3. The Visual Basic .NET Language Learning to Program Overview - - PowerPoint PPT Presentation

• 3. The Visual Basic

.NET Language

Learning to Program

Overview

The Common Language Runtime Variables, data types, constants and

literals

Identifiers Expressions, operators Statements and blocks Structured variables and enumerations

VB .NET and The CLR

.NET programs execute on a virtual machine

called the .NET CLR

CLR = Common Language Runtime This is so that a .NET program can be run on any

computer provided a version of CLR is implemented for it

Also allows programs to be written for an ‘ideal’

computer that was created by programmers for programmers

CLR supports many programming languages – not

just Visual Basic

VB Running on a .NET Platform

A Visual Basic program

The .NET Common Language Runtime environment Computer hardware and Operating System

CLR Features

CLR has a number of useful features for

programmers

Common Type System

A range of variable types that represent all of the common

types of computer data

Numbers, dates, times, characters, text, currency etc.

Since all .NET languages use it, all are directly

compatible with each other

Can use a .NET class in a C# program etc.

Data types are independent of the computer or

• perating system used

Variables in Programs

Programs need to store and use data

Values input by the user Intermediate results in calculations Data read from disk, CD-ROM, other programs or the Internet Information indicating the current state of a program

How far through a list of values it has got The name of a data file etc.

Variables are elements of computer memory made

accessible by a programming language

Storage for a data value is given a name, or Identifier The Identifier is used in program statements to access (read or

write) the contents of a variable

The values stored in variables can change over time

(that’s why they are called Variables)

Program statements can update a value as necessary

Example use of variables

Sub Main() Dim length, width, area As Single length = 12.5 width = 8.75 area = length * width Console.WriteLine("Area is {0}", area) End Sub 12.5 18.5 231.25 Length Width Area Variables can be used to

represent physical values

e.g. The Length, Width and Area of a

rectangular item

Until a value is assigned, a

variable will usually contain 0

Data Types

The CTS provides a wide range of types

Numbers

Integers, real (floating point) numbers Various precisions

Characters and Text Dates and Times True or False/Yes or No

This range allows almost any physical or

logical value to be stored and manipulated

Types in Detail

A high precision number stored in a direct decimal representation to improve the accuracy

• f calculations (29 digits
• f precision)

+/-79,228,162,514,264,337,593,543,950,335 with no decimal point; +/-7.9228162514264337593543950335 with 28 places to the right of the decimal; smallest non-zero number is +/-0.0000000000000000000000000001

Decimal

A double-precision floating point number stored in an efficient binary representation (15 digits

• f precision)
• 1.79769313486231E+308 to
• 4.94065645841247E-324 for negative values;

4.94065645841247E-324 to 1.79769313486231E+308 for positive values

Double

A single-precision floating- point number stored in an efficient binary representation (7 digits

• f precision)
• 3.402823E+38 to -1.401298E-45 for negative values;

1.401298E-45 to 3.402823E+38 for positive values

Single

A 64 bit signed number with no fractional part

• 9,223,372,036,854,775,808 to

9,223,372,036,854,775,807

Long

A 32 bit signed number with no fractional part

• 2,147,483,648 to 2,147,483,647

Integer

Description Range Type Name

Types in Detail II

Description Range Type Name Dates, times and combinations of both. A single date variable stores a point in time (Date and Time) January 1, 0001 to December 31, 9999

Date

Sequences of text characters, e.g. names, addresses, paragraphs of a document, whole documents etc. Up to 2 billion characters

String

Stores a single alphanumeric character (e.g. the letter ‘x’,

• r a digit or punctuation

symbol 0 to 65535

Char

Used to store simple true/false, on/off, up/down or

• ther 2-state values

True or False

Boolean

Type Compatibility

Numeric values are interchangeable between

different types, but:

There can be a loss of data

E.g. Store a double value 3.14159268 in an Integer variable

(result is 3)

General principle

Store contents of lower precision data type in higher

precision or wider range variable – no problem

Store contents of higher precision or wider range data type in

lower precision variable – loss of data

Constant

A Constant is like a variable with a fixed value

(an oxymoron if ever there was one)

Constants are used in programs

to give memorable names to fundamental values

e.g. pi, Gravitational_Constant, cm_per_inch

to provide for values that might change from version

to version of a program

e.g. VATRate, max_class_size, database_server_name

to provide for data that may have one value in a test

version and another in the final release of the software

e.g. sample_size (could be 10 in tests, 1000 in working

s/ware), max_redials (could be 10 in tests, 3 in real life)

Literals

A Literal is a value stated explicitly in program

code

e.g. 3.1415927, 100, “Fred Bloggs”,

“www.our_site.com/data_server”

i.e. any value that could be applied to a variable

Generally, try to eliminate literals from program

code

Define constants to replace them Not feasible to get rid of them all, but should aim to

minimize the need to change statements throughout a program if some assumed value changes

Identifiers

Names given to:

Variables Constants Structures and Classes Class members (member variables, methods) Code routines Modules, Namespaces Forms, controls etc.

The purpose of an identifier is to allow a programmer to

manipulate some value in code:

E.g.

FullPrice = Price + Price * VATRate

FullPrice and Price are variables, referring to some stored value.

Need to use the names to access these values

Rules for Identifiers

Must start with alphabetic or underscore

e.g. Cust_Name, PI, _data_Server

Must not be an existing keyword in the

language:

e.g. avoid – Integer, Sub, For, Next, Function etc.

Must contain one or more alphabetics or

numerics

e.g. X, A2, _d, _5

Must be no more than 16383 characters long

Not much of a restriction

Assigning literal values - rules

Numeric assignments

State values directly. e.g. 12.3, 10000, -125

Char assignments (single characters)

Use double quotes. e.g. “X”, “;”

String assignments (text)

Use double quotes. e.g. “Fred”, “ABCEDFG”

Dates and times

Enclose in double quotes. e.g. “14/02/2003”, “17:00”,

“21/08/2003 12:30:00”

Initialising Variables

A variable with no value assigned will contain the type

equivalent of zero:

0 or 0.0 for numbers “” for strings and characters “01/01/0001 00:00:00” for dates/times

Typically a variable is declared then assigned

Can lead to a situation where a zero value is in the variable

unintentionally for some time

Can lead to subtle errors Programmer can declare and forget to assign at all

Possible to Declare/Assign in one step

e.g. Dim myName As String = "Alistair" Prevents problem of unassigned variables

Expressions

An Expression is some combination of:

Variable references Operators Constants or literals Function calls

Legal expressions always reduce to a single value

e.g. 12/3 + 4*2 – 11 (expression) evaluates to 1 (4 + 8 –

11)

e.g. Sqr(16) (expression) evaluates to 4 (sqr root of

16)

Operators

Generally, expressions involve

• perators

A range of legal operators is available

for the range of types

e.g. Numeric (+, -, *, /, ^, +=, *=) e.g. String (&, &=) e.g. Dates (+, -, +=, -=)

String Operators

Strings can be added together

Two operators (plus modifications) available for this - + and &

The + operator used with strings joins them together

e.g. “Fred “ + “Bloggs” = “Fred Bloggs”

The & operator used with any data type will produce a

string result

e.g. “Catch ” & 22 = “Catch 22” e.g. "Strange but " & (2 = 2)

Also the ‘increment’ variants of these operators

e.g. s = “Farenheit “

s &= 451

e.g. name = “Charlie “

name += “Brown”

Statements and sequence

Statements organised in a sequence will

• perate in that sequence

This is crucial – it provides the mechanism for

performing a task in a number of steps

Price = 10.00 TaxRate = 0.175 Price = Price + Price * TaxRate Console.WriteLine(Price) These must come before this for the calculation to work.

Blocks

A sequence of statements

that is not interrupted by control statements (later) is a block

Can be considered to be a

unit of code: a block of statements performs a task

Blocks can be given a

name (identifier) by enclosing them as a Sub

• r Function

This allows us to Call a

block of code by using its name as a statement

Sub AddTax() Price = 10.00 TaxRate = 0.175 Price = Price + Price * TaxRate Console.WriteLine(Price) End Sub Sub Main() AddTax() End Sub

Structured Variables

Individual variables are

adequate for storing data in programs, but by combining variables, we can provide better structural integrity

Allows us to create our own,

meaningful, variable types

Organising data like this is

beneficial in programming

Keep related info together Improves clarity Simplifies moving data around a

program Structure Rectangle Dim Length As Single Dim Width As Single End Structure Sub Main() Dim R As Rectangle R.Length = 12.5 R.Width = 16.2 ‘ ‘ ‘ ‘ ‘ ‘ End Sub

Enumerations

Some types of

information fit most comfortably into restricted ranges

e.g. Colours, Shoe Sizes,

Types of class (Lecture, Lab, Tutorial, Seminar) etc.

An enumeration is an

efficient way of creating a list of possible values for a type

Reduces possibility of

error by assigning an invalid value to a variable (only the specified range is available)

Enum MusicStyles Classical Swing Jazz Pop Rock Techno End Enum Enum ClassType Lecture Lab Tutorial Seminar End Enum

Summary

CLR is a Virtual Computer that .NET programs run on Variables used to store items of data (and provide a name for them)

Range of predefined types available

Literals and Constants are specific values used in expressions Operators are used to form expressions (with variables, literals and

constants) to do calculations

Strings (sequences of text characters) can also be calculated and

manipulated

A sequence of statements specifies the order a process will be

done in

A block of statements performs some task, and can be named (as a

Sub or Function) to make it easy to use

Individual data values can be organised into groups as structures,

• r limited in range by enumerations