WWW / Internet
Aaron Bloomfield
CS 415
Fall 2005
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Impact of Internet on Programming
Languages
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Formatting Languages
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HTML Background
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HyperText Markup Language
Tim Berners-Lee– 1989
Subset/Instance of SGML
Developed to support WWW
Developed as a document-layout language
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Markup Languages
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Used to encode formatting
Markup tags embedded in text
Output device interpreted tags
Originally used in word processing
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SGML
• Standard General Markup Language
• Rules for defining logical structure of a
document
• Basis for creating markup languages
– Essential info easily transferred
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HTML
• HTML file processed and displayed by Web
Browser
• Passive language
– Program that displays the html code decides
how to interpret the description
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Tags
• Tell Web Browser how to format text on
screen
• Always enclosed in angle brackets: <tag>
• Not case sensitive
• Whitespace does not matter
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Anchors
• Allow jumping to different spots
• HREF
– Inserts a hyperlink
<A HREF=http://www.virginia.edu>UVA</A>
• Name
– Allows a different spot in the document to be
jumped to directly
<A NAME=“sec2”>Section 2</A>
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Images
• Only need image URL
• SRC tag
• Images can be placed anywhere
– Align
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HTML Evolution
• W3C & IETF: HTML standards
• Internet
Drafts:
published,
tested,
commented on, and become Document
Type Definitions
• Browser authors add to HTML
– Support it in their browsers
– Community accepts or abandons additions
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PDF
• Portable Document Format
• The second generation of PostScript
• PDF files retain their formatting across
different viewing environments
• Applications exist for converting documents
to PDF
• Can contain hyperlinks
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Tex
• Document preparation system
• Typesetting for math and technical material
• Very close control over document
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Tex: File Creation
• Tex file created with regular text editor
• Tex converts it into a DVI (DeVice
Independent) file
• DVI file read by another program
• Creating DVI file allows for 1 file to
produce the same output by different
reading programs
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LaTeX
• Macro package for Tex
• Used for document preparation
• Author does not format document
– Specify defaults for document classes
• You can create your own document classes
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Latex Example
\documentclass{article}
\title{Latex Example}
\begin{document}
Body text goes here.
\end{document}
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PL Concepts
• Formatting languages are not PLs
• But their use employs:
– Parsing
– Generating output in a different form
– Nesting
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XML
the eXensible Markup Language
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What is XML?
• XML is a markup languge used to represent
data in a structured, portable manner
– It allows programmers to use a standard parser
for many different types of data without having
to worry about small changes in document
structure affecting their parsing code
– It allows for easy interchange of data between
heterogenous systems
• Why do we care about XML? Its not a
programming language.
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Example: Gradebook
• Without XML:
Doe, John 80 75 60 92
Doe, Jane 81 90 54 84
• With XML:
<grades>
<student name=”Doe, John”>
<grade name=”hw1”>80</grade>
<grade name=”hw2”>75</grade>
...
</student>
...
</grades>
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XML isn't perfect
• For some applications the tags make up a
significant fraction of a documents size
– In general, this doesn't matter
• XML by itself doesn't provide a whole lot of
services
– Things like linking, templating, parsing, and defining
schemas are provided by other languages
• It isn't really that revolutionary
– It is very similar to SGML and ASN1
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XML DTD
<?xml version=”1.0”?>
<!ELEMENT grades (student+)>
<!ELEMENT student (grade+)>
<!ATTLIST student name #CDATA
#REQUIRED>
<!ATTLIST student id #CDATA>
<!ELEMENT grade (#CDATA)>
<!ATTLIST grade name #CDATA #REQUIRED>
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Why do we use DTDs
• They show an agreement between two
groups on a specific way of expressing some
data.
– There are tools that will verify a document to a
DTD
• They don't let you enforce restrictions
beyond the structure of the document
– Without another tool there isn't a way to require
that a grade tag only encloses a number.
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Real-world uses of XML
• Data exchange is great, but how is XML
really used?
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RSS/RDF
FOAF
SOAP/XMLRPC
Content Management/Syndication
Streaming Video (SML)
Microsoft Word
... and much, much more
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Content Mangement
• Many businesses like to write content once,
and format it for a variety of destinations
– We want to have a version for the web (HTML),
cellphones (WML), and print (SGML).
– We don't want to have to make each version by
hand
– Gererally it works like this:
• A story is written and marked up in XML
• CMS software uses stylesheets to create the different
output from the same source
• Everybody is happy
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Style sheets for XML
• The most common style sheet language is
XSLT: the eXensible StyLesheet and
Templating language.
– It allows us to transform one XML document
into another document.
– Unlike XML, XSLT is a programming language
– XSLT programs just are expressed in XML
– XSLT is interpreted
– XSLT is dynamically typed and type-safe, does
dynamic type checking, and is statically scoped
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Example: gradebook stylesheet
<xsl:stylesheet version = '1.0'
xmlns:xsl='http://www.w3.org/1999/XSL
/Transform'>
<xsl:template match=”/”>
<xsl:apply-templates />
<xsl:template />
<xsl:template match=”student”>
<h1><xsl:value-of
select=”@name”></h1>
<table><xsl:applytemplates />
</table>
</xsl:template>
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Gradebook continued
<xsl:template match=”grade”>
<tr>
<td><xsl:value-of
select=”@name”/></td>
<td><xsl:value-of
select=”.”/></td>
</tr>
</xsl:template>
</xsl:stylesheet>
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More XSLT
• We can also do conditional processing
– <xsl:variable name=”thisgrade”
select=”.”/>
<xsl:if test=”$thisgrade < 60”>
<h2><xsl:value-of select=”.”/></h2>
<xsl:if/>
<xsl:if test=”not ($thisgrade < 60)”>
<xsl:value-of select=”.”/>
<xsl:if/>
• ...and other things
<xsl:value-of “sum(./grade)
count(./grade)” />
div
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Internet Applications and
Multimedia
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History
• People like to be able to use their senses
• However, the space that digital pictures,
sounds, and video required was far more
than a disk could hold
• ~1994 the Internet became more popular in
the common household
• However, browsers were originally mostly
text based and connections were too slow to
support large files
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Flash
• Macromedia created Flash which, in one
simple program allowed people to develop
multimedia content
• Flash uses vector graphics, which is very
space efficient and incorporates other
methods for compressing media types
• Also provides streaming content, extremely
useful to an internet user
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ActionScript
• ActionScript is Flash’s scripting language
• From a programmer’s point of view, makes
Flash a lot easier to work with
• The programmer has complete control over
what is going on in their application
• Enhances one’s ability to create an
interactive
application,
be
that
a
presentation, a game, or a movie
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ActionScript cont.
• ActionScript and Flash are primarily used today as
a 2D graphics environment.
• However, people are also using ActionScript to
create interactive forms on the web
• Another useful application is that a company
could store something on it’s main server, and
when they have to give a presentation out in
Seattle, they could just access that and have real
time stream and interactivity
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More on ActionScript
• In ActionScript, we don’t need to
specify what type the variable is.
ex. favColor = “pink”; sets the variable named ‘favColor’
to a string with the value “pink”.
• Also, favColor can later be changed to
another type.
ex. (following first example) favColor = 1; sets the
variable ‘favColor’ to the number 1.
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More on ActionScript
• However, if you want a function to only
take a certain type and only return a certain
type, you can do something like this:
ex. Function doThis(myWord:String):Number { }
• Also, ActionScript (being part of Flash) was
designed with interaction in mind.
ex.
on (release)
{clip2._visible = false; }
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More on ActionScript
• There are currently other languages out
there which are intended to achieve the
same results as ActionScript.
• The popular ones include JavaScript and
upcoming Avalon.
• Using ActionScript, a programmer can
extremely enhance a presentation with
sound, graphics, video, and their overall
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layout
More on ActionScript
• Both ActionScript and JavaScript are based
on the ECMA-262 (The European
Computers Manufacturers Association)
• One big difference is that ActionScript
doesn’t support browser specific objects and
commands.
• Microsoft’s future OS is planning on
distributing Avalon as a feature with it. Will
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support 2D and 3D vector graphics.
Internet Applications
(Applets)
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Applets
• One common misconception is that an
applet refers to Java applets, which isn’t
necessarily the case
• Something to note about these applications
is that they can use the computing power of
the server or of the user
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Java Applets
• Even though we have stated that applets do not
necessarily have to be Java, this is a predominant
form
• One crucial feature of Java and Java’s Virtual
Machine is that it is machine-independent
• To do this, applets need to be able to access some
information about the user’s machine, but Java
Applets are not allowed/able to read or write files
on/to the users machine
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Interpreted/Compiled
• Originally, the Java Applets for the Java
Virtual Machine were intended to be
Interpreted
• This has its weaknesses, so there are two
other methods that the JVM can use
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More on the JVM
• One thing that the JVM can do is look at what part
of the program that is being most heavily used and
convert that into machine code
• The other option is the Just-in-Time (JIT)
Compiler
• This takes the Java bytecode and converts it to
instructions that can be directly performed by the
processor (also using machine code)
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More on the JVM
• Users of ActionScript like to point out that
whatever they create will look the same on any
machine (though you do need to download the
Flash viewer) whereas JavaScript can be
interpreted differently in different (especially the
less mainstream) browsers
• Along the same lines, Java applets (as previously
stated) were designed to be machine independent
and could potentially even work on someone’s cell
phone
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PERL & CGI
Practical Extraction & Reporting Language
(Pathologically Eclectic Rubbish Lister)
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Agenda
• Brief History
• Language Overview
• Benefits of using Perl (Efficiency,
Portability)
• Common Gateway Interface (CGI)
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Brief History
• Developed by Larry Wall
• Perl 1.000 released in 1987
• Perl 3.000 released in 1988 under GNU Public
License
• Current stable release: Perl 5.8.1
• Today, Perl comes standard on most operating
systems (e.g. Solaris, Redhat, Mac OS X)
• 3rd party companies offer pre-built Perl
distributions for other OSs (e.g. Windows)
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What is Perl?
• Interpreted language
• Wall originally intended Perl for:
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Scanning arbitrary text files
Extracting information
Printing reports from extracted info
System management tasks
• Expression syntax borrows heavily from
other languages (C, sed, awk, sh)
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Wall on Perl
• “Perl is a language for getting your job
done.”
• “Perl is designed to make easy jobs easy,
without making hard jobs impossible.”
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$str = “Hello, world!”;
sub PrintHello {
while(1) {
if ($str =~ m/o..w/) {
print “$str\n”;
}
else {
print “NO MATCH\n”;
break;
}
}
}
&PrintHello;
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Language Overview - Types
• 3 primitive types: $scalars, @arrays, and %hashes
(associative arrays)
• A $scalar variable may be a number or a string
– Conversion between the two is automatic pending on
your operator usage (e.g. “= =“ OR “eq”)
• @arrays and %hashes are dynamically sized
• A %hash consist of a set of key-data pairs
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Type Examples
$helloStr = “Hello, world!”;
$someNum = 415;
@myArray = ($helloStr,
$someNum, 1, 2.2, “3”);
%myHash{“key”} = $data;
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Control Statements
• Conditional branching:
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if (condition) { statement(s); }
else { statement(s); }
elsif (condition) { statement(s); }
unless (condition) { statement(s); }
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Control Statements - cont’d
• Iteration:
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while (condition) { statement(s); }
until (condition) { statement(s); }
for ( initialize; condition; re-initialize; ) { }
foreach $scalar (@list) { };
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Functions & Scopes
• Perl allows for recursion with proper
scoping declarations
• Defining a subroutine:
sub subname {
statement(s);
}
• Calling a subroutine:
&subname;
subname()
• Scoping keywords: my, our, and local
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Regular Expressions
• One of Perl’s primary purposes was to process text
• Perl has built-in pattern matching – useful for
searching and manipulating strings
• Perl’s regular expressions are derived from
established regular expressions found in popular tools
like sed, awk, grep, and vi
• regex operators:
=~ matches, !~ doesn’t match
m/ PATTERN /
s/ PATTERN / PATTERN /
tr/ PATTERN / PATTERN /
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Perl Benefits - Portability
• Perl programs are not platform dependant
– Comes standard on many different operating systems
– Avoid C’s #IFDEF statements
– Does not guarantee that a program uses platform
dependant tools
• regex widely used (grep, sed, awk, ed, vi, emacs,
etc)
• Perl is a semantic superset of the above
– any regex that can be described in the above can also be
described in Perl (but maybe using different characters
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Perl Benefits – Efficiency
• Use of hashes instead of linear searches
through an array
• Use of foreach instead of for loops
– No need to have extra operation on counter
• regex for fast pattern matching
searching large amounts of data
• Use of AutoLoader module
for
– Defines functions the first time they’re called
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Common Gateway Interface (CGI)
• CGI script – a program on the web server which
runs on demand to generate the content of a web
page
• You can find CGI scripts everywhere on-line:
– Used to access and manipulate databases
– On-line shopping
• Popular CGI scripting languages:
– Perl, PHP, ASP, Python
– But really, anything that is executable on the server
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What happens when a script runs?
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CGI with Perl
• Why use Perl to create CGI scripts?
– Free software
– Fast and powerful string manipulation (imagine
doing the same thing with C/C++)
– Built-in types and functions that are useful for
handling records and information (arrays and
hashes)
– Comes standard on almost all web servers
– Standard CGI module
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CGI Example – Server-side
#!/usr/bin/perl
use CGI ':standard';
print header, start_html('Hello
World'), b('And hello to you
too'), end_html;
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CGI – Example – Client-side
Content-type: text/html
<HTML><HEAD><TITLE>Hello
World</TITLE><BODY>
<STRONG>And hello to you
too</STRONG>
</BODY></HTML>
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Perl/CGI forms
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Example 1: Standard CGI script
• Purpose was to allow computations of the form ne
mod m
– In particular, where the numbers involved were 500 digit
integers
• Used in RSA encryption
– Provided to my CS 202 class
• The script creates the HTML page
– I’ve provided the HTML page separately to show what the
HTML code looks like
• Note that the script is synchronous
– It requires the page be re-loaded with the new data
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Example 2: AJAX script
• AJAX stands for Asynchronous Java And Xml
– Does not require the page be re-loaded to display new
data
• Google Maps is an example of an AJAX script
• Two files involved
– ajax.html: has the HTML and Javascript necessary to
load the content without reloading the page
– ajax: the Perl script itself, it’s pretty simple
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URLs
• Example 1
– Script can be run at http://www.cs.virginia.edu/cgibin/cgiwrap/cs415/modpow
– Script can be viewed at
http://www.cs.virginia.edu/~cs415/cgi-bin/modpow
• Example 2
– ajax.html:
http://www.cs.virginia.edu/~cs415/code/ajax.html
– ajax script: http://www.cs.virginia.edu/~cs415/cgibin/ajax
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