Lecture #2 xml

D e p a r t m e n t o f C o m p u t e r E n g i n e e r i n g
D o k u z E y l ü l U n i v e r s i t y
Lecture #2
XML Introduction
Dr.Adil Alpkocak
Dokuz Eylul University
Dept of Computer Engineering
CME2002
Data Organization and
Management

Today
• Field and record organization
• XML introduction
2

Files
A file can be seen as
1. A stream of bytes (no structure), or
2. A collection of records with fields
3

#1. File as stream bytes
• File is viewed as a sequence of bytes:
• Data semantics is lost: there is no way to
get it apart again.
4
87359CarrollAlice in wonderland38180FolkFile Structures ...

#2: File as a Collection of Records
Definitions
File: Collection of records.
Record: a collection of related fields.
Field: the smallest logically meaningful unit of
information in a file.
Key: a subset of the fields in a record used to
identify (uniquely) the record.
e.g. In the example file of books:
– Each line corresponds to a record.
– Fields in each record: ISBN, Author, Title
5

Different Notation
column row Table
Field Record File
Attribute Tuple Relation
Terms in the same column are synonyms, and can be used interchangeably.

Record Keys
• Primary key: a key that uniquely identifies a
record.
• Secondary key: other keys that may be used for
search
– Author name
– Book title
– Author name + book title
• Note that in general not every field is a key (keys
correspond to fields, or a combination of fields,
that may be used in a search).
7

Record Structures
1. Fixed-length records.
2. Fixed number of fields.
3. Begin each record with a length
indicator.
4. Use an index to keep track of
addresses.
5. Place a delimiter at the end of the
record.
8

1.Fixed-length records
Two ways of making fixed-length records:
1. Fixed-length records with fixed-length
fields.
2. Fixed-length records with variable-
length fields.
9
87359 Carroll Alice in wonderland
03818 Folk File Structures
87359|Carroll|Alice in wonderland| unused
38180|Folk|File Structures| unused

Fixed-Length Records
• e.g.
struct PERSON {
char last[11];
char first[11];
char address[16];
char city[16];
char state[3];
char zip[10];
} person;
Will produce a fixed size record of size 67
bytes.
10

Fixed-Length Records
• The fixed length record structure,
however, does NOT imply, the fixed -
length field structure.
• Fixed-length records are frequently
used as “containers” to hold variable
numbers of variable-length fields.
• Fixed-length record structures are
among the most commonly used
methods for organizing files.
11

Field Structures:
1. Fixing the Length of Fields
This method relies on creating fields
of predictable fixed size.
struct PERSON {
char last[11];
char first[11];
char address[16];
char city[16];
char state[3];
char zip[10];
} person
14

Field Structures:
2. Beginning Each Field with a Length
Indicator
 This method requires that each field data
be preceded with an indicator of its length
(in bytes).
 E.G.
04Ames04Mary09123 Maple10StillWater02OK0574075
 One of the disadvantages of this method is that it
is more complex since it requires extracting of
numbers and strings from a single string
representing a record.
15

Field Structures:
4. Using a “keyword = value” expression
 This method requires that each field data be
preceded with the field identifier (keyword).
last=Ames
first=Mary
address=123
Maplecity=StillWater
state=OK
zip=574075
 Can be used with the delimiter method to
mark the field ends.
last=Ames|first=Mary|address=123
Maple|City=StillWater|state=OK|zip=574075
17

Field Structures:
4. Using a “keyword = value”
expression
• Advantages:
• each field provides information about itself
• good format for dealing with missing fields
• Disadvantages:
• In some application a lot of space may be wasted
on field keywords (up 50%).
18

XML
EXtensible Markup Language

XML
• is a markup language that defines a set of
rules for encoding documents in a format
that is both human-readable and machine-
readable.

What is XML?
• XML stands for EXtensible Markup Language
• XML is a markup language much like HTML
• XML was designed to describe data
• XML tags are not predefined. You must define your
own tags
• XML uses a Document Type Definition (DTD) or
an XML Schema (XSD) to describe the data
• XML with a DTD or XSD is designed to be self-
descriptive
• XML is a W3C Recommendation.

Simple XML example
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this
weekend!</body>
</note>

The Main Difference Between XML
and HTML
XML was designed to carry data.
• XML and HTML were designed with
different goals:
– XML was designed to describe data and to
focus on what data is.
– HTML was designed to display data and to
focus on how data looks.
• HTML is about displaying information,
while XML is about describing data.
• XML is not a replacement for HTML.

XML is Used to Exchange Data
With XML, data can be exchanged between
incompatible systems
• In the real world, computer systems and databases
contain data in incompatible formats.
• One of the most time-consuming challenges for
developers has been to exchange data between
such systems over the Internet.
• Converting the data to XML can greatly reduce this
complexity and create data that can be read by
many different types of applications.

XML Can be Used to Share Data
With XML, plain text files can be used to share data
• Since XML data is stored in plain text format, XML
provides a software- and hardware-independent way
of sharing data.
• This makes it much easier to create data that
different applications can work with.
• It also makes it easier to expand or upgrade a
system to new operating systems, servers,
applications, and new browsers

XML Can be Used to Store Data
With XML, plain text files can be used to
store data
• XML can also be used to store data in files
or in databases.
• Applications can be written to store and
retrieve information from the store, and
generic applications can be used to
display the data

XML Can Make your Data More
Useful
With XML, your data is available to more users.
• Since XML is independent of hardware, software
and application, you can make your data available
to other than only standard HTML browsers.
• Other clients and applications can access your XML
files as data sources, like they are accessing
databases.
• Your data can be made available to all kinds of
"reading machines" (agents), and it is easier to
make your data available for blind people, or people
with other disabilities.

XML Can be Used to Create New
Languages
XML is the mother of WAP and WML
• The Wireless Markup Language (WML),
used to markup Internet applications for
handheld devices like mobile phones, is
written in XML.
• For a list of XML based languages refer to
wikipedia pages.

XML Syntax Rules
• The syntax rules of XML are very simple
and very strict.
• The rules are very easy to learn, and very
easy to use.
• Because of this, creating software that can
read and manipulate XML is very easy.

An Example XML Document
XML documents use a self-describing
and simple syntax
<?xml version="1.0" encoding="ISO-8859-1"?>
<note>
<to>Tove</to>
<from>Jani</from>
<body>Don't forget me this weekend!</body>
</note>

All XML Elements Must Have a
Closing Tag
With XML, it is illegal to omit the closing
tag
• In XML all elements must have a closing
tag, like this:
This is a paragraph
This is another paragraph

XML Tags are Case Sensitive
Unlike HTML, XML tags are case
sensitive
• With XML, the tag <Letter> is different
from the tag <letter>.
• Opening and closing tags must therefore
be written with the same case:
<Message>This is incorrect</message>
<message>This is correct</message>

XML Elements Must be Properly
Nested
Improper nesting of tags makes no sense to XML
• In HTML some elements can be improperly nested
within each other like this:
• In XML all elements must be properly nested within
each other like this:
This text is bold and italic
This text is bold and italic

XML Documents Must Have a Root
Element
All XML documents must contain a single tag pair
to define a root element
• All other elements must be within this root element.
• All elements can have sub elements (child
elements). Sub elements must be correctly nested
within their parent element:
<root>
<child>
<subchild>.....</subchild>
</child>
</root>

XML Attribute Values Must be
Quoted
With XML, it is illegal to omit quotation marks around attribute
values
<note date=12/11/2002>
<to>Tove</to>
<from>Jani</from>
</note>
<note date="12/11/2002">
<to>Tove</to>
<from>Jani</from>
</note>

With XML, White Space is Preserved
• With XML, the white space in your document is
not truncated.
• This is unlike HTML. With HTML, a sentence like
this:
Hello my name is Tove,
will be displayed like this:
Hello my name is Tove,
• because HTML reduces multiple, consecutive white
space characters to a single white space

With XML, CR / LF is
Converted to LF
With XML, a new line is always stored as LF
• In Windows applications, a new line is normally
stored as a pair of characters: carriage return (CR)
and line feed (LF).
• The character pair bears some resemblance to the
typewriter actions of setting a new line. In Unix
applications, a new line is normally stored as a LF
character.
• Macintosh applications use only a CR character to
store a new line.

Comments in XML
• The syntax for writing comments in XML is
similar to that of HTML.

There is Nothing Special About XML
• It is just plain text with the addition of some XML
tags enclosed in angle brackets.
• Software that can handle plain text can also handle
XML. In a simple text editor, the XML tags will be
visible and will not be handled specially.
• In an XML-aware application however, the XML tags
can be handled specially.
• The tags may or may not be visible, or have a
functional meaning, depending on the nature of the
application.

XML Elements
• XML Elements are extensible and they
have relationships.
• XML Elements have simple naming rules.

XML Elements are Extensible
• XML documents can be extended to carry more information.
<note>
<to>Tove</to>
<from>Jani</from>
</note>
MESSAGE
To: Tove
From: Jani
Don't forget me this weekend!

XML Elements are Extensible
• XML document added some extra information to it:
MESSAGE
To: Tove
From: Jani
Don't forget me this weekend!
<note>
<date>2002-08-01</date>
<to>Tove</to>
<from>Jani</from>
</note>

XML Elements have Relationships
• Imagine that this is a description of a book:
My First XML
Introduction to XML
What is HTML
What is XML
XML Syntax
Elements must have a closing tag
Elements must be properly nested

XML Elements have Relationships
• Imagine that this XML document describes the book:
<book>
<title>My First XML</title>
<prod id="33-657" media="paper"></prod>
<chapter>Introduction to XML
<para>What is HTML</para>
<para>What is XML</para>
</chapter>
<chapter>XML Syntax
<para>Elements must have a closing tag</para>
<para>Elements must be properly nested</para>
</chapter>
</book>

Elements have Content
Elements can have different content
types.
• An XML element is everything from
(including) the element's start tag to
(including) the element's end tag.
• An element can have element content,
mixed content, simple content, or empty
content. An element can also have
attributes.

Element Naming
XML elements must follow these naming
rules
• Names can contain letters, numbers, and
other characters
• Names must not start with a number or
punctuation character
• Names must not start with the letters xml
(or XML, or Xml, etc)
• Names cannot contain spaces

Element Naming
Take care when you "invent" element
names and follow these simple rules
• Any name can be used, no words are
reserved, but the idea is to make names
descriptive. Names with an underscore
separator are nice.

XML Attributes
• XML elements can have attributes in the
start tag, just like HTML.
• Attributes are used to provide additional
information about elements.

XML Attributes
• XML elements can have attributes.
• From HTML you will remember this: <IMG
SRC="computer.gif">. The SRC attribute provides
additional information about the IMG element.
• In HTML (and in XML) attributes provide additional
information about elements:
<img src="computer.gif">
<a href="demo.asp">

XML Attributes
• Attributes often provide information that is
not a part of the data.
• In the example below, the file type is
irrelevant to the data, but important to the
software that wants to manipulate the
element:
<file type="gif">computer.gif</file>

Quote Styles, "female" or 'female'?
• Attribute values must always be enclosed
in quotes, but either single or double
quotes can be used.
• For a person's sex, the person tag can be
written like this:
or
<person sex="female">
<person sex='female'>

Quote Styles in Attributes
• If the attribute value itself contains double quotes it
is necessary to use single quotes, like in this
example:
• If the attribute value itself contains single quotes it is
necessary to use double quotes, like in this
example:
<gangster name='George "Shotgun" Ziegler'>
<gangster name="George 'Shotgun' Ziegler">

Use of Elements vs. Attributes
• Data can be stored in child elements or in
attributes.
<person sex="female">
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>
<person>
<sex>female</sex>
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>

• Following XML documents contain exactly
the same information:
• Alternative #1
<note date="12/11/2002">
<to>Tove</to>
<from>Jani</from>
</note>

• Alternative #2
<note>
<date>12/11/2002</date>
<to>Tove</to>
<from>Jani</from>
</note>

• Alternative #3
<note>
<date>
<day>12</day>
<month>11</month>
<year>2002</year>
</date>
<to>Tove</to>
<from>Jani</from>
</note>

• Alternative #4
<note day="12" month="11" year="2002“ to="Tove"
from="Jani" heading="Reminder"
body="Don't forget me this weekend!">
</note>

Some of the problems with using
attributes
1. attributes cannot contain multiple values (child
elements can)
2. attributes are not easily expandable (for future
changes)
3. attributes cannot describe structures (child
elements can)
4. attributes are more difficult to manipulate by
program code
5. attribute values are not easy to test against a
Document Type Definition (DTD) - which is used
to define the legal elements of an XML
document

Rule of thumb
• If you use attributes as containers for data,
you end up with documents that are
difficult to read and maintain.
• Try to use elements to describe data.
• Use attributes only to provide information
that is not relevant to the data.

An Exception to Attribute Rule
 The ID in this example is just a counter, or a
unique identifier, to identify the different notes in
the XML file, and not a part of the note data.
<messages>
<note id="p501">
<to>Tove</to>
<from>Jani</from>
</note>
<note id="p502">
<to>Jani</to>
<from>Tove</from>
<heading>Re: Reminder</heading>
<body>I will not!</body>
</note>
</messages>

XML Validation
• XML with correct syntax is Well Formed
XML.
• XML validated against a DTD is Valid
XML.

Well Formed XML Documents
A "Well Formed" XML document has
correct XML syntax.
XML documents must have a root element
XML elements must have a closing tag
XML tags are case sensitive
XML elements must be properly nested
XML attribute values must always be
quoted

Valid XML Documents
A "Valid" XML document also conforms to a DTD
<!DOCTYPE note SYSTEM "InternalNote.dtd">
<note>
<to>Tove</to>
<from>Jani</from>
</note>

XML DTD
A DTD defines the legal elements of an
XML document
• The purpose of a DTD is to define the
legal building blocks of an XML document.
• It defines the document structure with a list
of legal elements.

XML Schema (XSD)
• XML Schema is an XML based alternative
to DTD
• W3C supports an alternative to DTD called
XML Schema.

Displaying XML with XSL
• XSL is the preferred style sheet
language of XML
• XSL (the eXtensible Stylesheet Language)
is far more sophisticated than CSS.
• One way to use XSL is to transform XML
into HTML before it is displayed by the
browser as demonstrated in these
examples:
• XML file, the XSL style sheet, and View
the result.

XML File
<?xml-stylesheet type="text/xsl"
href="simple.xsl"?>
<breakfast_menu>
<food>
<name>Belgian Waffles</name>
<price>$5.95</price>
<description>
two of our famous Belgian Waffles
</description>
<calories>650</calories>
</food>
</breakfast_menu>

Lecture #2 xml

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