 

• If a value of string variable is assigned within the program code, itenclosed in quoates e.g.

Name = “Agrippa” 

Name = “”

• The second example will be a null (un-initialised) string. It is diffe

space but is distinct in nature.

• Strings can be of fixed length or variable length 

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Variable length string

• Variable length string

Dim Surname As String

• Variable length does not specify the number ofcharacters, each data items occupies the numbeof spaces it requires.

• This is declared as follows

•   Dim Sname as string 

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Fixed length string• Fixed length string

Dim StudentNumber As String * 5

• Fixed length: has specified maximum number of charact

Dim Sname as string * 20  

• The field will be allocated 20 spaces in the computer me

• If a name shorter than 20 characters is entered, blank spremain unoccupied since they would have been assigned already.

• If too long than allocated space, the extra characters are

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Processing strings: Sorting

• Using relational operators.

if Name1< Name2 then

Print Name1 & “ “ & Name2 

else

Print Name2 & “ “ & Name1 

end if

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Processing Strings cont‟d • Count number of characters in a string using len

•The function Len returns the number of characters stored in a strinit.

Syntax: CharacterNumber = len(string) then Print CharacterNumber

Print Len(string)

• Question:

•Design a program in VB to capture five names of computer scienceand count the number of characters in each student name. the prodisplay the student name and in front of it, the number of charact

• E.g. Mungazi Davies This name has 14 characters

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VB codeDim Spaces As Integer

Dim EmployName As String

Dim Index As Integer

Dim Charater As String * 1EmployeeName = txtName.text

Spaces = 1

For index = 1 to Len(EmployeeName)

Character = Mid(EmployeeName, Index)

If character = “” then

Spaces = Spaces + 1

End if

Next Index

If Spaces >= 2 then

Msgbox (“too many spaces”), vbExclamation