Programming the Web

friendshomeopathInternet και Εφαρμογές Web

4 Δεκ 2013 (πριν από 3 χρόνια και 11 μήνες)

170 εμφανίσεις

Chapter 5


Programming the Web

Server
-
side Programming

ASP .NET Web
Forms



Yingcai Xiao

Programming the Internet



The Internet as a Computer





Every computer on the Internet is a “CPU”



Transmitting the data over the Internet is the Key



Analogy of Internet Protocols:




IP =>
“
Binary
”

(low
-
level Internet transmission protocol)



HTTP =>
“
Assembly
”

(high
-
level Internet transmission protocol)


HTML => High Level Language (for writing web
-
pages)



An international language everyone understand


Internet Protocol





IP: Internet Protocol

data transmission protocol (standard) for the Internet.

•
The Internet is a computer network based on the Internet Protocol.

•
Each computer on the Internet is identified by its Internet Address:
130.101.10.134 or wp.cs.uakron.edu

•
Data transmitted between computers on the Internet as packet.

•
Each packet has an IP header followed by data.

•
The first 2 entries in an IP header are the receiver’s address and the sender’s
address. IP is similar to postal mailing (Packet Switching), not phoning
(Circuit Switching).

•
IPv4 (32bit addressing, 4B), IPv6 (48bit addressing, 256T)

•
The headers are in the standard IP format, platform
-
independent.

•
Data (
any type
) is transmitted over the Internet bit
-
by
-
bit. No restrictions on
what can be transmitted.

•
Binary data are platform
-
dependent.

Binary data transmitted from one
computer to another computer may not be readable by the receiver if it has a
different binary data format than the sender.



Hypertext Transfer Protocol




Can we make it platform
-
independent?


The World Wide Web is an application built on the Internet using the Hypertext
Transfer Protocol in which only text data is permitted to be transmitted.



HTTP

(Hypertext Transfer Protocol)


•
Tim Berners
-
Lee ("father of the Web") and RFC 2068



(www.w3.org/Protocols/rfc2068/rfc2068).

•
Entirely text based: ASCII (8
-
bits) or Unicode (16
-
bits).

•
Platform independent

•
Defines how Web browsers and Web servers communicate.

•
7 instructions defined in HTTP 1.1.: GET, POST, …

•
Transmitted over TCP/IP (Transport Control Protocol/Internet Protocol).

•
Web applications are implemented over the Internet using HTML and other
Web languages.






HTML

(Hypertext Markup Language)

•

Defines syntax and semantics of the Web language.

•

Entirely text based (platform independent)

•

Hypertexts are tagged in < >, not to be displayed. They are
metadata describing regular text.
(http://www.w3schools.com/tags/)

•

Browsers are GUI
-
based HTML interpreters.


HTML 4.01 became XHTML 1.0 in 2000


simple.html:

<html>


<body>



Hello, world


</body>

</html>


HTML


Client
-
Side:

•
Thin client

1.
Only a browser is needed.

2.
The browser renders a html page on the client
computer.

3.
HTML (static), DHTML (dynamic), HTML5 (dynamic
and multi
-
medium).

4.
Programming with embedded scripts: JavaScript, .
Dynamic and computational.

5.
Ajax (asynchronous JavaScript & XML):
asynchronous.

6.
Plugins: VRML (3D Web)

7.
Java Applet: transmit the application to the client and
run it there. Too much to transmit for large distribution
lists.

Computing over the Internet


Client
-
Side:

•
Thick client

1.
Client program installed prior, no need to download at
runtime.

2.
Runs like any other program on the client, but can talk
to the server when needed.

3.
.NET
Remoting
,
Windows Communication Foundation
(WCF),

4.
Java's Remote Method Invocation (RMI)

5.
Common Object Request Broker Architecture
(CORBA)

Computing over the Internet


Server
-
Side:

•
Web pages are generated by server side programs at
runtime. Dynamic contents and heavy
-
duty computing.

•
Server Scripts

1.
Perl, PHP, ….

2.
Does not scale well. (each client needs a process to
service)

•
Server VMs

1.
.NET ASP (Active Server Page), JSP (Java Server
Page),

2.
All clients are served by a single process. The process
will create a thread to serve each client.



Computing over the Internet


Server
-
Side:

•
CMS (Content Management Systems): web contents are
managed at server side on demand.

•
Dejango

CMS,
Joomla
.



Server
-
Client Communication:

•
Important for Internet based computing.

•
HTTP channel (slower, works for both thin and thick
clients).

•
TCP/IP channel (faster, works only for thick clients).

•
HTTP next, TCP/IP later.




Computing over the Internet

What happens when browsing a web page on a server?

http://www.wintellect.com/simple.html

•

Start a client (a browser).

•

Type in the URL (Unified Resource Locator).

•

Internet
’
s Domain Name System (DNS) converts

www.wintellect.com into an IP address (66.45.26.25).

•

The browser opens a socket (IP) connection to the server

using default port 80 (mailbox #).

http://www.wintellect.com:80/simple.html


What happens when browsing a web page on a server?

•

The browser transmits an HTTP request to the server.

GET /simple.html HTTP/1.1

Accept: */*

Accept
-
Language: en
-
us

Accept
-
Encoding: gzip, deflate

User
-
agent: Mozilla/4.0.(compatible; MSIE.6.0; Windows NT 5.1)

Host: www.wintellect.com

Connection: Keep
-
Alive

[blank line]




What happens when browsing a web page on a server?


•

The server transmits an HTTP response back.


HTTP/1.1 200 OK

Server: Microsoft
-
IIS/5.0

Date: Wed, 24 Oct 2001 14:12:37 GMT

Content
-
Type: text/html

Accept
-
Ranges: bytes

Last
-
Modified: Wed, 24 Oct 2001 14:00:53 GMT

ETag: "d02acf81975cc11:a78"

Content
-
Length: 46

[blank line]

<html>

<body>

Hello, world

</body>

</html>


What happens when browsing a web page on a server?


•
Upon receiving the response, the browser parses the
HTML and displays the resulting Web page.

•
This is just a static page.

•
To compute, we need get the data from the client user. A
client form is needed. HTML form serves the purpose.






http://winserv1.cs.uakron.edu/xiaotest/Calc.html

<html>


<body>


<form method=
“
post
”
>


<input type="text" name="op1" />


+


<input type="text" name="op2" />


<input type="submit" value=" = " />


</form>


</body>

</html>



HTML Forms

What
’
猠桡灰敮楮朿

No action to send anything to the server when the user types values
in the text fields. But when the submit button (<input
type=
“
submit
”
>) is clicked, the browser submits the form along
with any input in the form
’
s controls.

POST /calc.html HTTP/1.1

…

Content
-
Length: 11

[blank line]

op1=2&op2=2

Web Server



Deploying a Web Page on an IIS server.


IIS:
Internet Information Services


Deploying Directory:

C:
\
Inetpub
\
wwwroot


Access URL: http://
localhost
/ (case insensitive)




Server
-
Side Processing

Common Gateway Interface (CGI)

•

CGI applications write HTTP responses to standard output (
stdout
) on

the server, which are then forwarded to the client browser by the web

server.

•

Defines a low
-
level programmatic interface between Web servers and

applications that run on Web servers.

•

CGI applications can be written in any programming language.

•

CGI applications read the input accompanying
postbacks

through server

environment variables and standard input (
stdin
).

•

Slow, restarts a process on every request.


ISAPI



ISAPI
: Internet Server Application Programming Interface

•

ISAPI extensions are Windows DLLs hosted by IIS.

•

They
’
re referenced by URL just like HTML files (for

example, http://www.wintellect.com/calc.dll).

•

IIS forwards HTTP requests to an ISAPI DLL by calling a

special function exported from the DLL.

•

The DLL generates HTTP responses.

•

Faster than CGI (run in the same process as IIS).

•

Once loaded, they remain in memory.

•

They
’
re difficult to write.




Active Server Pages (ASP)




Active Server Pages (ASP)



Introduced in 1996.


Dynamically generates HTML on Web servers.


Allows HTML and scripts / languages to be mixed.


The embedded
s
cript
code are
between the <% and %> tags.


When an Active Server Page is requested, ASP server parses the
page and executes any scripts contained inside it.


Scripts access the input by using the Request object.


Scripts write HTML to the HTTP response using the Response
object, which is sent to the client by the Web server along with
the static HTML.


ASP integrates with ActiveX Data Objects (ADO).


Interpreted, slow.


Lacks encapsulation.

<%@ Language="VBScript" %>

<html>


<body>


<form>


<input type="text" name="op1" value="<%= Request ("op1")

%>"/>


+


<input type="text" name="op2" value="<%= Request ("op2")

%>" />


<input type="submit" value=" = " />


<%


If Request ("op1") <> "" And Request ("op2") <> "" Then


a =
CInt

(Request ("op1"))


b =
CInt

(Request ("op2"))


Response.Write

(
CStr

(a + b))


End If


%>


</form>


</body>

</html>


Calc.asp

APS.NET


ASP:

Microfoft
’
s equivalent of JSP

Code on the server that dynamically generates HTML for the
clients at runtime.




JSP: Java Server Page

Java code on the server that generates HTML for the clients.

HTML contents can be dynamically generated at runtime



ASP.NET:

ASP based on the .NET framework, one of the most popular
web programming techniques. Code on the server that
dynamically generates HTML for the clients at runtime.












ASP.NET



WEB


S

E

R

V

E

R


IIS



WEB


C

L

I

E

N

T

Application 3

Application 2

Application 1

ASP.NET

HTTP


Web Forms




Web Forms are
GUI
-
based
EDP

web pages built around controls and event
handlers.


.NET web forms are processed on the
server side
.


Web forms use
HTML
,
HTTP

and IP to transmit and display GUI into a client
web
-
browser.




ASP.NET Web Forms

•

GUI

for Web applications

•

Object
-
oriented
(encapsulation, inheritance, polymorphism)

•

Event driven (
EDP
)

•

Server
-
side
processing (dynamic generation of HTML)

•

Compiled code, faster

•

HTML embedding tag: <asp:
\
>


(processed on the server side)

•

File extension: .aspx

•

To deploy on winserv1:


copy Examples/c5/calc.aspx to


C:
\
Inetpub
\
wwwroot
\
xiaotest

•

To access:


http://winserv1.cs.uakron.edu/xiaotest/calc.aspx


ASP.NET Web Forms


<html>


<body>


<form RunAt="server">


<asp:TextBox ID="op1" RunAt="server" />


+


<asp:TextBox ID="op2" RunAt="server" />


<asp:Button Text=" = " OnClick="OnAdd" RunAt="server" />


<asp:Label ID="Sum" RunAt="server" />


</form>


</body>

</html>

<script language="C#" RunAt="server">


void OnAdd (Object sender, EventArgs e)


{


int a = Convert.ToInt32 (op1.Text);


int b = Convert.ToInt32 (op2.Text);


Sum.Text = (a + b).ToString ();


}

</script>

Calc.aspx

When Calc.aspx is accessed by a client


An ASP.NET server page contains two parts: static HTML
code and dynamic embedded APS.NET code.



ASP.NET
-
enabled web server processes the ASP.NET code in
the server page
.
(Tomcat does not support ASP.NET but
Mono does)



It instantiates TextBox, Button, and Label objects (the

classes are defined in System.Web.UI.WebControls).


Each object dynamically renders itself into HTML.




The dynamically generated HTML is merged into the static

HTML
.




The resulting HTML is returned to the client as an HTTP

response
.

Calc.aspx as seen by the client

<html>


<body>


<form name="_ctl0" method="post"


action="calc.aspx" id="_ctl0">


<input type="hidden" name="__VIEWSTATE"

value="dDwxOTE0NDY4ODE2Ozs+" />


<input name="op1" type="text" id="op1" />


+


<input name="op2" type="text" id="op2" />


<input type="submit" name="_ctl1" value=
”

= " />


<span id="Sum"></span>


</form>


</body>

</html>


note: there are also
“
hidden
”

and
“
div
”

(section) tags.

•

When the
“

=
“

button is clicked, the input from
“
op1
”

and

“
op2
”

are posted to the server and directed to calc.aspx

(since action="calc.aspx").

•

On the server side, ASP.NET notifies the Button object in
calc.aspx and the Button responds by firing a Click event.

•

ASP.NET receives the event and subsequently invokes its
handler: OnAdd ( ).

•

“
Sum.Text = (a + b).ToString ();
”

renders the result into the

“
Sum
”

label in <span id="Sum"></span> as HTML.

•

The dynamically generated HTML is merged with the static
HTML (parts not in server side tags).

•

The resulting HTML is sent to the client browser.


Use the client browser
’
s View
-
>Page Source menu to see the final
HTML code received by the client.

(Mac: Tools
-
>Web Developer
-
>Page Source)


When Calc.aspx is accessed by a client

ASP.NET Controls



Two types: Web Controls and HTML Controls




Web Controls

•

Class names are prefixed with asp:.

•

Classes are from System.Web.UI.WebControls.

•

The name of the object is defined by the ID attribute.

•

ASP Web controls are rendered into HTML.

•

ASP Web controls are highly programmable.


They support methods, properties, events.



<asp:TextBox Text="2" ID="op1" runat="server" />



This web control initializes the textbox to display 2. Any

public property of a control can be used this way.


ASP.NET Controls

•

Control properties can be accessed from server
-
side scripts (code

between the <script> and </script> tags).


Read a control property by scripts:



int a = Convert.ToInt32 (op1.Text);


Write a control property by scripts:



Sum.Text = (a + b).ToString ();


•

Event
-
driven programming. Controls fire events when users click

on them. Wiring an event to an event handler is accomplished by

prefixing the event name with
“
On
”
.


<asp:Button Text=" = " RunAt="server" OnClick="OnAdd" />


•

Exception handling can be added in the handlers.


void OnAdd (Object sender, EventArgs e)


{ try {




int a = Convert.ToInt32 (op1.Text);




int b = Convert.ToInt32 (op2.Text);




Sum.Text = (a + b).ToString ();




}



catch (FormatException) {




Sum.Text =
“
Format Error";



}


}


ASP.NET Controls

Three principles of the Web Forms programming model:

•

A Web form
’
s user interface is
“
declared
”

using a

combination of HTML and server controls.

•

Server controls fire events that can be handled by server
-
side

scripts.

•

Server
-
side scripts in ASP.NET are compiled to CIL and

executed by the CLR on the server.

RunAt=
“
server
”

must be used in every tag that ASP.NET is to
process



The Web Forms Programming Model


Web Controls


AdRotator

Displays rotating banners in Web
forms


Button

Generates submit buttons

Calendar

Displays calendars with selectable
dates

CheckBox

Displays a check box in a Web form

CheckBoxList

Displays a group of check boxes


CompareValidator

Validates user input by comparing it to
another value

CustomValidator

Validates user input using the
algorithm of your choice


Web Controls

DataGrid

Displays data in tabular format


DataList

Displays items in single
-
column or
multicolumn lists using HTML
templates


DropDownList

Generates HTML drop
-
down lists


HyperLink

Generates hyperlinks

Image

Displays images in Web forms

ImageButton

Displays graphical push buttons

Label

Generates programmable text fields

LinkButton

Generates hyperlinks that post back to
the server

Web Controls

ListBox

Generates HTML list boxes


Literal

Generates literal text in a Web form

Panel

Groups other controls

RadioButton

Displays a radio button in a Web form

RadioButtonList

Displays a group of check boxes

RangeValidator

Verifies that user input falls within a
specified range


RegularExpressionValidator

Validates user input using regular
expressions


Repeater

Displays items using HTML templates

Web Controls

RequiredFieldValidator

Verifies that an input field isn
’
t empty

Table

Generates HTML tables


TextBox

Generates text input fields

ValidationSummary

Displays a summary of validation
errors

Xml

Displays XML documents and
optionally formats them using XSLT



ASP.NET HTML
controls

mimic the original HTML
tags
.
(They are processed on the server side. They are not HTML tags
which are processed by the client browser.)

e.g
. <input type="text" RunAt="server" />

•

HTML controls are used like HTML tags.

•

An HTML control is an instance of

System.Web.UI.HtmlControls.HtmlInputText.

•

ASP.NET sees the RunAt=
“
server
”

attribute and creates an

HtmlInputText object.

•

The HtmlInputText object, in turn, emits an <input

type=
“
text
”
> tag that
’
s ultimately returned to the browser.

•

HTML controls (server side) have properties and can

generate events which make them more powerful than

HTML tags (client side).



Without RunAt="server
“
, HTML controls become HTML tags.

HTML Controls


HTML Controls

Tag

Corresponding HTML Control

<a runat=
“
server
”
>

HtmlAnchor

<button runat=
“
server
”
>

HtmlButton

<form runat=
“
server
”
>

HtmlForm

<img runat=
“
server
”
>

HtmlImage

<input type=
“
button
”

runat=
“
server
”
>

HtmlInputButton

<input type=
“
reset
”

runat=
“
server
”
>

HtmlInputButton

<input type=
“
submit
”

runat=
“
server
”
>

HtmlInputButton

HTML Controls

<input type=
“
checkbox
”

runat=
“
server
”
>

HtmlInputCheckBox

<input type=
“
file
”

runat=
“
server
”
>

HtmlInputFile

<input type=
“
hidden
”

runat=
“
server
”
>

HtmlInputHidden

<input type=
“
image
”

runat=
“
server
”
>

HtmlInputImage

<input type=
“
radio
”

runat=
“
server
”
>

HtmlInputRadioButton

<input type=
“
password
”

runat=
“
server
”
>

HtmlInputPassword

<input type=
“
text
”

runat=
“
server
”
>

HtmlInputText

<select runat=
“
server
”
>

HtmlSelect

HTML Controls

<table runat=
“
server
”
>

HtmlTable

<td runat=
“
server
”
>

HtmlTableCell

<th runat=
“
server
”
>

HtmlTableCell

<tr runat=
“
server
”
>

HtmlTableRow

<textarea runat=
“
server
”
>

HtmlTextArea

Any other tag with runat=
“
server
”


HtmlGenericControl

<html>


<body>


<form runat="server">


<input type="text" id="op1" runat="server" />


+


<input type="text" id="op2" runat="server" />


<input type="submit" value=" = " OnServerClick="OnAdd" runat="server" />


<span id="Sum" runat="server" />


</form>


</body>

</html>


<script language="C#" runat="server">

void OnAdd (Object sender, EventArgs e)

{ int a = Convert.ToInt32 (op1.Value);


int b = Convert.ToInt32 (op2.Value);


Sum.InnerText = (a + b).ToString (); }

</script>

The HTML controls version of Calc.aspx


When ASP.NET processes the
first

HTTP request for an ASPX file:

1.
It creates class representing the dynamic code of the file.

2.
It compiles it, loads it, creates an object for it.

3.
The object initializes itself and processes page
-
level events.

4.
The object generates HTML output to be sent to the client.

5.
The object remains in the server memory to process further
requests to the page.

6.
When the ASPX file is modify, ASP.NET automatically repeats
steps 1
-
4.

How does an ASP.NET enabled server process an ASPX file?

When ASP.NET processes the first HTTP request for an ASPX file:

1.

It creates a Page class deriving from class

System.Web.UI.Page.

2.

It copies the code in the ASPX file to the Page
-
derived class.

3.

A method (e.g. OnAdd) in a <script> block becomes a

member method of the derived class.

4.

ASP.NET compiles the derived class and places the

resulting

DLL in a system folder and caches it there.

C:
\
WINNT
\
Microsoft.NET
\
Framework
\
vn.n.nnnn
\
Temporary

ASP.NET Files.

5.

ASP.NET instantiates an object of the derived Page class

and
“
executes
”

it by calling a series of methods on it.

6.

The Page object instantiates any controls declared inside it

and solicits their output.

How does a ASP.NET enabled server process an ASPX file?

(Details)

7.
As the Page object executes, it fires a series of events (page
-
level events) that
can be processed by server
-
side scripts:


“
Init
”
, which is fired when the page is first instantiated, and


“
Load
”
, which is fired after the page
’
s controls are initialized but before
the page renders any output.

8.
Implicit wiring of Page events to handlers by defining handlers
’

names

as Page_EventName.


<asp:DropDownList ID="MyList" RunAt="server" />


. . .

<script language="C#" runat="server">


void Page_Load (Object sender, EventArgs e)


{ if (!IsPostBack) { // if it is not from the client


for (int i=0; i<5; i++) {// list 5 days


DateTime date =


DateTime.Today + new TimeSpan (i, 0, 0, 0);


MyList.Items.Add (date.ToString ("MMMM dd, yyyy"));


} } } </script>


Page
-
Level Events

ASP.NET Compilation Directives @



@ Page

Defines general attributes and compilation settings for
ASPX files (e.g. Language)

@ Control

Defines general attributes and compilation settings for
ASCX files

@ Import

Imports a namespace


@ Assembly

Enables linkage to assemblies, not linked to by default

@ Register

Registers user controls and custom controls for use in a
Web form

@ OutputCache

Exerts declarative control over page caching and
fragment caching

@ Reference

Adds a reference to an external ASPX or ASCX file

@ Implements

Identifies an interface implemented by a Web page


Must be positioned at the top of an ASPX file.

1.
ASPX files are in text files, anyone can read it.

2.
When a company sells its ASPX server programs, it does not want
people to see their source code.

3.
How? Code
-
behind is designed to protect the source code.

4.
For the static code in HTML, we can
’
t do anything about it.

5.
Dynamic code in C# or other .NET languages can be separated out
and compiled into DLLs.

6.
Only the static HTML code and the DLLs are delivered to the
customs.

Separating Static Code from Dynamic Code

1
.

Create

a CS file (e.g. Examples/C5/Lander/Lander.cs) containing
code that would normally appear between <script> and </script> tags.
Make each of these source code elements members of a class (e.g.
LanderPage) derived from System.Web.UI.Page.

2.

In your Page
-
derived class, declare protected fields whose names
mirror

the IDs of the controls declared in the ASPX file (e.g.
Lander.aspx). At run time, ASP.NET maps these fields to the controls
of the same name.

3.
Compile

the CS file into a DLL.

In a
Visual Studio Command Prompt

window, run the following:


csc /target:library Lander.cs

Code
-
behind with C#

4.

Place

the HTML portion of the Web form
—
everything

between
the <html> and </html> tags
—
in an ASPX file

(e.g.
Lander.aspx).

5.

Include in the ASPX file an @ Page directive containing an
Inherits

attribute that identifies the Page
-
derived class in the
DLL. When a request arrives for that page at run
-
time,
ASP.NET derives a child class from this class to create the form.


e.g.

<%@ Page Inherits="LanderPage%>

6.

To use the source code without recompiling:


<%@ Page Inherits="LanderPage" Src="Lander.cs" %>

7.

Code embedded has to be in C#, VB .NET, or JScript. Code
-
behind can be in any other language supported by .NET.


Code
-
behind in Web forms coded in C#

8.
To
deploy

the program:



move

the ASPX file to Inetpub/wwwroot/xiaotest/Lander
and



the DLL to Inetpub/wwwroot/xiaotest/Lander/bin.

9.
The program can be accessed the same way as other

ASP.NET programs:



http://winserv1.cs.uakron.edu/xiaotest/Lander/Lander.aspx

9.
In order for


http://winserv1.cs.uakron.edu/xiaotest/Lander/Lander.aspx


to work, the Lander directory has to be converted to a Web
Application, which instructs ASP.NET to find Lander.dll under
the application’s bin directory.






Code
-
behind in Web forms coded in C#

Create a Web Application in IIS



* You need to be an administrator to use IIS



winserv1
-
> Sites
-
> Default Web Site



Right
-
click
-
>Add Application



Alias: xiaotestLander



Application pool: defualtAppPool



Physical path: C:
\
inetpub
\
wwwroot
\
xiaotest
\
Lander

http://winserv1.cs.uakron.edu/xiaotestLander/Lander.aspx


Existing directories can be converted to Web Applications

•
winserv1
-
> Sites
-
> Default Web Site
-
>xiaotest

•
Right
-
click on Lander

•
Convert to Application

•
Application pool: defualtAppPool

•
http://winserv1.cs.uakron.edu/xiaotest/Lander/Lander
.aspx



Development of Web forms using Visual Studio .NET

•

Create a Web Application project

•

Use forms designer to design forms by drag
-
and
-
drop

controls from a palette onto the forms.

•

Edit the HTML.

•

Write event handlers by double
-
clicking controls and filling

in empty method bodies.

•

Compile and run applications by executing simple menu

commands.


Web Forms and Visual Studio .NET

Summary

Web Programming, Client Side, Server Side, HTTP, HTML, HTML
Forms, CGI, ISAPI, ASP, ASP.NET, ASP.NET Forms, ASP.NET
Controls, IIS, Web Forms Programming Model, Page
-
Level Events,
Code Behind.




Principles:



What are they?


How do they work? (internals, static structures, dynamic
data/event flow.)



Practice:


Explain the logic, output, and internal working of a given
code segment.


Find errors in a given code segment.


Write a simple program to perform a given task.