(PERL) Introduction What is PERL? Practical Report and Extraction - - PDF document

Practical Report and Extraction Language

(PERL)

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Introduction

• What is PERL?

– Practical Report and Extraction Language. – It is an interpreted language optimized for scanning arbitrary text files, extracting information from them, and printing reports based on that information. – Very powerful string handling features. – Available on all platforms.

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Main Advantages

• Speed of development

– You can enter the program in a text file, and just run it. It is an interpretive language; no compiler is needed.

• It is powerful

– The regular expressions of Perl are extremely powerful. – Uses sophisticated pattern matching techniques to scan large amounts of data very quickly.

• Portability

– Perl is a standard language and is available on all platforms. – Free versions are available on the Internet.

• Editing Perl programs

– No sophisticated editing tool is needed. – Any simple text editor like Notepad or vi will do.

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• Flexibility

– Perl does not limit the size of your data. – If memory is available, Perl can handle the whole file as a single string. – Allows one to write simple programs to perform complex tasks.

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How to run Perl?

• Perl can be downloaded from the Internet.

– Available on almost all platforms.

• Assumptions:

– For Windows operating system, you can run Perl programs from the command prompt.

• Run “cmd” to get command prompt window.

– For Unix/Linux, you can run directly from the shell prompt.

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• Recommended steps:

– Create a directory/folder where you will be storing the Perl files. – Using any text editor, create a file “test.pl” with the following content: print “Good day\n”; print “This is my first Perl program\n”; – Execute the program by typing the following at the command prompt: perl test.pl

Working through an example

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• On Unix/Linux, an additional line has to be given at

the beginning of every Perl program.

#!/usr/bin/perl print “Good day\n”; print “This is my first Perl program \n”;

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Variables

• Scalar variables

– A scalar variable holds a single value. – Other variable types are also available (array and associative array) – to be discussed later. – A ‘$’ is used before the name of a variable to indicate that it is a scalar variable. $xyz = 20;

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• Some examples:

$a = 10; $name=“Indranil Sen Gupta”; $average = 28.37; – Variables do not have any fixed types. – Variables can be printed as: print “My name is $name, the average temperature is $average\n”;

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• Data types:

– Perl does not specify the types of variables.

• It is a loosely typed language.
• Languages like C or java are strongly typed.

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• A powerful feature

– Variable names are automatically replaced by values when they appear in double-quoted strings.

• An example:

$stud = “Rupak”; $marks = 75; print “Marks obtained by $stud is $marks\n”; print ‘Marks obtained by $stud is $marks\n’;

Variable Interpolation

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– The program will give the following output: Marks obtained by Rupak is 75 Marks obtained by $stud is $marks – What do we see:

• If we need to do variable interpolation, use double

quotes; otherwise, use single quotes.

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• Another example:

$Expense = ‘$100’; print “The expenditure is $Expense.\n”;

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Expressions with Scalars

• Illustrated through examples (syntax similar to C)

$abc = 10; $abc++; $total- -; $a = $b ** 10; # exponentiation $a = $b % 10; # modulus $balance = $balance + $deposit; $balance += $deposit;

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• Operations on strings:

– Concatenation: the dot (.) is used. $a = “Good”; $b = “ day”; $c = “\n”; $total = $a.$b.$c; # concatenate the strings $a .= “ day\n”; # add to the string $a

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– Arithmetic operations on strings $a = “bat”; $b = $a + 1; print $a, “ and ”, $b; will print bat and bau – Operations carried out based on ASCII codes.

• May not always be meaningful.

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– String repetition operator (x). $a = $b x3; will concatenate three copies of $b and assign it to $a. print “Ba”. “na”x2; will print the string “banana”.

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String as a Number

• A string can be used in an arithmetic expression.

– How is the value evaluated? – When converting a string to a number, Perl takes any spaces, an optional minus sign, and as many digits it can find (with dot) at the beginning of the string, and ignores everything else. “23.54” evaluates to 23.54 “123Hello25” evaluates to 123 “banana” evaluates to 0

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• The character ‘\’ is used as the escape character.

– It escapes all of Perl’s special characters (e.g., $, @, #, etc.). $num = 20; print “Value of \$num is $num\n”; print “The windows path is c:\\perl\\”;

Escaping

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Line Oriented Quoting

• Perl supports specification of a string spanning

multiple lines.

– Use the marker ‘<<’. – Follow it by a string, which is used to terminate the quoted material.

• Example:

print << terminator; Hello, how are you? Good day. terminator

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• Another example:

print “<HTML>\n”; print “<HEAD><TITLE>Test page </TITLE></HEAD>\n”; print “<BODY>\n”; print “<H2>This is a test document.<H2>\n”; print “</BODY></HTML>”;

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print << EOM; <HTML> <HEAD><TITLE>Test page </TITLE></HEAD> <BODY> <H2>This is a test document.<H2> </BODY></HTML> EOM

Lists and Arrays

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Basic Difference

• List is an ordered list of scalars.
• Array is a variable that holds a list.
• Each element of an array is a scalar.
• The size of an array:

– Lower limit: 0 – Upper limit: no specific limit; depends on virtual memory.

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List Literal

• Examples:

(10, 20, 50, 100) (‘red', “blue", “green") (“a", 1, 2, 3, ‘b') ($a, 12) () # empty list (10..20) # list constructor function (‘A’..’Z’) # same, for lettere\s

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Specifying Array Variable

• We use the special character ‘@’.

@months # denotes an array The individual elements of the array are scalars, and can be referred to as:

$months[0] # first element of @months $months[1] # second element of @months ……

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Initializing an Array

• Two ways:

– Specify values, separated by commas. @color = (‘red’, ‘green’, “blue”, “black”); – Use the quote words (qw) function, that uses space as the delimiter: @color = qw (red green blue black);

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Array Assignment

– Assign from a list of literals @numbers = (1, 2, 3); @colors = (“red”, “green”, “blue”); – From the contents of another array. @array1 = @array2; – Using the qw function: @word = qw (Hello good morning); – Combination of above: @allcolors = (“white”, @colors, “brown”);

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– Some other examples: @xyz = (2..5); @xyz = (1, @xyz); @xyz = (@xyz, 6);

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Multiple Assignments

($x, $y, $y) = (10, 20, 30); ($x, $y) = ($y, $x); # swap elements ($a, @col) = (‘red’, ‘green’, ‘blue’); # $a gets the value ‘red’ # @col gets the value (‘green’, ‘blue’) ($first, @val, $last) = (1, 2, 3, 4); # $first gets the value 1 # @val gets the value (2, 3, 4) # $last is undefined

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Number of Elements in Array

• Two ways:

$size = scalar @colors; $size = @colors;

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Accessing Elements

@list = (1, 2, 3, 4); $first = $list[0]; $fourth = $list[3]; $list[1]++; # array becomes (1, 3, 3, 4) $x = $list[5]; # $x gets the value undef $list[2] = “Go”;# array becomes (1, 2, “Go”, 4)

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• The $# is the index of the last element of the array.

@value = (1, 2, 3, 4, 5); print “$#value \n”; # prints 4

• An empty array has the value

$#value = -1;

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shift and unshift

• They operate on the front of the array.

– ‘shift’ removes the first element of the array. – ‘unshift’ replaces the element at the start of the array.

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• Example:

@color = qw (red, blue, green, black); $first = shift @color; # $first gets “red”, and @color becomes # (blue, green, black) unshift (@color, “white”); # @color becomes (white, blue, green, black)

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pop and push

• They operate on the bottom of the array.

– ‘pop’ removes the last element of the array. – ‘push’ replaces the last element of the array.

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• Example:

@color = qw (red, blue, green, black); $first = pop @color; # $first gets “black”, and @color becomes # (red, blue, green) push (@color, “white”); # @color becomes (red, blue, green, white)

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Reversing an Array

• By using the ‘reverse’ keyword.

@names = (“Mina”, “Tina”, ‘Rina”) @rev = reverse @names; # Reversed list stored in ‘rev’. @names = reverse @names; # Original array is reversed.

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Printing an Array

• Example:

@colors = qw (red, green, blue); print @colors; # prints without spaces – redgreenblue print “@colors”; # prints with spaces – red green blue

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Sort the Elements of an Array

• Using the ‘sort’ keyword, by default we can sort the

elements of an array lexicographically.

– Elements considered as strings. @colors = qw (red blue green black); @sort_col = sort @colors # Array @sort_col is (black blue green red)

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– Another example: @num = qw (10 2 5 22 7 15); @new = sort @num; # @new will contain (10 15 2 22 5 7) – How do sort numerically? @num = qw (10 2 5 22 7 15); @new = sort {$a <=> $b} @num; # @new will contain (2 5 7 10 15 22)

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The ‘splice’ function

• Arguments to the ‘splice’ function:

– The first argument is an array. – The second argument is an offset (index number of the list element to begin splicing at). – Third argument is the number of elements to remove. @colors = (“red”, “green”, “blue”, “black”); @middle = splice (@colors, 1, 2); # @middle contains the elements removed

File Handling

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Interacting with the user

• Read from the keyboard (standard input).

– Use the file handle <STDIN>. – Very simple to use. print “Enter your name: ”; $name = <STDIN>; # Read from keyboard print “Good morning, $name. \n”; – $name also contains the newline character.

• Need to chop it off.

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The ‘chop’ Function

• The ‘chop’ function removes the last character of

whatever it is given to chop.

• In the following example, it chops the newline.

print “Enter your name: ”; chop ($name = <STDIN>); # Read from keyboard and chop newline print “Good morning, $name. \n”;

• ‘chop’ removes the last character irrespective of

whether it is a newline or not.

– Sometimes dangerous.

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Safe chopping: ‘chomp’

• The ‘chomp’ function works similar to ‘chop’, with

the difference that it chops off the last character

• nly if it is a newline.

print “Enter your name: ”; chomp ($name = <STDIN>); # Read from keyboard and chomp newline print “Good morning, $name. \n”;

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File Operations

• Opening a file

– The ‘open’ command opens a file and returns a file handle. – For standard input, we have a predefined handle <STDIN>. $fname = “/home/isg/report.txt”;

• pen XYZ , $fname;

while (<XYZ>) { print “Line number $. : $_”; }

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– Checking the error code: $fname = “/home/isg/report.txt”;

• pen XYZ, $fname or die “Error in open: $!”;

while (<XYZ>) { print “Line number $. : $_”; } – $. returns the line number (starting at 1) – $_ returns the contents of last match – $i returns the error code/message

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• Reading from a file:

– The last example also illustrates file reading. – The angle brackets (< >) are the line input operators.

• The data read goes into $_

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• Writing into a file:

$out = “/home/isg/out.txt”;

• pen XYZ , “>$out” or die “Error in write: $!”;

for $i (1..20) { print XYZ “$i :: Hello, the time is”, scalar(localtime), “\n”; }

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• Appending to a file:

$out = “/home/isg/out.txt”;

• pen XYZ , “>>$out” or die “Error in write: $!”;

for $i (1..20) { print XYZ “$i :: Hello, the time is”, scalar(localtime), “\n”; }

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• Closing a file:

close XYZ;

where XYZ is the file handle of the file being closed.

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• Printing a file:

– This is very easy to do in Perl. $input = “/home/isg/report.txt”;

• pen IN, $input or die “Error in open: $!”;

while (<IN>) { print; } close IN;

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Command Line Arguments

• Perl uses a special array called @ARGV.

– List of arguments passed along with the script name on the command line. – Example: if you invoke Perl as: perl test.pl red blue green then @ARGV will be (red blue green). – Printing the command line arguments: foreach (@ARGV) { print “$_ \n”; }

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Standard File Handles

• <STDIN>

– Read from standard input (keyboard).

• <STDOUT>

– Print to standard output (screen).

• <STDERR>

– For outputting error messages.

• <ARGV>

– Reads the names of the files from the command line and

• pens them all.

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– @ARGV array contains the text after the program’s name in command line.

• <ARGV> takes each file in turn.
• If there is nothing specified on the command line, it

reads from the standard input. – Since this is very commonly used, Perl provides an abbreviation for <ARGV>, namely, < > – An example is shown.

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$lineno = 1; while (< >) { print $lineno ++; print “$lineno: $_”; } – In this program, the name of the file has to be given on the command line. perl list_lines.pl file1.txt perl list_lines.pl a.txt b.txt c.txt

Control Structures

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Introduction

• There are many control constructs in Perl.

– Similar to those in C. – Would be illustrated through examples. – The available constructs:

• for
• foreach
• if/elseif/else
• while
• do, etc.

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Concept of Block

• A statement block is a sequence of statements

enclosed in matching pair of { and }.

if (year == 2000) { print “You have entered new millenium.\n”; }

• Blocks may be nested within other blocks.

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Definition of TRUE in Perl

• In Perl, only three things are considered as FALSE:

– The value 0 – The empty string (“ ”) – undef

• Everything else in Perl is TRUE.

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if .. else

• General syntax:

if (test expression) { # if TRUE, do this } else { # if FALSE, do this }

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• Examples:

if ($name eq ‘isg’) { print “Welcome Indranil. \n”; } else { print “You are somebody else. \n”; } if ($flag == 1) { print “There has been an error. \n”; } # The else block is optional

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elseif

• Example:

print “Enter your id: ”; chomp ($name = <STDIN>); if ($name eq ‘isg’) { print “Welcome Indranil. \n”; } elseif ($name eq ‘bkd’) { print “Welcome Bimal. \n”; } elseif ($name eq ‘akm’) { print “Welcome Arun. \n”; } else { print “Sorry, I do not know you. \n”; }

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while

• Example: (Guessing the correct word)

$your_choice = ‘ ‘; $secret_word = ‘India’; while ($your_choice ne $secret_word) { print “Enter your guess: \n”; chomp ($your_choice = <STDIN>); } print “Congratulations! Mera Bharat Mahan.”

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for

• Syntax same as in C.
• Example:

for ($i=1; $i<10; $i++) { print “Iteration number $i \n”; }

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foreach

• Very commonly used function that iterates over a

list.

• Example:

@colors = qw (red blue green); foreach $name (@colors) { print “Color is $name. \n”; }

• We can use ‘for’ in place of ‘foreach’.

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• Example: Counting odd numbers in a list

@xyz = qw (10 15 17 28 12 77 56); $count = 0; foreach $number (@xyz) { if (($number % 2) == 1) { print “$number is odd. \n”; $count ++; } print “Number of odd numbers is $count. \n”; }

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Breaking out of a loop

• The statement ‘last’, if it appears in the body of a

loop, will cause Perl to immediately exit the loop.

– Used with a conditional. last if (i > 10);

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Skipping to end of loop

• For this we use the statement ‘next’.

– When executed, the remaining statements in the loop will be skipped, and the next iteration will begin. – Also used with a conditional.

Relational Operators

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The Operators Listed

le <= Less or equal ge >= Greater or equal lt < Less than gt > Greater than ne != Not equal eq == Equal String Numeric Comparison

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Logical Connectives

• If $a and $b are logical expressions, then the

following conjunctions are supported by Perl:

– $a and $b $a && $b – $a or $b $a || $b – not $a ! $a

• Both the above alternatives are equivalent; first one

is more readable.

String Functions

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The Split Function

• ‘split’ is used to split a string into multiple pieces using a

delimiter, and create a list out of it.

$_=‘Red:Blue:Green:White:255'; @details = split /:/, $_; foreach (@details) { print “$_\n”; }

– The first parameter to ‘split’ is a regular expression that specifies what to split on. – The second specifies what to split.

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• Another example:

$_= “Indranil isg@iitkgp.ac.in 283496”; ($name, $email, $phone) = split / /, $_;

• By default, ‘split’ breaks a string using space as

delimiter.

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The Join Function

• ‘join’ is used to concatenate several elements into a

single string, with a specified delimiter in between.

$new = join ' ', $x1, $x2, $x3, $x4, $x5, $x6; $sep = ‘::’; $new = join $sep, $x1, $x2, $w3, @abc, $x4, $x5;

Regular Expressions

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Introduction

• One of the most useful features of Perl.
• What is a regular expression (RegEx)?

– Refers to a pattern that follows the rules of syntax. – Basically specifies a chunk of text. – Very powerful way to specify string patterns.

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An Example: without RegEx

$found = 0; $_ = “Hello good morning everybody”; $search = “every”; foreach $word (split) { if ($word eq $search) { $found = 1; last; } } if ($found) { print “Found the word ‘every’ \n”; }

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Using RegEx

$_ = “Hello good morning everybody”; if ($_ =~ /every/) { print “Found the word ‘every’ \n”; }

• Very easy to use.
• The text between the forward slashes defines the

regular expression.

• If we use “!~” instead of “=~”, it means that the

pattern is not present in the string.

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• The previous example illustrates literal texts as

regular expressions.

– Simplest form of regular expression.

• Point to remember:

– When performing the matching, all the characters in the string are considered to be significant, including punctuation and white spaces.

• For example, /every / will not match in the previous

example.

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Another Simple Example

$_ = “Welcome to IIT Kharagpur, students”; if (/IIT K/) { print “’IIT K’ is present in the string\n”; { if (/Kharagpur students/) { print “This will not match\n”; }

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Types of RegEx

• Basically two types:

– Matching

• Checking if a string contains a substring.
• The symbol ‘m’ is used (optional if forward slash used

as delimiter). – Substitution

• Replacing a substring by another substring.
• The symbol ‘s’ is used.

Matching

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The =~ Operator

• Tells Perl to apply the regular expression on the

right to the value on the left.

• The regular expression is contained within

delimiters (forward slash by default).

– If some other delimiter is used, then a preceding ‘m’ is essential.

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Examples

$string = “Good day”; if ($string =~ m/day/) { print “Match successful \n"; } if ($string =~ /day/) { print “Match successful \n"; }

• Both forms are equivalent.
• The ‘m’ in the first form is optional.

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$string = “Good day”; if ($string =~ m@day@) { print “Match successful \n"; } if ($string =~ m[day[ ) { print “Match successful \n"; }

• Both forms are equivalent.
• The character following ‘m’ is the delimiter.

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Character Class

• Use square brackets to specify “any value in the list
• f possible values”.

my $string = “Some test string 1234"; if ($string =~ /[0123456789]/) { print "found a number \n"; } if ($string =~ /[aeiou]/) { print "Found a vowel \n"; } if ($string =~ /[0123456789ABCDEF]/) { print "Found a hex digit \n"; }

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Character Class Negation

• Use ‘^’ at the beginning of the character class to

specify “any single element that is not one of these values”.

my $string = “Some test string 1234"; if ($string =~ /[^aeiou]/) { print "Found a consonant\n"; }

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Pattern Abbreviations

• Useful in common cases

Not a space character \S Not a word character \W Not a digit, same as [^0-9] \D A space character (tab, space, etc) \s A word character, [0-9a-zA-Z_] \w A digit, same as [0-9] \d Anything except newline (\n)

.

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$string = “Good and bad days"; if ($string =~ /d..s/) { print "Found something like days\n"; } if ($string =~ /\w\w\w\w\s/) { print "Found a four-letter word!\n"; }

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Anchors

• Three ways to define an anchor:

^ :: anchors to the beginning of string $ :: anchors to the end of the string \b :: anchors to a word boundary

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if ($string =~ /^\w/) :: does string start with a word character? if ($string =~ /\d$/) :: does string end with a digit? if ($string =~ /\bGood\b/) :: Does string contain the word “Good”?

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Multipliers

• There are three multiplier characters.

* :: Find zero or more occurrences + :: Find one or more occurrences ? :: Find zero or one occurrence

• Some example usages:

$string =~ /^\w+/; $string =~ /\d?/; $string =~ /\b\w+\s+/; $string =~ /\w+\s?$/;

Substitution

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Basic Usage

• Uses the ‘s’ character.
• Basic syntax is:

$new =~ s/pattern_to_match/new_pattern/; What this does?

• Looks for pattern_to_match in $new and, if found,

replaces it with new_pattern.

• It looks for the pattern once. That is, only the first
• ccurrence is replaced.
• There is a way to replace all occurrences (to be

discussed shortly).

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Examples

$xyz = “Rama and Lakshman went to the forest”; $xyz =~ s/Lakshman/Bharat/; $xyz =~ s/R\w+a/Bharat/; $xyz =~ s/[aeiou]/i/; $abc = “A year has 11 months \n”; $abc =~ s/\d+/12/; $abc =~ s /\n$/ /;

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Common Modifiers

• Two such modifiers are defined:

/i :: ignore case /g :: match/substitute all occurrences $string = “Ram and Shyam are very honest"; if ($string =~ /RAM/i) { print “Ram is present in the string”; } $string =~ s/m/j/g; # Ram -> Raj, Shyam -> Shyaj

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Use of Memory in RegEx

• We can use parentheses to capture a piece of

matched text for later use.

– Perl memorizes the matched texts. – Multiple sets of parentheses can be used.

• How to recall the captured text?

– Use \1, \2, \3, etc. if still in RegEx. – Use $1, $2, $3 if after the RegEx.

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Examples

$string = “Ram and Shyam are honest"; $string =~ /^(\w+)/; print $1, "\n"; # prints “Ra\n” $string =~ /(\w+)$/; print $1, "\n"; # prints “st\n” $string =~ /^(\w+)\s+(\w+)/; print "$1 $2\n"; # prints “Ramnd Shyam are honest”;

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$string = “Ram and Shyam are very poor"; if ($string =~ /(\w)\1/) { print "found 2 in a row\n"; } if ($string =~ /(\w+).*\1/) { print "found repeat\n"; } $string =~ s/(\w+) and (\w+)/$2 and $1/;

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Example 1

• validating user input

print “Enter age (or 'q' to quit): "; chomp (my $age = <STDIN>); exit if ($age =~ /^q$/i); if ($age =~ /\D/) { print "$age is a non-number!\n"; }

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Example 2: validation contd.

• File has 2 columns, name and age, delimited by one
• r more spaces. Can also have blank lines or

commented lines (start with #).

• pen IN, $file or die "Cannot open $file: $!";

while (my $line = <IN>) { chomp $line; next if ($line =~ /^\s*$/ or $line =~ /^\s*#/); my ($name, $age) = split /\s+/, $line; print “The age of $name is $age. \n"; }

Some Special Variables

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$&, $` and $’

• What is $&?

– It represents the string matched by the last successful pattern match.

• What is $`?

– It represents the string preceding whatever was matched by the last successful pattern match.

• What is $‘?

– It represents the string following whatever was matched by the last successful pattern match .

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– Example: $_ = 'abcdefghi'; /def/; print "$\`:$&:$'\n"; # prints abc:def:ghi

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• So actually ….

– S` represents pre match – $& represents present match – $’ represents post match

Associative Arrays

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Introduction

• Associative arrays, also known as hashes.

– Similar to a list

• Every list element consists of a pair, a hash key and a

value.

• Hash keys must be unique.

– Accessing an element

• Unlike an array, an element value can be found out by

specifying the hash key value.

• Associative search.

– A hash array name must begin with a ‘%’.

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Specifying Hash Array

• Two ways to specify:

– Specifying hash keys and values, in proper sequence. %directory = ( “Rabi”, “258345”, “Chandan”, “325129”, “Atul”, “445287”, “Sruti”, “237221” );

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– Using the => operator. %directory = ( Rabi => “258345”, Chandan => “325129”, Atul => “445287”, Sruti => “237221” ); – Whatever appears on the left hand side of ‘=>’ is treated as a double-quoted string.

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Conversion Array <=> Hash

• An array can be converted to hash.

@list = qw (Rabi 258345 Chandan 325129 Atul 445287 Sruti 237221); %directory = @list;

• A hash can be converted to an array:

@list = %directory;

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Accessing a Hash Element

• Given the hash key, the value can be accessed

using ‘{ }’.

• Example:

@list = qw (Rabi 258345 Chandan 325129 Atul 445287 Sruti 237221); %directory = @list; print “Atul’s number is $directory{“Atul”} \n”;

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Modifying a Value

• By simple assignment:

@list = qw (Rabi 258345 Chandan 325129 Atul 445287 Sruti 237221); %directory = @list; $directory{Sruti} = “453322”; $directory{‘Chandan’} ++;

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Deleting an Entry

• A (hash key, value) pair can be deleted from a hash

array using the “delete” function.

– Hash key has to be specified. @list = qw (Rabi 258345 Chandan 325129 Atul 445287 Sruti 237221); %directory = @list; delete $directory{Atul};

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Swapping Keys and Values

• Why needed?

– Suppose we want to search for a person, given the phone number. @list = qw (Rabi 258345 Chandan 325129 Atul 445287 Sruti 237221); %directory = @list; %revdir = reverse %directory; print “$revdir{237221} \n”;

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Using Functions ‘keys’, ‘values’

• ‘keys’ returns all the hash keys as a list.
• ‘values’ returns all the values as a list.

@list = qw (Rabi 258345 Chandan 325129 Atul 445287 Sruti 237221); %directory = @list; @all_names = keys %directory; @all_phones = values %directory;

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An Example

• List all person names and telephone numbers.

@list = qw (Rabi 258345 Chandan 325129 Atul 445287 Sruti 237221); %directory = @list; foreach $name (keys %directory) { print “$name \t $directory{$name} \n”; }

Subroutines

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Introduction

• A subroutine …..

– Is a user-defined function. – Allows code reuse. – Define ones, use multiple times.

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How to use?

• Defining a subroutine

sub test_sub { # the body of the subroutine goes here # …….. }

• Calling a subroutine

– Use the ‘&’ prefix to call a subroutine. &test_sub; &gcd ($val1, $val2); # Two parameters – However, the ‘&’ is optional.

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Subroutine Return Values

• Use the ‘return’ statement.

– This is also optional. – If the keyword ‘return’ is omitted, Perl functions return the last value evaluated.

• A subroutine can also return a non-scalar.
• Some examples are given next.

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Example 1

$name = ‘Indranil'; welcome(); # call the first sub welcome_namei(); # call the second sub exit; sub welcome { print "hi there\n"; } sub welcome_name { print "hi $name\n"; # uses global $name variable }

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Example 2

# Return a non-scalar sub return_alpha_and_beta { return ($alpha, $beta); } $alpha = 15; $beta = 25; @c = return_alpha_and_beta; # @c gets (5,6)

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Passing Arguments

• All arguments are passed into a Perl function

through the special array $_.

– Thus, we can send as many arguments as we want.

• Individual arguments can also be accessed as $_[0],

$_[1], $_[2], etc.

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Example 3

# Two different ways to write a subroutine to add two numbers sub add_ver1 { ($first, $second) = @_; return ($first + $second); } sub add_ver2 { return $_[0] + $_[1]; # $_[0] and $_[1] are the first two # elements of @_ }

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Example 4

$total = find_total (5, 10, -12, 7, 40); sub find_total { # adds all numbers passed to the sub $sum = 0; for $num (@_) { $sum += $num; } return $sum; }

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‘my’ variables

• We can define local variables using the ‘my’

keyword.

– Confines a variable to a region of code (within a block { } ). – ‘my’ variable’s storage is freed whenever the variable goes

• ut of scope.

– All variables in Perl is by default ‘global’.

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Example 5

$sum = 7; $total = add_any (20, 10, -15); # $total gets 15 sub add_any { # local variable, won't interfere # with global $sum my $sum = 0; for my $num (@_ ) { $sum += $num; } return $sum; }

Writing CGI Scripts in Perl

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Introduction

• Perl provides with a number of facilities to facilitate

writing of CGI scripts.

– Standard library modules.

• Included as part of the Perl distribution.
• No need to install them separately.

#!/usr/bin/perl use CGI qw (:standard);

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• Some of the functions included in the CGI.pm (.pm

is optional) are:

– header

• This prints out the “Content-type” header.
• With no arguments, the type is assumed to be

“text/html”. – start_html

• This prints out the <html>, <head>, <title> and <body>

tags.

• Accepts optional arguments.

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– end_html

• This prints out the closing HTML tags, </body>, >/html>.
• Typical usages and arguments would be illustrated

through examples.

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Example 1 (without using CGI.pm)

#!/usr/bin/perl print <<TO_END; Content-type: text/html <HTML> <HEAD> <TITLE> Server Details </TITLE> </HEAD> <BODY> Server name: $ENV{SERVER_NAME} <BR> Server port number: $ENV{SERVER_PORT} <BR> Server protocol: $ENV{SERVER_PROTOCOL} </BODY> </HTML> TO_END

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Example 2 (using CGI.pm)

#!/usr/bin/perl -wT use CGI qw(:standard); print header (“text/html”); print start_html ("Hello World"); print "<h2>Hello, world!</h2>\n"; print end_html;

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Example 3: Decoding Form Input

sub parse_form_data { my %form_data; my $name_value; my @nv_pairs = split /&/, $ENV{QUERY_STRING}; if ( $ENV{REQUEST_METHOD} eq ‘POST’ ) { my $query = “”; read (STDIN, $query, $ENV{CONTENT_LENGTH}); push @nv_pairs, split /&/, $query; }

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foreach $name_value (@nv_pairs) { my ($name, $value) = split /=/, $name_value; $name =~ tr/+/ /; $name =~ s/%([\da-f][\da-f])/chr (hex($1))/egi; $value =~ tr/+/ /; $value =~ s/%([\da-f][\da-f])/chr (hex($1))/egi; $form_data{$name} = $value; } return %form_data; }

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Using CGI.pm

• The decoded form value can be directly accessed

as:

$value = param (‘fieldname’);

• An equivalent Perl code as in the last example using

CGI.pm

– Shown in next slide.

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Example 4

#!/usr/bin/perl -wT use CGI qw(:standard); my %form_data; foreach my $name (param() ) { $form_data {$name} = param($name); }

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Example 5: sending mail

#!/usr/bin/perl -wT use CGI qw(:standard); print header; print start_html (“Response to Guestbook”); $ENV{PATH} = “/usr/sbin”; # to locate sendmail

• pen (MAIL, “| /usr/sbin/sendmail –oi –t”);

# open the pipe to sendmail my $recipient = ‘xyz@hotmail.com’; print MAIL “To: $recipient\n”; print MAIL “From: isg\@cse.iitkgp.ac.in\n”; print MAIL “Subject: Submitted data\n\n”;

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foreach my $xyz (param()) { print MAIL “$xyz = “, param($xyz), “\n”; } close (MAIL); print <<EOM; <h2>Thanks for the comments</h2> <p>Hope you visit again.</p> EOM print end_html;