Get documents about "XML And XPath
Document Sample
XML And XPath
DSA Term 2
Week 14
DSA/2006/week 14 1
Lecture overview
• Matters arising
– Character coding
• Well-formed XML
• Creating simple XML files
• Placename to BBC code
• Introduction to XPath
DSA/2006/week 14 2
Character Coding
• Character set
– ISO 8549 - 1 Byte
• 0 - 127 are ASCII
• 128- 255 vary depending on the part of the standard
• 15 different character maps
– ISO-8859-1 - Latin -1 - the default for HTML
– ISO-8859-2 – Central European
• A document must be on one encoding
– problem of mixing characters e.g. an Arabic quotation in a Cyrillic text
– UTF-8 - Unicode 1- 4 byte variable length to support a huge range of
international languages in a single code
• ASCII is included as characters 0-127
• Ensures that the internet is truly multi-lingual
• Key invention by Ken Thompson of self-synchronisation allowing character
boundaries to be detected
• Character references in HTML
– Named °
– decimal &176;
– Hexadecimal &#B0;
DSA/2006/week 14 3
Defining the Encoding
• Encodings in HTML
– In a meta-tag
• <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
– In the xml processing instruction
• <?xml version="1.0" encoding="ISO-8859-1"?>
– In the HTTP content header
• Content-Type: text/html; charset=ISO-8859-1
• Setting Encoding in PHP
– header("Content-type: text/html; charset=UTF-8");
• Setting encoding in the Browser
– Firefox
• View/Character Encoding
DSA/2006/week 14 4
Design a simple XML file
• Design an XML vocabulary to represent
pairs of place names and codes
– Bristol 1263
– Bath 1123
• First review XML structure
DSA/2006/week 14 5
Example
<MapSet>
<Map id="P2" desc="P Block level 2">
<room id="2P2">
<area shape="rect" coords="118,39,138,68"/>
<type>Staff Room</type>
<occupant>Tony Solomonides</occupant>
</room>
<room id="2P3">
<area shape="rect" coords="141,40,162,69"/>
<type>Staff Room</type>
<occupant>Richard Lawson</occupant>
</room>
<room id="2P4">
<area shape="poly" coords="201,40,234,40,234,118,164,119,163,71,200,71"/>
<type>Office</type>
<occupant>Eleanor Gibbons</occupant>
<occupant>Dee Evans</occupant>
<occupant>Ali Jack</occupant>
</room>
….
</Map>
</MapSet> DSA/2006/week 14 6
Well-formed XML documents (1)
Every XML document must be well-formed and must therefore adhere to
the following rules (among others):
1. Every start-tag must have a matching end tag.
2. Elements may nest but must not overlap.
<name>Anna<em>Coffey</em></name> - √
<name><em>Anna</name>Coffey</em> - ×
3. There must be exactly one root element.
4. Attribute values must be quoted.
5. An element must not be quoted.
6. Comments and processing instructions may not appear inside tags.
7. No unescaped < or & signs may occur in the character data of an
element.
DSA/2006/week 14 7
Well-formed XML documents (2)
Element names are case sensitive - <NAME>, <name>, <Name> & <NaMe>
are four different element types.
No white spaces in element name - <First Name> not allowed; <First_Name>
OK.
Element names cannot start with the letters “XML” or “xml” – reserved terms.
Element names must start with a letter or a underscore. Element names
cannot start with a number but numbers may be embedded within an element
name - <2you> not allowed; <me2you> is OK.
Attribute names are constrained by the above rules for element names.
Entity references are used to substitute specific characters. There are five
predefined entities built into XML:
Entity Char Notes
& & Do not use inside processing instructions
< < Use inside attribute values quoted with “.
> > Use after ]] in normal text and inside processing instruction.
" “ Use inside attribute values quoted with “.
' „ Use inside attribute values quoted with „.
DSA/2006/week 14 Map
8
Errors
• Look at the listing of the XML file and
identify all the places which prevent this
XML from being well-formed
DSA/2006/week 14 9
<Map id=P2 desc="P Block level 2'>
<room id="2P2">
This is a nice big office
<area rect coords="118,39,138,68">
<typo>Staff Room</typo>
<occupant>Tony Solomonides</occupant>
</Room>
<room id="2P3">
<area rect coords="141,40,162,69"></area>
<typo>Staff Room</typo>
<occupant>”Richard Lawson”</occupant>
</Room>
<room id="2P4">
<area poly coords="201,40,234,40,234,118,164,119,163,71,200,71"/>
<typo>Office</typo>
<occupant>Eleanor Gibbons</occupant>
<person>Dee Evans</person>
<occupant>Ali Jack</occupant
</Room>
--- DSA/2006/week 14 10
Task
• Draw the structure
– Use ER notation
• Attributes in the Entity
• Cross-foot notation for one-many, optional
• Identify any restricted sets of values (ennumerated
types)
– In the lab, QSEE will allow you to define the
structure and generate the schema definition
(XML Schema or DTD)
DSA/2006/week 14 11
DSA/2006/week 14 12
XPATH
• Core language for selecting nodes in XML
• Version 1.0 used in XSLT 1.0
– client-side in Browsers
– xalan engine
– w3.schools Tutorial is for XPath 1.0
– SimpleXML in PHP
• Version 2.0 used in XSLT 2.0
– Saxon parser
– XQuery 1.0
• Differences
– Code data structure in 2.0 is a node sequence
– Full support for all XML schema datatypes
– Two kinds of equality operators
– Larger function library
DSA/2006/week 14 13
XPath Language
• Not a programming language
• Expressions to be evaluated
• Focus on
– Navigation in a tree structure
• Multiple directions or „axes‟
– Down to children (child axis)
– Up to parent (parent axis)
– Down to attributes (attribute axis)
– Across to siblings (sibling axis)
– Operators
– Functions
DSA/2006/week 14 14
XPATH DOM
Declarative Procedural (navigational)
Root / document
Context Node .
Current
Directory
(self)
Local Name n
Child * n.childNodes
[1] n.firstChild
[last()] n.lastChild
Parent .. f.parentNode
Sibling ../g f.nextSibling
../* f.previousSibling
Attribute f/@att f.getAttribute(att)
Locate node by ID //[@id='f1'] document.getElementById('f1')
Locate nodes by //img document.getElementsByTagName('img')
Tag
Number of nodes in count(s) s.length
sequence
Select by predicate [@size= 10 and . ='fred']
DSA/2006/week 14 15
XPath operators
• Arithmetic operators
+ - * div idiv mod
• Value comparisons
eq, le, ge, gt, lt
• Sequence comparisons
= , !=
= is true if there are common elements
!= is true if there are no common elements
(1,2,3) = (2,3,4) is true
(1,2,3) != (2,3,4) is also true
not ((1,2,3) = (2,3,4) ) is false
• Logical operators
and, or, not()
DSA/2006/week 14 16
large function library
• count (seq) , max((seq)) ,min((seq)),
average
– count(1,2,3) = 3
• max, min
• string functions
– string-length(„abc‟)
– tokenize(„a,b,c‟,‟,‟)
– string-join((a,b,c),‟, „)
DSA/2006/week 14 17
Using the eXist database
• eXist database as an XPath / XQuery engine.
– Rest interface
• ..exist/rest/db/chriswallace/rooms?_query=//Map
– Java client
– Sandbox (using Ajax to do dynamic syntax checking)
• Context is the whole database
• The demo database includes
– the whole text for Romeo and Juliet
– the mondial world database
DSA/2006/week 14 18
Examples
• all Rooms
– /MapSet/Map/room
– //room
• room 2P5
– //room[@id=„2P5‟]
• the occupants of room 2P4
– //room[@id=„2P4‟]/occupant
• the roomNo of the room which Colin Fudge occupies
– //room[occupant = „Colin Fudge‟]/@id
• the number of occupants of 2P4
– count(//room[@id=„2P4‟]/occupant)
• The floor of Ali Jack‟s room
– //room[occupant = „Ali Jack‟]/../@desc
DSA/2006/week 14 19
Notes
• Note how = tests if a person is amongst
the occupants
• To „serialise‟ an attribute use string()
• See how ../ allows navigation to the parent
element
DSA/2006/week 14 20
Examples for you
• The room number for Richard Lawton
• The coordinates of room 2P2
• All rooms with poly shape
• Who are Ali Jack‟s office mates?
DSA/2006/week 14 21
XML design
• Rooms is a mixture of text elements and
attributes.
• Could be all attributes – what would change?
• Could be no attributes – what would change?
• For the workshop exercise use elements instead
of attributes – its simpler even if more verbose
• Generally, what do the experts recommend?
DSA/2006/week 14 22
Workshop
• Create a simple XML file containing pairs
of Place names and BBC codes
• Change the PHP script to accept a
placename
• Read the new xml file and decode the
name to get the code using PHP
SimpleXML interface and xpath(„‟)
DSA/2006/week 14 23
Related docs
