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

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

ObjectsIn 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 dataMethods 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 ArchitecturePresentation(Interface)

Data LayerDatabase

Middle TierBusiness 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 teamsBoth teams to count from 1 – 50One team to write the numbers on paperThe other team to count in their headsEverybody standRaise your hand when you have finished

The Assignment Operator

The symbol for copying data around the systemPossibly the most important concept in programmingFail to grasp how this works and you won’t progress

How it workstxtMessage.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 RAMComputer RAM is VERY COMPLICATEDStores data as binary values

“hello” stored as … “0100100001000101010011000100110001001111”

Variables spare us a great deal of pain!

Variables are a very simple kind of objectAllow us to control a section of RAMThree things we want to do…

Allocate a section of RAMGive that section a name (a word)Set the rules for the type of data it will storeStore some data in the variable

Creating Variables

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

Data TypesNotice 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 TypesInteger 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 timeBoolean true or falseDecimal any whole or decimal number

Declare Variables at the top of your code…

Makes them easier to findMust declare a variable before we may use it

Think as Variables a Boxes in RAMThe seven variables declared above have the

following names…ErrMsgOfferTitleDescriptionSwapNoUsernoEmailUserName

A series of boxes are created…

Rules for Variable NamesWe do not have any spaces in the variable names

Err Msg BadErrMsg GoodUse underscore if you want a space e.g. Err_Msg

The variable names use pascal or camel caseswapno BadSwapNo Good (public variable)swapNo Good (private variable)

The names must be meaningfulA, 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 ApplicationTo finish off we will create the following

application…

Common Variable ErrorsWe shall look at some common errors in

Visual StudioLearn to recognise the errors

Missing variable declarationMismatched Variable NamesSelection of incorrect data typeRounding errorsOverflow errors