1. Variables Data Types and Assignment

2. Stuff to memorise this Week
"A variable is a named section of RAM that stores data of a specific data type"

3. Objects
In the first lecture we introduced the idea of classes and objects
One way of looking at an object is that it is a word in our code that links to the functions in a class
Once we have an instance of an object (the word) we may access the methods and properties defined in the class
Properties allow us to change or find out about some aspect of the data
Methods allow us to perform some action on the data
There are lots of different objects within the computer system all allowing us to control data in different ways

4. Screen Objects
Event driven

5. System Architecture Presentation (Interface) Data Layer Database
Middle Tier
Business Logic
(Objects/Classes)

6. Layers Split in to Other Layers
Interface and code linked by events

7. Click Event Handler
Event Handler = Function that responds to an event

8. Interface v RAM Exercise
Split into two teams
Both teams to count from 1 – 50
One team to write the numbers on paper
The other team to count in their heads
Everybody stand
Raise your hand when you have finished

9. The Assignment Operator
The symbol for copying data around the system
Possibly the most important concept in programming
Fail to grasp how this works and you won’t progress

10. How it works txtMessage.Text = “Hello world”;
The assignment operator copies data from right to left
Destination = Source
All together now!

11. Computer Technology
We need objects & classes to control all this!

12. Where the RAM fits in
The RAM

13. Variables and RAM Computer RAM is VERY COMPLICATED
Stores data as binary values
“hello” stored as … “ ”
Variables spare us a great deal of pain!
Variables are a very simple kind of object
Allow us to control a section of RAM
Three things we want to do…
Allocate a section of RAM
Give that section a name (a word)
Set the rules for the type of data it will store
Store some data in the variable

14. Creating Variables
Variables are “declared” using the key word “Dim”

15. Data Types
Notice in declaring variables we give them a data type, e.g.
string
Sets the rule as to the type of data we may put in the box

16. Data Types Integer whole number
Types of Int
Int to
Int32 (or just Int) -2,147, to +2,147,483683
Int to
String any combination of numbers and letters
DateTime any valid date or time
Boolean true or false
Decimal any whole or decimal number

17. Declare Variables at the top of your code…
Makes them easier to find
Must declare a variable before we may use it

18. Think of Variables as Boxes in RAM
The seven variables declared above have the following names…
ErrMsg
OfferTitle
Description
SwapNo
Userno
UserName
A series of boxes are created…

19. Rules for Variable Names
We do not have any spaces in the variable names
Err Msg Bad
ErrMsg Good
Use underscore if you want a space e.g. Err_Msg
The variable names use pascal or camel case
swapno Bad
SwapNo Good (public variable)
swapNo Good (private variable)
The names must be meaningful
A, B & C are normally bad names as they give no clue as to their usage

20. Things we do with Variables
Assign literal constants

21. Perform Calculations

22. Get data off the Interface
string FirstName;
FirstName = txtFirstName.Text;

23. A Simple Application
To finish off we will create the following application…

24. Common Variable Errors
We shall look at some common errors in Visual Studio
Learn to recognise the errors
Missing variable declaration
Mismatched Variable Names
Selection of incorrect data type
Rounding errors
Overflow errors