CSE3325: Perl Programming

In the previous lecture:

• CGI programming
  
  
• Some basic Perl Scripts

In this lecture:

• Perl Syntax overview
  
  
• CGI.pm introduction

References:

• Stein, L.D.: How To Set Up and Maintain a Web Site 2nd edn, Addison Wesley 1997, Chpt9.
  
  
• Wall, Larry, Tom Christiansen & Randal L. Schwartz: Programming Perl, 2nd edn., O'Reilly and Associates, Inc., 1996 (the 'Camel' book).
  
  
• It is essential that some reference other than the above (eg. the WWW) be consulted on this topic, particularly in learning elementary Perl.
  
  
• It is not necessary that you become a proficient Perl programmer, but an understanding of the basic syntax is required.

Perl at light speed

• Perl - Practical Extraction and Report Language (Affectionately: Pathologically Eclectic Rubbish Lister)
  
  
• Perl is an interpretted language which is compiled by the interpretter before execution. (This speeds up execution because the code is not interpretted during execution.)
  
  
• Perl is a powerful, sometimes messy, programming language, a bit of a hacker's dream... good at many of the things that C makes very difficult or impossible.
  
  
• Perl has very advanced string processing and manipulation facilities — imagine the best features of csh, awk and sed all rolled into one and you begin to see the world of Perl.

• $variable a scalar variable
  
  
  • $variable = 3;
  
  
• @variable an array (a list indexed by integers)
  
  
  • $variable[0][0] = 4;
    
    
  • First element of @variable is assigned 4.
  • The $ prefix indicates the type of element stored.
  • Braces [ ] [] used to access regular array.
  • @list = ('cat', 'dog', 'pig'); print $list[1]; – prints "dog"
  
  
• %variable an associative array (a list indexed by strings)
  
  
  • $variable{'string'} = 4; OR
    $variable{ string } = 4;
    
    
  • 'string' element of %variable is assigned 4.
  • The $ prefix indicates the type of element stored.
  • Braces used to access associative array.

• Strings, integers and floating point values may be interconverted as necessary as Perl is not strict with types.
  
  $x = "7" * 2.0 + 3;
  
  
• Lists can be assigned to associative arrays, the even-numbered elements become keys / indices and the odd-numbered elements become their values.
  
  %ROMAN_TO_ARABIC = ('I',1,'II',2,'III',3,'IV',4,'V',5);
  
  or in Perl v5 where => is a synonym for a comma (,)
  
  %ROMAN_TO_ARABIC = ('I'=>1,'II'=>2,'III'=>3,'IV'=>4,'V'=>5);
  
  
• Built in associative array %ENV holds the current environment variables.
  
  eg. $ENV{PATH} and $ENV{HOME}

• open(INPUT, "<$fileName");
  
  Associate the file whose name is in $fileName with the file handle INPUT and attempt to open it for reading.
  
  
• $variable = <FILEHANDLE>;
  
  Grab a line from the file indicated by filehandle and put it in the variable.
  
  
• $variable = <>;
  
  Grab a line from standard input (the default filehandle).
  
  
• <>;
  
  A line from standard input is placed in the magic variable $_ which stores the result of the most recent operation.
  
  
• while (<>) {
  
  print;
  }
• while (<>) {
  
  print $_;
  }
• while ($line = <>) {
  
  print $line;
  }
  The three examples above all do the same thing. A line from the default input stream (STDIN) is placed in the magic variable $_ which stores the result of the most recent operation. While a line is successfully retrieved, print from the default variable ($_) to the default output stream (STDOUT).
  
  
• print $variable;
  
  Print the contents of the variable
  
  
• $model = 9000
  print "Hi Dave! There's no way to get rid of your HAL $model!\n";
  
  Print the contents of the text string, evaluating the variable $model.
  
  
• print <<LABEL
  
  
  Text that appears here will be printed out
  with line breaks until the text LABEL is reached.
  Variables (variable = $variable) will also be evaluated.
  
  LABEL
  
  
• open(OUTPUT, ">$fileName")
  
  Associate the file whose name is in $fileName with the file handle OUTPUT and attempt to open it for writing.
  
  

• $var = 4;
  
  
• $var++;
  
  
• $var += 4;
  
  
• if (EXPR) { ... } elsif { ... } else { ... } # note elsif NOT elseif
  
  
• if (($var >= 4) || ($var < 0))
  
  

Functions / Subroutines

Functions are declared and called as illustrated by this simple example.

#!/usr/monash/bin/perl -w
$val1 = 7;
$val2 = 9;

$answer = &maximum($val1, $val2);      # assign global $answer to maximum of 9 and 7
                                       # use the '&' symbol to call a function
print "answer = $answer\n";            # print the value of $answer

sub maximum                            # define a subroutine maximum
{
        my ($arg1, $arg2) = @_;        # receive the arguments passed in through @_
                                       # and place them in the local variables $arg1 & 2
        
        my ($result);                  # declare a local variable $result

        if ($arg1 >= $arg2)
        { $result = $arg1; }

        else
        { $result = $arg2; }

        return $result;                # return $result
}

A more complex example (from the Camel book):

Suppose we have a list of student names and their assignment scores:

Consider the following Perl program:

#!/usr/monash/bin/perl -w
open (GRADES, "grades") or die "Can't open grades: $!\n";

while ($line = <GRADES>) {
	($student, $grade) = split(" ", $line);
	$grades{$student} .= $grade . " ";
}

foreach $student (sort keys %grades) {
	$scores = 0;
	$total = 0;
	@grades = split(" ", $grades{$student});
	foreach $grade (@grades) {
		$total += $grade;
		$scores++;
	}
	$average = $total / $scores;
	print "$student: $grades{$student}\tAverage: $average\n";
}

Run the program on your local account and look at the results. Modify the script so the results are displayed on a web page via CGI.

CGI.pm

• CGI.pm is a Perl Module specifically designed for writing CGI scripts with Perl.
  
  
• It makes it easy to design web fill-out forms and dynamically created HTML
  
  
• It also has advanced features such as support for file uploads, cookies, cascading style sheets (CSS), server push and frames.
  

Example:

#!/usr/monash/bin/perl

use CGI qw(:standard);

print header;
print start_html('A Simple Example'),
    h1('A Simple Example'),
    start_form,
    "What's your name? ",textfield('name'),
    p,
    "What's the combination?",
    p,
    checkbox_group(-name=>'words',
                   -values=>['eenie':'meenie','minie','moe'],
                   -defaults=>['eenie':'minie']),
    p,
    "What's your favorite color? ",
    popup_menu(-name=>'color',
               -values=>['red':'green','blue','chartreuse']),
    p,
    submit,
    end_form,
    hr;

if (param()) {
    print 
        "Your name is",em(param('name')),
        p,
        "The keywords are: ",em(join(", ",param('words'))),
        p,
        "Your favorite color is ",em(param('color')),
        hr;
}
print end_html;

#!/usr/local/bin/perl
use CGI qw/:standard/;
print header(),
      start_html(-title=>'Wow!'),
      h1('Wow!'),
      'Look Ma, no hands!',
      end_html();

#!/usr/local/bin/perl
use CGI;
$q = new CGI;
print $q->header(),
      $q->start_html(-title=>'Wow!'),
      $q->h1('Wow!'),
      'Look Ma, no hands!',
      $q->end_html(); 

For more information see: http://stein.cshl.org/WWW/CGI/.

Where to find more:

This Page of Perl Pearls is only the bare minimum required to understand the examples given in lectures.

Please read a book or website on Perl to become more familiar with the language. Above all, practice writing a few Perl scripts!

Perl web sites:

• www.perl.com - a good place to start to find out about Perl
• CPAN - Comprehensive Perl Archive Network: a huge collection of Perl modules, scripts and documentation, the place for Perl software.
• www.perl.org - News and information about Perl.
• www.macperl.com - Perl port for the Macintosh (yes, there's a windoz port too).

• Perl can do a lot of things, many seem like magic. There is an old saying: Any technology sufficiently advanced is indistinguishable from a Perl script.
  
  
• Remember also the Perl motto: TMTOWTDI (There's more than one way to do it)!

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©Copyright Alan Dorin 2005