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 Words
while ($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 Oriented
use TestModule;
$test = TestModule->new()
$test->test_me()

Standard
use TestModule
test_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
pod2html
pod2man
file
converts to a text file
converts to an html file
converts to a man page
Thank you 