Rote Learning of the Week "A variable is a named section of RAM that stores data of a specific data type"

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

Screen Objects Event driven

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

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

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

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

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

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

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

Where the RAM fits in The RAM

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

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

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

Data Types Integer whole number Types of Int Int to Int32 (or just Int)-2,147, to +2,147, Int to String any combination of numbers and letters DateTimeany valid date or time Booleantrue or false Decimalany whole or decimal number

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

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

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

Things we do with Variables Assign literal constants

Perform Calculations

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

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

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