Perl

Let’s Learn Perl
An introduction to Perl

Sreejith S
NLP Process Analyst
365Media Pvt. Ltd.
srssreejith@gmail.com

January 22, 2011

Sreejith S Let’s Learn Perl

Just a word about me !!

Working in Natural Language Processing (NLP), Machine Learning,
Data Mining
Passionate about Free and Open source :-)
Works for 365Media Pvt. Ltd. Coimbatore India.


What’s Perl ??

Practical Extraction and Reporting Language


What’s Perl ??

Perl is a Portable Scripting Language


What’s Perl ??

Fast and easy text processing capability


What’s Perl ??

Fast and easy ﬁle handling capability


What’s Perl ??

It aims to be practical (easy to use, eﬃcient, complete) rather than
beautiful (tiny, elegant, minimal)


What’s Perl ??

It aims to be practical (easy to use, eﬃcient, complete) rather than
beautiful (tiny, elegant, minimal)
Perl was developed by Larry Wall.


To install...

Perl Comes by Default on Linux, Cygwin, MacOSX


To install...

www.perl.com has rpm’s for Linux


To install...

www.activestate.com has binaries for Windows


To install...

Latest Version is 5.10


To install...

To check if Perl is working and the version number


To install...

To check if Perl is working and the version number
perl -v or perl -version


Basic Syntax of Perl

Comments



Comments
begin with #



Comments
begin with #
dont try to use // or /* ... */



Comments
begin with #
Print



Comments
begin with #
Print
print is simple function to display/output something on
monitor/stdout



Comments
begin with #
Print
monitor/stdout
e.g :- print “hello world“;



Comments
begin with #
Print
monitor/stdout
Statement terminator ;



Comments
begin with #
Print
monitor/stdout
All Statements end with ; like C language



Comments
begin with #
Print
monitor/stdout
Delimiter



Comments
begin with #
Print
monitor/stdout
Delimiter
{} delimit blocks, loops, subroutines



Comments
begin with #
Print
monitor/stdout
Delimiter
No main() function



Comments
begin with #
Print
monitor/stdout
Delimiter
No main() function
Top-down programming approach


Structure of a Perl Program

#! /usr/bin/perl -w



#! /usr/bin/perl -w
It tells where to ﬁnd Perl



#! /usr/bin/perl -w
.pl extension is optional but is commonly used



#! /usr/bin/perl -w
You can write perl programs without extension also.



#! /usr/bin/perl -w
You can write perl programs without extension also.
-w switches on warning : not required but a really good idea


Customary Hello World Program

#! /usr/bin/perl -w
print "Hello World!n";



#! /usr/bin/perl -w

Save this as hello.pl



#! /usr/bin/perl -w

To run the program



#! /usr/bin/perl -w

To run the program
perl hello.pl or perl hello


Variables

Variables are memory location to store information.


Variables

Variables are type less


Variables

There is no data type like int,char,ﬂoat


Variables

Variables do not need to be declared


Variables

Every variable is a string


Variables

Variable type is decided at run time


Variables

Depending on the context will be treated as int, ﬂoat etc.


Variables

There are 4 kinds of variables namely


Variables

Scalars


Variables

Scalars
Lists


Variables

Scalars
Lists
Arrays


Variables

Scalars
Lists
Arrays
Hashes.


Scalars

Scalar meaning single value


Scalars

Name of scalar variable must begin with $ symbol.


Scalars

next character must a letter or


Scalars

remaining characters: letters, numbers, or


Scalars

name can be up to 255 characters long


Scalars

Right


Scalars

Right
$foo, $a, $zebra1, $F87dr df3


Scalars

Right
Wrong


Scalars

Right
Wrong
$24da, $hi&bye, $bar$foo


Scalars

Right
Wrong
Examples


Scalars

Right
Wrong
Examples
$name=”ram”;


Scalars

Right
Wrong
Examples
$name=”ram”; # in string context


Scalars

Right
Wrong
Examples
$age=30;


Scalars

Right
Wrong
Examples
$age=30; # in numerical context


Scalars

Right
Wrong
Examples
$value = 4.43;


Scalars

Right
Wrong
Examples
$value = 4.43; # in ﬂoat context


Scalars

Right
Wrong
Examples
$foo = Z;


Scalars

Right
Wrong
Examples
$foo = Z; # in char context


Variable & Character Interpolation

Perl looks for variables inside strings and replaces them with their
value



value
$name = "Sree";
print "$name is my namen";



value
$name = "Sree";

This does not happen when you use single quotes



value
$name = "Sree";

print ’$name is my name’; and it will print



value
$name = "Sree";

$name is my name



value
$name = "Sree";

$name is my name
List of character escapes that are recognized when using double
quoted strings
All escape characters are recognized



value
$name = "Sree";

$name is my name
List of character escapes that are recognized when using double
quoted strings
All escape characters are recognized
# It prints Hello World and then a return


Lists

List variables are denoted by symbol ‘()‘


Lists

List is just a list of values may be constants, scalars etc


Lists

(a,b,c) or ($name,$age,$sex)


Lists


The index are speciﬁed inside a square bracket ‘[]‘


Lists


$first=(a,b,c)[0];
print "$firstn";


Lists


$first=(a,b,c)[0];
print "$firstn";

List variables can be assigned like this


Lists


$first=(a,b,c)[0];
print "$firstn";

List variables can be assigned like this
($name,$age)=(’Raman’,20);
print "name=$namen";


Arrays

Array variables should have preﬁx ‘@‘


Arrays

Arrays are used to store ordered values


Arrays

Much more dynamic than C/C++


Arrays

no declaration of size, type


Arrays

can hold any kind of value, and multiple kinds of values


Arrays

Array Indexes start at 0


Arrays

@array=(1,2,3);

To access the whole array, use the whole array


Arrays

@array=(1,2,3);

print @array;


Arrays

@array=(1,2,3);

print @array;
To access one element of the array : use $


Arrays

@array=(1,2,3);

print @array;
To access one element of the array : use $
print $array[0]; # prints : 1


Operations on Arrays
To append to the end of an array : PUSH


@array=(1,2,3);
push @array,’4’;
print "@array n";


@array=(1,2,3);
print "@array n";

To remove the last element of the array : POP


@array=(1,2,3);
print "@array n";

@array=(1,2,3);
$last=pop @array;
print "last=$lastn";


@array=(1,2,3);
print "@array n";

@array=(1,2,3);
$last=pop @array;

To prepend to the beginning of an array : UNSHIFT


@array=(1,2,3);
print "@array n";

@array=(1,2,3);
$last=pop @array;

@array=(3,4,5);
unshift @array,’2’;
print "array=@arrayn";


@array=(1,2,3);
print "@array n";

@array=(1,2,3);
$last=pop @array;

@array=(3,4,5);

To remove the ﬁrst element of the array : SHIFT


@array=(1,2,3);
print "@array n";

@array=(1,2,3);
$last=pop @array;

@array=(3,4,5);

To remove the ﬁrst element of the array : SHIFT
@array=(3,4,5);

Operations on Arrays Contind...

$#array is a special variable containing index of last element.



print $#array; # it will print 2



Another way to ﬁnd the number of elements in the array:



Another way to ﬁnd the number of elements in the array:
$array_size = @array;
scalar(@array) is a function to return the size of array


Hashes

Hash variables will have % as preﬁx


Hashes

Key-Value pair


Hashes

Key-Value pair
The contents of hash are called values and index is called key. the
array:


Hashes

Key-Value pair
array:
%fruits= ( ’apple’=>’red’,’banana’=>’yellow’,
’grape’ =>’black’);
print "colour of apple =$fruits{apple}n";


Hashes

Key-Value pair
array:
Other way of populating a hash


Hashes

Key-Value pair
array:
%fruits =(’apple’,’red’,’banana’,’yellow’,
’grape’,’black’);
print "colour of banana =$fruits{banana}n";


Hashes

Key-Value pair
array:
%fruits =(’apple’,’red’,’banana’,’yellow’,
’grape’,’black’);
print "colour of banana =$fruits{banana}n";
Note the {} instead of [ ] as in the case of array;


Conditionals - IF

The if statement is similar to if in C language, except


Conditionals - IF

ﬂower brace is required even for single statement
else if is noted by elsif (note missing e)


Conditionals - IF

The syntax of if statement is


Conditionals - IF

The syntax of if statement is
if (condition) {
}
elsif (condition){
}
else {
}


Control Structures - Loops

Perl supports four main loop types



while, until, for, and foreach



while loop is used to iterate and has syntax similar to C.



while loop is used to iterate and has syntax similar to C.
$i = 0;
while ( $i <= 1000 )
{
print $in;
$i++;
}


Control Structures - Until Loop

The until function evaluates an expression repeatedly until a speciﬁc
condition is met


Control Structures - Until Loop

The until function evaluates an expression repeatedly until a speciﬁc
condition is met
$i = 0;
until ($i == 1000)
{
print $in;
$i++;
}


Control Structures - For Loop

For loop syntax is similar to c.



There are variations



Syntax 1:



Syntax 1:
for ( $i = 0; $i <= 1000; $i=$i+2 )
{
print $i;
}



Syntax 1:
for ( $i = 0; $i <= 1000; $i=$i+2 )
{
print $i;
}
Syntax 2:



Syntax 1:
for ( $i = 0; $i <= 1000; $i=$i+2 )
{
print $i;
}
Syntax 2:
for $i(0..1000)
{
print $i;
}


Control Structures - ForEach Loop

Syntax 3:


Control Structures - ForEach Loop

Syntax 3:
foreach $i (a,b,c)
{
print uc $i;
}


Default scalar variable

$ is called default variable


Default scalar variable

$ is called default variable
It will be used if no other variable is speciﬁed
foreach (a,b,c){
print uc ;
}


Accepting input

Keyboard inputs can be accepted using STDIN


Accepting input

Keyboard inputs can be accepted using STDIN
print "enter your name ";
$name=<STDIN>;
chomp $name;
# chomp function is similar to fflush in C.
# It removes trailing newline if any
print "Welcome $namen";


Subroutines

Subroutines can be deﬁned using sub keyword


Subroutines

Subroutines can be deﬁned using sub keyword
’@ ’ default array
$v1=10;$v2=20;
add($v1,$v2);
sub add {
($a,$b)=@_;
print $a+$b;
}


Scope of variables

By default all variables are global


Scope of variables

By default all variables are global
You can limit scope to a block/sub by using my.
$v1=10; $v2=30; #v1,v2 global
my $i = 10;


File handling
File handling can be done very easily


File handling
To open a ﬁle in read mode


File handling
open( $fh, < , data.txt);


File handling
File reading line by line can be done like


File handling
$line=<$fh>;


File handling
$line=<$fh>;
File writing can be done using print and ﬁlehandle like this
print $fh "hello";


File handling
$line=<$fh>;
print $fh "hello";
Example


File handling
$line=<$fh>;
print $fh "hello";
Example
open ( $fh, "<", "data.txt" ); #open file read only
open ($fh1,">","udata.txt"); #Open file write mode
while ( $line = <$fh>; ) { #read line by
print "line=$line"; #display content on screen
print $fh1 uc($line); #write upper cased content t
}
close($fh);
close($fh1);


Some Common String Functions

There are many built in string functions



Convert to upper case



$name = uc($name);



$name = uc($name);
Convert only the ﬁrst char to upper case



$name = uc($name);
$name = ucfirst($name);



$name = uc($name);
Convert to lower case



$name = uc($name);
$name = lc($name);



$name = uc($name);
$name = lc($name);
Convert only the ﬁrst char to lower case



$name = uc($name);
$name = lc($name);
$name = lcfirst($name);



$name = uc($name);
$name = lc($name);
Concatenation: ’.’ similar to strcat



$name = uc($name);
$name = lc($name);
Concatenation: ’.’ similar to strcat
$first_name = Larry;
$last_name = Wall;
$full_name = $first_name . . $last_name;


Some Common Functions

There are many built in functions



Strings to Arrays : split



@array = split( /;/, "Larry;Curly;Moe" );
@array= ("Larry", "Curly", "Moe");



Split into characters



@stooge = split( //, "curly" );
# array @stooge has 5 elements: c, u, r, l, y



Join with any character you want



Join with any character you want
@array = 10, 20, 30, 40);
$string = join(->, @array);
# string = 10->20->30->40


Web Resource for Perl

Web resources for Perl


Web Resource for Perl

Web resources for Perl
www.perl.com
www.perl.org
www.perlmonks.org


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