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--- tanszek:oktatas:techcomm:dtd [2024/10/07 17:33] – created knehez
+++ tanszek:oktatas:techcomm:dtd [2025/12/02 08:01] (current) – knehez
@@ Line 14: / Line 14: @@
 === Internal declaration ===
-<sxh dtd>
+<sxh>
-<!DOCTYPE uzenet [
+<!DOCTYPE message [
         ....
 ]>
@@ Line 21: / Line 21: @@
 === External URI ===
-<sxh dtd>
+<sxh>
 <!DOCTYPE html PUBLIC "-//W3//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/......dtd">
 </sxh>
 === External file ===
-<sxh dtd>
+<sxh>
-<!DOCTYPE uzenet SYSTEM "filename.dtd">
+<!DOCTYPE message SYSTEM "filename.dtd">
 </sxh>
 The DTD defines the structure and rules that an XML document must follow to be considered valid.
+==== DTD Through Examples ====
+Here is a DTD file named ''message_syntax.dtd'' and the corresponding XML on the right:
+<sxh>
+<!ELEMENT message ( text )>
+<!ELEMENT text ( #PCDATA )>
+</sxh>
+<sxh xml>
+<?xml version = "1.0" encoding = "UTF-8"?>
+<!DOCTYPE message SYSTEM "message_syntax.dtd">
+<message>
+   <text>Hello XML</text>
+</message>
+</sxh>
+In the XML structure, a **comma** indicates the required order of elements:
+<sxh>
+<!DOCTYPE studygroup [
+        <!ELEMENT group (teacher, student)>
+        <!ELEMENT teacher ( #PCDATA ) >
+        <!ELEMENT student ( #PCDATA ) >
+]>
+</sxh>
+<sxh xml>
+<studygroup>
+    <teacher>Kiss Janos</teacher>
+    <student>Gipsz Jakab</student>
+</studygroup>
+</sxh>
+The **pipe symbol `|`** expresses an **either-or** relationship. For example, the storage device can be either a **pendrive** or an **SSD**, but not both:
+<sxh>
+<!DOCTYPE datastore [
+    <!ELEMENT datastorage  (pendrive | SSD)>
+    <!ELEMENT pendrive ( #PCDATA ) >
+    <!ELEMENT SSD ( #PCDATA ) >
+]>
+</sxh>
+<sxh xml>
+<datastorage>
+    <pendrive>64MB</pendrive>
+</datastorage>
+</sxh>
+There are three ways to express **frequency**:
+^ Symbol ^ Meaning ^
+| ''+''    | The element appears at least once. |
+| ''*''    | The element can appear any number of times, including zero. |
+| ''?''    | The element appears zero or one time. |
+==== Example: A music CD contains **tracks**: ====
+<sxh>
+<!DOCTYPE datastorage [
+    <!ELEMENT CD (recording + )>
+    <!ELEMENT recording ( #PCDATA ) >
+]>
+</sxh>
+<sxh xml>
+<CD>
+    <recording>Song 1</recording>
+    <recording>Song 2</recording>
+</CD>
+</sxh>
+----
+==== Example: An album has at least one title, followed by at least one **track title** and **duration**: ====
+<sxh>
+<!DOCTYPE datastorage [
+    <!ELEMENT album (title+, (tracktitle, duration)+)>
+    <!ELEMENT title ( #PCDATA ) >
+    <!ELEMENT tracktitle ( #PCDATA ) >
+    <!ELEMENT duration ( #PCDATA ) >
+]>
+</sxh>
+**Syntactically correct XML**:
+<sxh xml>
+<album>
+    <title>Title 1</title>
+    <title>Subtitle</title>
+    <tracktitle>Track Title 1</tracktitle>
+    <duration>3.42</duration>
+    <tracktitle>Track Title 2</tracktitle>
+    <duration>2.32</duration>
+</album>
+</sxh>
+----
+==== Example: A library may contain 'books' (zero or more)====
+<sxh>
+<!DOCTYPE datastorage  [
+    <!ELEMENT library (book*) >
+    <!ELEMENT book (author, title) >
+    <!ELEMENT author ( #PCDATA ) >
+    <!ELEMENT title ( #PCDATA ) >
+]>
+</sxh>
+**Syntactically correct XML**:
+<sxh xml>
+<library>
+    <book >
+        <author>Orwell, George</author>
+        <title >1984</title >
+    </book>
+    <book>
+        <author>Brown, Dan</author>
+        <title >The Da Vinci Code</title >
+    </book >
+</library>
+</sxh>
+===== Defining Attributes in DTD =====
+If a class has an attribute such as "number of students," it can be specified as follows:
+<sxh>
+<!ELEMENT class (student *) >
+<!ATTLIST class number CDATA #REQUIRED>
+</sxh>
+Attributes can be:
+  * **#IMPLIED**: not mandatory
+  * **#REQUIRED**: mandatory
+  * **#FIXED**: fixed value
+=== Specifying a Default Attribute Value ==
+Example:
+<sxh>
+<!ATTLIST paymentType type CDATA "bankTransfer">
+</sxh
+The XML could look like this:
+<sxh>
+<paymentType/> or <paymentType type="bankTransfer">
+</sxh>
+=== Enumerated Values ===
+**Syntax**:
+<sxh>
+<!ATTLIST element-name attribute-name (eval | eval | ..) default-value>
+</sxh>
+**DTD Example**:
+<sxh>
+<!ATTLIST payment type (check | cash) "cash">
+</sxh>
+**XML Example**:
+<sxh xml>
+<payment type="check"/> or <payment type="cash"/>
+</sxh>
+==== Complex DTD Example for a Hypothetical Mail Processing System ====
+**Task**: Provide an XML example that satisfies the following DTD:
+<sxh>
+<!ELEMENT mails (email*, postcard*)>
+<!ELEMENT email (address, sender, message?, attachment?)>
+<!ELEMENT postcard (address, sender?, message?)>
+<!ELEMENT address (name, postalcode, city, country)>
+<!ATTLIST address nick CDATA #IMPLIED>
+<!ATTLIST sender nick CDATA #IMPLIED>
+<!ATTLIST attachment type CDATA #REQUIRED>
+<!ATTLIST postcard scanimage CDATA #IMPLIED>
+<!ATTLIST sender name CDATA #REQUIRED>
+</sxh>
+=== A Possible XML for the given DTD ===
+<sxh xml>
+<mails>
+    <email>
+        <address nick="Alice">alice@usa.com</address>
+        <sender nick="Bob">bob@jp.com</sender>
+        <message>Hash code</message>
+        <attachment type="text/doc"></attachment>
+    </email>
+    <postcard scanimage="kep.jpg">
+       <address>
+         <name>John Doe</name>
+         <postalcode>1234</postalcode>
+         <city>Miskolc</city>
+         <country>Hungary</country>
+       </address>
+       <sender name="Bob Cat" />
+       <message>Happy Name Day</message>
+    </postcard>
+</mails>
+</sxh>
+This provides an example of how attributes, including mandatory, optional, and default values, can be defined in a DTD and represented in an XML document.