Variables Data Types and Assignment

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 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

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

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 … “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 (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 • 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 as Variables a 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

Variables Data Types and Assignment