PERL PROGRAMMING LANGUAGE

GO TO LINK & DOWNLOADO'REILLY_LEARNING_PERL_EBOOK PDF

GO TO LINK & DOWNLOAD:PERL_QUICK_LEARNING PDF

GO TO LINK & DOWNLOAD:PERL_EXAMPLES

Perl - Introduction

Perl is a general-purpose programming language originally developed for text manipulation and now used for a wide range of tasks including system administration, web development, network programming, GUI development, and more.

What is Perl?

Perl is a stable, cross platform programming language.
Though Perl is not officially an acronym but few people used it as Practical Extraction and Report Language.
It is used for mission critical projects in the public and private sectors.
Perl is an Open Source software, licensed under its Artistic License, or the GNU General Public License (GPL).
Perl was created by Larry Wall.
Perl 1.0 was released to usenet's alt.comp.sources in 1987
At the time of writing thi tutorial, latest version of perl is 5.16.2
Perl is listed in the Oxford English Dictionary.

PC Magazine named Perl a finalist for its 1998 Technical Excellence Award in the Development Tool category.

Perl Features

Perl takes the best features from other languages, such as C, awk, sed, sh, and BASIC, among others.
Perls database integration interface DBI supports third-party databases including Oracle, Sybase, Postgres, MySQL and others.
Perl works with HTML, XML, and other mark-up languages.
Perl supports Unicode.
Perl is Y2K compliant.
Perl supports both procedural and object-oriented programming.
Perl interfaces with external C/C++ libraries through XS or SWIG.
Perl is extensible. There are over 20,000 third party modules available from the Comprehensive Perl Archive Network (CPAN).
The Perl interpreter can be embedded into other systems.

Perl and the Web

Perl used to be the most popular web programming language due to its text manipulation capabilities and rapid development cycle.
Perl is widely known as " the duct-tape of the Internet".
Perl can handle encrypted Web data, including e-commerce transactions.
Perl can be embedded into web servers to speed up processing by as much as 2000%.
Perl's mod_perl allows the Apache web server to embed a Perl interpreter.
Perl's DBI package makes web-database integration easy.

Perl is Interpreted

Perl is an interpreted, which means that your code can be run as is, without a compilation stage that creates a non portable executable program.

Traditional compilers convert programs into machine language. When you run a Perl program, it's first compiled into a byte code, which is then converted ( as the program runs) into machine instructions. So it is not quite the same as shells, or Tcl, which are strictly interpreted without an intermediate representation.

Neither it is like most versions of C or C++, which are compiled directly into a machine dependent format. It is somewhere in between, along with Python and awk and Emacs .elc files.

Perl - Environment Setup

Before we start writing our Perl programs, let's understand how to setup our Perl environment. Perl is available on a wide variety of platforms:

Unix (Solaris, Linux, FreeBSD, AIX, HP/UX, SunOS, IRIX etc.)
Win 9x/NT/2000/
WinCE
Macintosh (PPC, 68K)
Solaris (x86, SPARC)
OpenVMS
Alpha (7.2 and later)
Symbian
Debian GNU/kFreeBSD
MirOS BSD
And many more...

Perl - Syntax Overview

Perl borrows syntax and concepts from many languages: awk, sed, C, Bourne Shell, Smalltalk, Lisp and even English. However, there are some definite differences between the languages. This chapter is designed to quickly get you up to speed on the syntax that is expected in Perl.

A Perl program consists of a sequence of declarations and statements which run from the top to the bottom. Loops, subroutines, and other control structures allow you to jump around within the code. Every simple statement must end with a semicolon (;).

Perl is a free-form language: you can format and indent it however you like. Whitespace serves mostly to separate tokens, unlike languages like Python where it is an important part of the syntax, or Fortran where it is immaterial.

Perl - Data Types

Perl is loosely typed language and there is no need to specify a type for your data while using in your program. The Perl interpreter will choose the type based on the context of the data itself.

Perl has three basic data types: scalars, arrays of scalars, and hashes of scalars, also known as associative arrays. Here is little detail about these data types.

S.N.	Types and Description
1	Scalar: Scalars are simple variables. They are preceded by a dollar sign ($). A scalar is either a number, a string, or a reference. A reference is actually an address of a variable which we will see in upcoming chapters.
2	Arrays: Arrays are ordered lists of scalars that you access with a numeric index which starts with 0. They are preceded by an "at" sign (@).
3	Hashes: Hashes are unordered sets of key/value pairs that you access using the keys as subscripts. They are preceded by a percent sign (%).

Numeric Literals

Perl stores all the numbers internally as either signed integers or double-precision floating-point values. Numeric literals are specified in any of the following floating-point or integer formats:

Type	Value
Integer	1234
Negative integer	-100
Floating point	2000
Scientific notation	16.12E14
Hexadecimal	0xffff
Octal	0577

String Literals

Strings are sequences of characters. They are usually alphanumeric values delimited by either single (') or double (") quotes. They work much like UNIX shell quotes where you can use single quoted strings and double quoted strings.

Double-quoted string literals allows variable interpolation, and single-quoted strings are not. There are certain characters when they are proceeded by a back slash they will have special meaning and they are used to represent like newline (\n) or tab (\t).

You can embed newlines or any of the following Escape sequences directly in your double quoted strings:

Escape sequence

Meaning

Backslash

Single quote

Double quote

Alert or bell

Backspace

Form feed

Newline

Carriage return

Horizontal tab

Vertical tab

\0nn

Creates Octal formatted numbers

\xnn

Creates Hexideciamal formatted numbers

\cX

Control characters, x may be any character

Force next character to uppercase

Force next character to lowercase

Force all following characters to uppercase

Force all following characters to lowercase

Backslash all following non-alphanumeric characters

End \U, \L, or \Q

Perl - Variables

Variables are nothing but reserved memory locations to store values. This means that when you create a variable you reserve some space in memory.

Based on the data type of a variable, the interpreter allocates memory and decides what can be stored in the reserved memory. Therefore, by assigning different data types to variables, you can store integers, decimals, or strings in these variables.

We have learnt that Perl has following three basic data types:

Scalars
Arrays
Hashes

Accordingly we are going to use three types of variables in Perl. A scalar variable will precede by a dollar sign ($) and it can store either a number, a string, or a reference. A array variable will precede by sign @ and it will store ordered lists of scalars. Finaly Hash variable will precede by sign % and will be used to store sets of key/value pairs.

Perl maintains every variable type in a separate namespace. So you can, without fear of conflict, use the same name for a scalar variable, an array, or a hash. This means that $foo and @foo are two different variables.

Creating Variables

Perl variables do not have to be explicitly declared to reserve memory space. The declaration happens automatically when you assign a value to a variable. The equal sign (=) is used to assign values to variables.

Keep a note that this is mandatory to declare a varibale before we use it if we use use strictstatement in our program.

The operand to the left of the = operator is the name of the variable, and the operand to the right of the = operator is the value stored in the variable. For example:

$age = 25;             # An integer assignment
$name = "John Paul";   # A string 
$salary = 1445.50;     # A floating point

Here 25, "John Paul" and 1445.50 are the values assigned to $age, $name and $salary variables, respectively. Shortly we will see how we can assign values to arrays and hashes.

Scalar Variables

A scalar is a single unit of data. That data might be a integer number, floating point, a character, a string, a paragraph, or an entire web page. Simply saying it could be anything, but only a single thing.

Here is a simple example of using scalar variables:

#!/usr/bin/perl

$age = 25;             # An integer assignment
$name = "John Paul";   # A string 
$salary = 1445.50;     # A floating point

print "Age = $age\n";
print "Name = $name\n";
print "Salary = $salary\n";

This will produce following result:

Age = 25
Name = John Paul
Salary = 1445.5

Array Variables

An array is a variable that stores an ordered list of scalar values. Array variables are preceded by an "at" (@) sign. To refer to a single element of an array, you will use the dollar sign ($) with the variable name followed by the index of the element in square brackets.

Here is a simple example of using array variables:

#!/usr/bin/perl

@ages = (25, 30, 40);             
@names = ("John Paul", "Lisa", "Kumar");

print "\$ages[0] = $ages[0]\n";
print "\$ages[1] = $ages[1]\n";
print "\$ages[2] = $ages[2]\n";
print "\$names[0] = $names[0]\n";
print "\$names[1] = $names[1]\n";
print "\$names[2] = $names[2]\n";

Here we used escape sign (\) before $ sign just to print it other Perl will understand it as a variable and will print its value. When exected, this will produce following result:

$ages[0] = 25
$ages[1] = 30
$ages[2] = 40
$names[0] = John Paul
$names[1] = Lisa
$names[2] = Kumar

Hash Variables

A hash is a set of key/value pairs. Hash variables are preceded by a percent (%) sign. To refer to a single element of a hash, you will use the hash variable name followed by the "key" associated with the value in curly brackets.

Here is a simple example of using hash variables:

#!/usr/bin/perl

%data = ('John Paul', 45, 'Lisa', 30, 'Kumar', 40);

print "\$data{'John Paul'} = $data{'John Paul'}\n";
print "\$data{'Lisa'} = $data{'Lisa'}\n";
print "\$data{'Kumar'} = $data{'Kumar'}\n";

This will produce following result:

$data{'John Paul'} = 45
$data{'Lisa'} = 30
$data{'Kumar'} = 40

Variable Context

Perl treats same variable differently based on Context ie. situation where a variable is being used. Let's check following example:

#!/usr/bin/perl

@names = ('John Paul', 'Lisa', 'Kumar');

@copy = @names;
$size = @names;

print "Given names are : @copy\n";
print "Number of names are : $size\n";

This will produce following result:

Given names are : John Paul Lisa Kumar
Number of names are : 3

Here @names is an array, which has been used in two different contexts. First we copied it into anyother array ie. list so it returned all the elements assuming that context is list context. Next we used same array and tried to store this array in a scalar, so in this case it returned just number of elements in this array assuming that context is scalar context. Following table lists down various contexts:

S.N.

Context and Description

Scalar:
Assignment to a scalar variable evaluates the right-hand side in a scalar context.

List:
Assignment to an array or a hash evaluates the right-hand side in a list context.

Boolean:
Boolean context is simply any place where an expression is being evaluated to see whether it's true or false

Void:
This context not only doesn't care what the return value is, it doesn't even want a return value.

Interpolative:
This context only happens inside quotes, or things that work like quotes.

Perl - Arrays

Here is a simple example of using array variables:

#!/usr/bin/perl

@ages = (25, 30, 40);             
@names = ("John Paul", "Lisa", "Kumar");

print "\$ages[0] = $ages[0]\n";
print "\$ages[1] = $ages[1]\n";
print "\$ages[2] = $ages[2]\n";
print "\$names[0] = $names[0]\n";
print "\$names[1] = $names[1]\n";
print "\$names[2] = $names[2]\n";

Here we used escape sign (\) before $ sign just to print it other Perl will understand it as a variable and will print its value. When exected, this will produce following result:

$ages[0] = 25
$ages[1] = 30
$ages[2] = 40
$names[0] = John Paul
$names[1] = Lisa
$names[2] = Kumar

In Perl, List and Array terms are often used as if they're interchangeable. But the list is the data, and the array is the variable.

Array Creation

Array variables are prefixed with the @ sign and are populated using either parentheses or the qw operator. For example:

@array = (1, 2, 'Hello');
@array = qw/This is an array/;

The second line uses the qw// operator, which returns a list of strings, separating the delimited string by white space. In this example, this leads to a four-element array; the first element is 'this' and last (fourth) is 'array'. This means that you can use different lines as follows:

@days = qw/Monday
Tuesday
...
Sunday/;

You can also populate an array by assigning each value individually as follows:

$array[0] = 'Monday';
...
$array[6] = 'Sunday';

Accessing Array Elements

When accessing individual elements from an array, you must prefix the variable with a dollar sign ($) and then append the element index within square brackets after the name of the variable. For example:

#!/usr/bin/perl

@days = qw/Mon Tue Wed Thu Fri Sat Sun/;

print "$days[0]\n";
print "$days[1]\n";
print "$days[2]\n";
print "$days[6]\n";
print "$days[-1]\n";
print "$days[-7]\n";

This will produce following result:

Mon
Tue
Wed
Sun
Sun
Mon

Array indices start from zero, so to access first element you need to give 0 as indices. You can also give a negative index, in which case you select the element from the end, rather than the beginning, of the array. This means that

print $days[-1]; # outputs Sun
print $days[-7]; # outputs Mon

Sequential Number Arrays

Perl offers a shortcut for sequential numbers and letters. Rather than typing out each element when counting to 100 for example, we can do something like as follows:

#!/usr/bin/perl

@var_10 = (1..10);
@var_20 = (10..20);
@var_abc = (a..z);

print "@var_10\n";   # Prints number from 1 to 10
print "@var_20\n";   # Prints number from 10 to 20
print "@var_abc\n";  # Prints number from a to z

Here double dot (..) is called range operator. This will produce following result:

1 2 3 4 5 6 7 8 9 10
10 11 12 13 14 15 16 17 18 19 20
a b c d e f g h i j k l m n o p q r s t u v w x y z

Array Size

The size of an array can be determined using scalar context on the array - the returned value will be the number of elements in the array:

@array = (1,2,3);
print "Size: ",scalar @array,"\n";

The value returned will always be the physical size of the array, not the number of valid elements. You can demonstrate this, and the difference between scalar @array and $#array, using this fragment as follows:

#!/uer/bin/perl

@array = (1,2,3);
$array[50] = 4;

$size = @array;
$max_index = $#array;

print "Size:  $size\n";
print "Max Index: $max_index\n";

This will produce following result:

Size: 51
Max Index: 50

There are only four elements in the array that contain information, but the array is 51 elements long, with a highest index of 50.

Adding and Removing Elements in Array

Perl provides a number of useful functions to add and remove elements in an array. You may have a question what is a function? So far you have used print function to print various values. Similarly there are various other functions or sometime called sub-routines which can be used for various other functionalities.

S.N.	Types and Description
1	push @ARRAY, LIST Pushes the values of the list onto the end of the array.
2	pop @ARRAY Pops off and returns the last value of the array.
3	shift @ARRAY Shifts the first value of the array off and returns it, shortening the array by 1 and moving everything down
4	unshift @ARRAY, LIST Prepends list to the front of the array, and returns the number of elements in the new array.

#!/usr/bin/perl

# create a simple array
@coins = ("Quarter","Dime","Nickel");
print "1. \@coins  = @coins\n";

# add one element at the end of the array
push(@coins, "Penny");
print "2. \@coins  = @coins\n";

# add one element at the beginning of the array
unshift(@coins, "Dollar");
print "3. \@coins  = @coins\n";

# remove one element from the last of the array.
pop(@coins);
print "4. \@coins  = @coins\n";

# remove one element from the beginning of the array.
shift(@coins);
print "5. \@coins  = @coins\n";

This will produce following result:

1. @coins = Quarter Dime Nickel
2. @coins = Quarter Dime Nickel Penny
3. @coins = Dollar Quarter Dime Nickel Penny
4. @coins = Dollar Quarter Dime Nickel
5. @coins = Quarter Dime Nickel

Slicing Array Elements

You can also extract a "slice" from an array - that is, you can select more than one item from an array in order to produce another array.

#!/usr/bin/perl

@days = qw/Mon Tue Wed Thu Fri Sat Sun/;

@weekdays = @days[3,4,5];

print "@weekdays\n";

This will produce following result:

Thu Fri Sat

The specification for a slice must a list of valid indices, either positive or negative, each separated by a comma. For speed, you can also use the .. range operator:

#!/usr/bin/perl

@days = qw/Mon Tue Wed Thu Fri Sat Sun/;

@weekdays = @days[3..5];

print "@weekdays\n";

This will produce following result:

Thu Fri Sat

Replacing Array Elements

Now we are going to introduce one more function called splice(), which has following syntax:

splice @ARRAY, OFFSET [ , LENGTH [ , LIST ] ]

This function will remove the elements of @ARRAY designated by OFFSET and LENGTH, and replaces them with LIST, if specified. Finally it returns the elements removed from the array. Following is the example:

#!/usr/bin/perl

@nums = (1..20);
print "Before - @nums\n";

splice(@nums, 5, 5, 21..25); 
print "After - @nums\n";

This will produce following result:

Before - 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
After - 1 2 3 4 5 21 22 23 24 25 11 12 13 14 15 16 17 18 19 20

Here actual replacement begins with the 6th number after that five elements are then replaced from 6 to 10 with the numbers 21, 22, 23, 24 and 25.

Transform Strings to Arrays

Let's look into one more function called split(), which has the following syntax:

split [ PATTERN [ , EXPR [ , LIMIT ] ] ]

This function splits a string into an array of strings, and returns it. If LIMIT is specified, splits into at most that number of fields. If PATTERN is omitted, splits on whitespace. Following is the example:

#!/usr/bin/perl

# define Strings
$var_string = "Rain-Drops-On-Roses-And-Whiskers-On-Kittens";
$var_names = "Larry,David,Roger,Ken,Michael,Tom";

# transform above strings into arrays.
@string = split('-', $var_string);
@names  = split(',', $var_names);

print "$string[3]\n";  # This will print Roses
print "$names[4]\n";   # This will print Michael

This will produce following result:

Roses
Michael

Transform Arrays to Strings

We can use the join() function to rejoin the array elements and form one long scalar string. This function has following syntax:

join EXPR, LIST

This function joins the separate strings of LIST into a single string with fields separated by the value of EXPR, and returns the string. Following is the example:

#!/usr/bin/perl

# define Strings
$var_string = "Rain-Drops-On-Roses-And-Whiskers-On-Kittens";
$var_names = "Larry,David,Roger,Ken,Michael,Tom";

# transform above strings into arrays.
@string = split('-', $var_string);
@names  = split(',', $var_names);

$string1 = join( '-', @string );
$string2 = join( ',', @names );

print "$string1\n";
print "$string2\n";

This will produce following result:

Rain-Drops-On-Roses-And-Whiskers-On-Kittens
Larry,David,Roger,Ken,Michael,Tom

Sorting Arrays

The sort() function sorts each element of an array according to ASCII Numeric standards. This function has following syntax:

sort [ SUBROUTINE ] LIST

This function sorts the LIST and returns the sorted array value. If SUBROUTINE is specified then specified logic inside the SUBTROUTINE is applied while sorting the elements.

#!/usr/bin/perl

# define an array
@foods = qw(pizza steak chicken burgers);
print "Before: @foods\n";

# sort this array
@foods = sort(@foods);
print "After: @foods\n";

This will produce following result:

Before: pizza steak chicken burgers
After: burgers chicken pizza steak

Please note that sorting is performed based on ASCII Numeric value of the words. So the best option is to first transform every element of the array into lowercase letters and then perform the sort function.

The $[ Special Variable

So far you have seen simple variable we defined in our programs and used them to store and print scalar and array values. Perl provides numerous special variables which have their predefined meaning.

We have a speciall variable which is written as $[. This special variable is a scalar containing the first index of all arrays. Because Perl arrays have zero-based indexing, $[ will almost always be 0. But if you set $[ to 1 then all your arrays will use on-based indexing. It is recommended not to use any other indexing other than zero. However, let's take one example to show the usage of $[ variable:

#!/usr/bin/perl

# define an array
@foods = qw(pizza steak chicken burgers);
print "Foods: @foods\n";

# Let's reset first index of all the arrays.
$[ = 1;

print "Food at \@foods[1]: $foods[1]\n";
print "Food at \@foods[2]: $foods[2]\n";

This will produce following result:

Foods: pizza steak chicken burgers
Food at @foods[1]: pizza
Food at @foods[2]: steak

Merging Arrays

Because an array is just a comma-separated sequence of values, you can combine them together as shown below:

#!/usr/bin/perl

@numbers = (1,3,(4,5,6));

print "numbers = @numbers\n";

This will produce following result:

numbers = 1 3 4 5 6

The embedded arrays just become part of the main array as shown below:

#!/usr/bin/perl

@odd = (1,3,5);
@even = (2, 4, 6);

@numbers = (@odd, @even);

print "numbers = @numbers\n";

This will produce following result:

numbers = 1 3 5 2 4 6

Selecting Elements from Lists

The list notation is identical to that for arrays - you can extract an element from an array by appending square brackets to the list and giving one or more indices:

#!/usr/bin/perl

$var = (5,4,3,2,1)[4];

print "value of var = $var\n"

This will produce following result:

value of var = 1

Similarly, we can extract slices, although without the requirement for a leading @ character:

#!/usr/bin/perl

@list = (5,4,3,2,1)[1..3];

print "Value of list = @list\n";

This will produce following result:

Value of list = 4 3 2

FOR MORE DOWNLOAD EBOOK,EXAMPLES

AND QUICK LERNING PDF

Monday, 29 December 2014