Variables Data Types and Assignment

Stuff to memorise this 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 as the name we give to a class file so that we may access its methods and properties Once we have an instance of an object we may access the methods and properties defined in the class Property allows us to read or change the settings of an object Methods tell an object to perform an action   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

Function for 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 … “0100100001000101010011000100110001001111” 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 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 Int16 -32768 to +32787 Int32 (or just Int) -2,147,483684 to +2,147,483683 Int64 -9223372036854775808 to +9223372036854775807 String any combination of numbers and letters DateTime any valid date or time Boolean true or false Decimal any 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 of Variables as Boxes in RAM The seven variables declared above have the following names… ErrMsg OfferTitle Description SwapNo Userno Email UserName A series of boxes are created…

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

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