Introduction to Perl

Part III

By: Cedric Notredame

Adapted from (BT McInnes)

Hashes

Hashes are like array, they store collections of scalars

... but unlike arrays, indexing is by name (just like ine real life!!!)

Two components to each hash entry: Key example : name Value example : phone number

Hashes denoted with % Example : %phoneDirectory

Elements are accessed using {} (like [] in arrays)

Hashes continued ...

Adding a new key-value pair $phoneDirectory{“Shirly”} = 7267975

Note the $ to specify “scalar” context! Each key can have only one value

$phoneDirectory{“Shirly”} = 7265797 # overwrites previous assignment

Multiple keys can have the same value

Accessing the value of a key $phoneNumber =$phoneDirectory{“Shirly”};

Hashes and Foreach Foreach works in hashes as well!

foreach $person (keys (%phoneDirectory) ) {

print “$person: $phoneDirectory{$person}”;}

Never depend on the order you put key/values in the hash! Perl has its own magic to make hashes amazingly fast!!

Hashes and Sorting The sort function works with hashes as well Sorting on the keys

foreach $person (sort keys %phoneDirectory) {

print “$person : $directory{$person}\n”;

} This will print the phoneDirectory hash table in

alphabetical order based on the name of the person, i.e. the key.

Hash and Sorting cont... Sorting by value

foreach $person (sort {$phoneDirectory{$a} <=> $phoneDirectory{$b}} keys %phoneDirectory)

{print “$person :

$phoneDirectory{$person}\n”;

}

Prints the person and their phone number in the order of their respective phone numbers, i.e. the value.

Exercise

Chose your own test or use wget “http://www.trinity.edu/~mkearl/family.html”

Identify the 100 most frequent words

Counting Wordswhile ($l=<F>)

{

@w=split (/\s+/, $l);

foreach $word (@w)

{

$word=~s/[sx]$//;#plurial elimination

$seen{$word}++;

}

}

foreach $word (sort {$seen{$a}<=>$seen{$b}} keys %seen)

{

print “Word $word N: $seen{$word}\n”;

}

Command Line Arguments

Command line arguments in Perl are extremely easy. @ARGV is the array that holds all arguments passed

in from the command line. Example:

% ./prog.pl arg1 arg2 arg3 @ARGV would contain ('arg1', arg2', 'arg3)

$#ARGV returns the number of command line arguments that have been passed. Remember $#array is the size of the array!

Quick Program with @ARGV

Simple program called log.pl that takes in a number and prints the log base 2 of that number;

#!/usr/local/bin/perl -w$log = log($ARGV[0]) / log(2);print “The log base 2 of $ARGV[0] is $log.\n”;

Run the program as follows: % log.pl 8

This will return the following: The log base 2 of 8 is 3.

$_

Perl default scalar value that is used when a variable is not explicitly specified.

Can be used in For Loops File Handling Regular Expressions

$_ and For Loops

Example using $_ in a for loop

@array = ( “Perl”, “C”, “Java” );for(@array) { print $_ . “is a language I know\n”;}

Output : Perl is a language I know. C is a language I know. Java is a language I know.

$_ and File Handlers

Example in using $_ when reading in a file;

while( <> ) { chomp $_; # remove the newline char @array = split/ /, $_; # split the line on white space

# and

stores data in an array}

Note: The line read in from the file is automatically store in

the default scalar variable $_

$_ and File Handling cont..

Another example similar to the previous example:

while(<>) { chomp; # removes trailing newline chars @array = split/ /; # splits the line on white

# space and stores the data

# in the array }

Notes: The functions chomp and split automatically perform their

respective operations on $_.

Example Program

Count the number of words in a text and display the top 10 most frequency words.#!/usr/local/bin/perl

%vocab = (); $counter = 0;

while(<>) {

chomp;

foreach $element (split/ /) { $vocab{$element}++; }

}

foreach $word (sort {$vocab{$b}<=>$vocab{$a}} %vocab) {

print “$word $vocab{$word}\n”;

if($counter == 10) { last; } $counter++;

}

$_ and Regular Expressions

Example in using $_ when using regular expressions

while( <> ) { chomp;

$_=~s/[.!,;]/ /; $_=~s/I am/Why are you/;

print “$_?\n”;}

Input line : I am feeling down today.

Output : Why are you feeling down today?

Perl Modules

What are Perl Modules? Batches of reusable code Allow for object oriented Perl Programming

Comprehensive Perl Archive Network (CPAN) Perl utilities Perl Modules Perl documentation Perl distribution

CPAN Organization

CPAN Material is organized by Modules Distributions Documentation Announcements Ports Scripts Authors

Distributions are ‘tar-gzipped’ tar gzip

Categories of Modules

by-author Modules are organized by author’s registered CPAN

name by-category

Modules categorized by subject matter by-module

Modules categorized by module name

Installing a Module

After you have gunziped and untared your module you have two options on installing your module depending upon if you have root privileges to the location where perl modules are installed or you don’t.

If you have root privileges or write access: perl Makefile.PL make make test make install

Local Install

Need to specify where you would like the module to be installed by setting the PREFIX argument when generating a Makefile from Makfile.PL

perl Makefile.PL PREFIX=/home/Perl/Modules make make test make install

Local Install cont…

Perl usually looks in system wide areas for modules, therefore it will not find your local module unless you tell Perl where to find it.

In your perl program where you will be using your module

#!/usr/local/bin/perl

use lib ‘<module location>’

use ModuleName;

‘using’ a Perl Module

If we have a module called TestModule that contains a function test_me(). To use this module we have two options:

Object Orienteduse TestModule;$test = TestModule->new()$test->test_me()

Standarduse TestModuletest_me();

Example Program

#!/usr/local/bin/perl

use lib ‘home/cs/bthomson/PerlModules/Suffix.pm’

use Suffix.pm

$sarray->Array::Suffix->new();$sarray->create_files(“hamlet.txt”);$sarray->get_ngrams();

Module Documentation

Perl Module Documentation is provided by the module author and usually written in pod format

To view the documentation on the command line %perldoc modulename

Convert the pod document to the format of your choice: pod2text converts to a text file pod2html converts to an html file pod2man converts to a man page file

Thank you