Introduction to Programming the WWW I

CMSC 10100-1

Winter 2003

A Crash Course in Perl

• Variables:– scalar, array, hash

• HTML forms

• CGI

• Files -- reading and writing

Variables

• All variables in Perl are prefixed with a special character:– $ for scalar values: $x– @ for list values: @list– % for hash values: %hash

• Both numbers and strings can be scalars• Arrays are very similar to Javascript• Hashes are “associative arrays”

Scalar variables

• For numerical values, we can add, subtract, multiply, divide, assign as in Javascript:

$x = 42;

$y = 63;

$z = $x + $y - 32;

String variables

$foo=“Hello world”;

$bar=“I’m joining the circus”;• String concatentation: $foo . $bar• String repitition: $foo x 3

Printing scalar data

• Double quotes vs. single quotes:print “$foo”; #prints Hello World

print ‘$foo’; #prints $foo

print “\$foo”; #prints $foo

• Can print result of operations:print “Hello “ . “World”;

print $foo x 3;

List variables

• Creating a list:

@mylist = (42,64,”foo”);• Accessing elements of a list:

$mylist[0] = 33;• How long is a list?

$length = $#mylist + 1;

print “$length\n”;

Adding and removing items

@classes=(‘Math’,’CS’,’History’);

• To add or remove from beginning:$class = shift(@classes);

unshift(@classes,’Dance’);

• To add or remove from end$class = pop(@classes);

push(@classes,’English’);

Doing stuff to each element

@classes = (“Math”,”English”,”CS”);

for (my $i=0;$i<=$#classes;$i++) {

$classes[$i] = “Honors “ .

$classes[$i];

}

foreach $class (@classes) {

print $class . “\n”;

}

Split and join

• If you have a line of text like:$foo = “Herbert::Smith::555-1234”;

• You can split it into three parts by($last,$first,$num) = split /::/,$foo;

• Or store the results in an array@mydata = split /::/,$foo;

• You can reverse the process$packed = join (“::”,@mydata);

Hash Variables

• A list of name/value pairs (dictionary)%grades = (“Bob”=>42,”Jane”=>45,”Jim”=>90);

• Lookup/edit in the list by key:print $grades{“Bob”} . “\n”;

$grades{“Jane”} = 48;

Other techniques

• Adding an entry:$grades{“Jill”} = 75;

• Deleting an entrydelete $grades{“Jill”};

• Does an entry exist?if exists($grades{“Jill”}) {

print “Jill is in the class.\n”;

}

Getting keys/values

• Getting the list of all keys

@students = keys(%grades);

• Getting the list of all values

@grades = values(%grades);

What’s the average grade?$num_students = $#students + 1;

$total_points = 0;

foreach $grade (@grades) {

$total_points += $grade;

}

$avg = $total_points / $num_students;

print “The class mean is $avg\n”;

HTML Forms

• All widgets are contained in the <form> tag.

<form action=“url” method=“post”>

…

</form>

Widgets

• Buttons: submit, reset, action

• Text: input, text areas, passwords

• Selectors: check, radio, lists

• Hidden fields

• Each widget (or group) is transmitted as a name/value pair to the server.

Buttons• Basic syntax<input type=“type” value=“value” name=“name”>

• submit, reset, or button specified by type• text appearing on the button given by value• can have multiple submit buttons by naming

each one

Getting text

<input type=“text” size=“”, maxlength=“”, name=“”>

• Allows user to enter a line of text up to maxlength

• Set type to password to use asterisks– not secure

Text areas

<textarea rows=“” cols=“” name=“”>

• Allows free-form entry spanning multiple lines

• Comments, input on NSF, conference registration sites

Radio buttons

• A group of several buttons, of which at most one may be checked

<input type=“radio” name=“foo” value=“a”> a

<input type=“radio” name=“foo” value=“b”> b

• All have same name• Can specify a default value by setting

one to be checked

Check boxes

• Group with same name, several can be checked

<input type=“checkbox” name=“foo” value=“a”> a

<input type=“checkbox” name=“foo” value=“b”> b

• Multiple values sent as a list

Selection lists<select name=“states” size=“5”><option> AL<option> AK<option> AR…</select>

• gives one value • can set one option to be selected• can also allow multiple items

Hidden fields

<input type=“hidden” name=“foo” value=“bar”>

• In multi-form applications, passes information from one page to the next

Common Gateway Interface

• A standard for interfacing external applications with information servers

• Not a particular language, but a rule for passing information

• Many languages provide a module/library for handling this automagically.

The Perl/CGI module

use CGI ‘:standard’;• gives access to lots of functions, pre-

defined strings

• also gives parameter-passing tools

Example of code generation#!/usr/local/bin/perl

use CGI ‘:standard’;

print header;

print start_html({-title=>’Hello’,

bgcolor=>“pink”});

print p(“Hello world”);

print end_html;

Getting data from forms

• Set the action of a form to some perl script

• Write the Perl script

• use the param() function to get the value of the form items

• Example: piping survey

Testing scripts

• Command-line mode:perl myprog.pl name=value

• Passing arguments via URL:http://<path>/myprog.pl?name=value

Environment variables

• When Perl is started from a browser, a special hash list is created describing how it was invoked. This is called the environmental hash list and is called %ENV

• Referring Web site, what browser, language, method, remote address, remote host

Environment variables

HTTP_REFERRER, REQUEST_USER_AGENT, HTTP_ACCEPT_LANGUAGE, REQUEST_METHOD, REMOTE_ADDRESS, REMOTE_HOST

• Can configure response to browser or disallow/ allow certain domains

What next?

• Scripts to write the forms

• Scripts to validate the form and spit it back if the user didn’t enter everything correctly

• Example: newform.pl

Files

• Files are opened via the open command:

open(FILE,’filename’);

• First argument is the “handle” – Similar to FILE* in C

• Second argument is a string -- the name of the file (perhaps including path)

Options for opening files

• Read (default):open(HANDLE,’<filename’);

• Write:open(HANDLE,’>filename’);

• Appendopen(HANDLE,’>>filename’);

• Uses redirection operators from Unix

What about errors?

• Errors can occur if a file that doesn’t exist is opened for reading, etc

open(HANDLE,’<filename’) || die “Can’t open file $!”;

• sends error message to STDERR• The variable $! contains the latest error

message returned by a system call• open returns 0 or 1

Reading from files (and STDIN)

• Use the syntax <HANDLE> to get either a line or the whole file

$aline = <MYFILE>;

@lines = <MYFILE>;• By not specifying a location, the line of

input appears in $_.

Example

• To read in a whole file and print it back to the screen, we use the code

open(FILE,’filename’) || die $!;while(<FILE>) { print $_;}close FILE;

• An EOF is interpreted as false in the loop

Even more arcaneopen(FILE,’filename’) || die $!;

while(<FILE>) {

print;

}

close FILE;

• The default argument of print (and some other functions) is $_.

More implicit variables• Records in a file (normally lines) are separated by $/

– changing this from “\n” to “” reads in paragraph mode, to “undef();” reads in the whole file

• Output field separator: $,– print “one”, “two” equivalent to– print “one” . $, . “two

• Output record separator: $\– Typically blank– Changing it changes the terminal value of print statements

Example

• Read and print email addresses

• Read, sort, and print email addresses

• Read and print links to email addresses

Editing a file

• Open file for reading

• Read everything into memory

• Close the file

• Make the changes (in memory)

• Open the file for writing

• Write the file

• Close the file