Perl Session

7/28/2019 Perl Session

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Practical Extraction and Report Language

or

Pathologically Eclectic Rubbish Lister


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Perl topics

Why Perl scripting

Basic Structure of Perl Program First Perl Program An example

Operators

Variables & Scope, Scalar Variables & stdin

Perl List Output Functions

Strings and various string functions

Control Flows, Arrays,Hashes

Functions ( Subroutines)

Regular Expressions & Patter matching. File handling.

Perl special variables

CPAN Expect scripting in perl Perl module example

Perl exercises


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Why Perl

Perl syntactically looks like C Perl Open source

Reg. Expressions are easy to implement

Great Community with support

CPAN provides tens of thousands of open sourced modules. Third party support.


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Basic Structure

Barces resemblances to C or Java

semi-colons separate executable statements

{ } delimit blocks, loops, subroutines

Comments begin with # and extend to end of line

No main() function code executed top-down.

Function arguments are comma-separated


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First Perl Program

#!/usr/bin/perlwprint "Now Iam a PERL programmer \n";

Standard: #!/usr/bin/perl[MUST be first line of the file no comments, no spaces

prior to the shebang]

Change the permissions to the file with chmod.

Chmod 755 example.pl

Execution : ./example.pl


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Perl has a range of operators, many of which are similar to the

operators used in C

Assignment Operator

= $n=22; $n2=$n;

Auto increment and decrements operators

++, --

$a++;$b--;

Arithematic operators

+, -, *, /, ** $test = 10 *20 +67 -45

Concatenation Operator

. $test1 = How;$test2=are;$test3=you;

Print $test1.test2.$test3

Operators


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Repetition Operator

X

Print Hay x 9

Relational Operators

Numerical data operators : =, ==, !=, String data operators :

lt gt le ge eq ne cmp(returns -1, 0 , 1)

logical Operators

AND - && or and

OR - || or or NOT - not

Range Operator

..

Print 250..300


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Binding Operator

=~, !~ Used in pattern matching

If ($a =~ m/there/) { print $a; }

Arrow operator

-> Also called reference operator, Used to create objects.

File I/O Operator

Used to read data from files or from standard input.


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Perl variables

Three types of variables

Scalars Defined by $ symbol, used to store either a number or a string

Arrays

Defined by @ symbol, internally List of scalar variables

Hashes Defined by % symbol and each item is identified by a key

value

Variable names are case sensitive Variable name can have alphabets, numbers and underscore (_)

All normal variable rules applicable as in C language Ex : $a=20; $fresh_value=50.58;


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Variables can be scoped either local or global

Global variables can be imported from/exported to other modules

Local variables are restricted to the blocks they are declared in

Variables define with my

The variables declared with my() are visible only within the scope ofthe block which names them. They are not visible outside of thisblock, not even in routines or blocks that it calls. If more than onevalue is listed, the list must be placed in parentheses.

Variables defined with local

'Local' variables, are visible to routines that are called from the

block where they are declared

Scope of variables


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Example on variables scope

my $y=2;

local $x=10;first();print "The value of local variable $x";

sub first {

local $x = 1;my $y = 1;

second(); }sub second {

print "x=", $x, "\n";

print "y=", $y, "\n"; }Output :

x=1

y= 2

The value of local variable 10


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Scalar Variables

A Scalar variable contains a single value. All scalar variables arepreceded by $ Symbol.

All of the standard types from C can be stored in a scalar variable

int, float, char, double, etc

No declaration of type of variable These scalar variables stores either a number or string.

Pre declaration is not required. If used strict module, pre declarationis required.

Example

$Hello = Hi How are you; $pvalue=7.568;

$ivalue=19;


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Reading data from keyboard as input

or is input operator, used to read data from keyboardincluding \n (From user)

Perl special variable $_ (@_) will stores this input value.

Example:#!/usr/bin/perl

print Enter your name ;

$name = ;

print \n Hi $name how are you;

STDIN


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Lists

A list is a comma-separated sequence of scalar values.

Collection of scalar data, Arrays and Hashes Represents a logical group of items

Any number, and any types of scalars can be held in a list:

(42, krishna kumar', sample,75.5); Lists are passed to functions, stored in arrays, and used in assignments my ($foo, $bar, $baz) = (35, 43.4, 'hello');

An assignment of a list of variables need not containthe same number of values on the left and right:

my ($foo,$bar)=(34,'hello',98.3);

98.3 simply discarded my ($alpha, $beta) = 5;

$alpha gets 5, $beta remains undefined


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List assignments can be used to 'swap' variables:

my ($one, $two) = ('alpha', 'beta'); ($one, $two) = ($two, $one);

Example

print (1,2,3) displays 123


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Output functions

Perl has print and printf functions.

print Is Unformatted output statement which can print a string, Number or

a List

Default outputs to STDOUT, can be redirected to STDERR or a File

print $myvalue; print FP I am going to print in File;

printf

Formatted output statement

By defaults outputs to STDOUT. Can be redirected to STDERR or aFile printf Format, variable list printf FP, variable List


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Perl strings

String is the Sequence of characters. Perl strings are two types Single quoted string

Variable values are not interpolated.

Example : $a = 10; print \n The value of a is $a; o/p : \n The value of a is $a

Double quoted string Variable values are interpolated

Example : $a = 10; print The value of a is $a;

o/p : The value of a is 10


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String operators

Concatenation & repetition Operator can be applied

on strings

Some String functions lc STRING Used to convert to lower case uc STRING Used to convert to Upper case

lcfirst STRING Used to convert initial case to lower

ucfirst STRING Used to convert initial case to upper

length (STRING) Used to get length of string

substr STRING,OFFSET,[LENGTH], REPLACEMENT

Used to extract/replace substring from main string


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Perl control structures

Blocks

Branching Statements if

if else

if elsif

Looping Statements while / until

do while/ until

for

foreach

Other Control Statements last, next, redo

exit, die


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Blocks

Blocks are set of statements enclosed between pair of

curling braces and each statement is delimited withsemicolon (;)

Example :

{

an expression;

another expression;

another expression;

last one;}


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If - block

if (expression) {

an expression;

another expression;

last one

}

If else - block

if (expression) {

an expression;

another expression;

last one

} else {

}

Branching statements


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If-elsif block

if (expression) {

an expression;

another expression;

last one} elsif (expression) {

}


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while loopwhile (condition) {

#while block statements

}

Control comes out of the loop when the condition is FALSE

until loopuntil (condition) {

#until block statements

}Control comes out of the loop when the condition is TRUE

Looping statements


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do while loop

do{

#while block statements

}while (condition)

Control comes out of the loop when the condition is FALSE

do until loop

do {

#until block statements

}until (condition)

Control comes out of the loop when the condition is TRUE


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for

for(init expr ; condition ; increment/decrement ) {

#for block

}

foreachmostly used when iteration has to be done for different values in a list

foreach Variable (LIST) {

#Block of Statements

}


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Other Control statements

last

exits the loop

continue block is not executed, if present

Will not evaluate the value of the control variable but repeats theloop

continue flow control statement follows while or foreach loop

continue block will be executed after every successful completion ofan iteration

next

used to continue with the next iteration of the loop by stopping

current iteration cannot be used in side do-while/do-until loop

if continue block is present it will also be executed

redo

restarts the loop without evaluating the condition again

continue block is not executed, if present


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exit

used to terminate the execution of a script

exit 0 denotes success and exit 1 denotes failure

cannot display error messages while exiting

$Choice= if ($Choice==4) { exit 0};

die

displays the error message to STDERR stream

and terminates the execution of a script

copy($TargetFile,$SourceFile) or die File cannot be copied


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Arrays

Arrays are variables that contain a list.

Arrays are declared with any size or type. They can hold lists containing anynumber or type of values.

Size can grow/shrink dynamically.

All array variable names begin with @

my (@s, @stuff, $size); @s =('a', 'list', 'of', 'strings');

@stuff = (32, 'Paul', 54.09);


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Accessing array elements

To get at a specific element of the array, place the index number in []after the array name, and replace the @ with a $ You're accessing a*single* value

my @foo = ("Hello", "World");my $greeting = $foo[0];

my $location = $foo[1]; my @age =('I am', 25, 'years old');

$age[1] = 26;@age now => ('I am', 26, 'years old')


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Array flattening

Remember that lists (and therefore arrays) can only contain scalar values.

Trying to place one array in another will result in array flattening:

my @in = (25, 50, 75);

my @out= ('a', 'b', 'c', @in, 'd', 'e');

@out contains eight elements:

a, b, c, 25, 50, 75, d, e Arrays on LHS of assignment will 'eat' remaining elements:

my ($foo, @bar, $baz) = (5, 6, 7, 8);

$foo = 5, @bar = (6, 7, 8), $baz undefined


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Special array variable

for any array @foo, there exists a scalar variable $#foo, which containsthe last index of @foo

@stuff = (5, 18, 23, 10);

$#stuff 3

This variable can be used to dynamically alter the size of the array:

$#stuff = 10; #@stuff now has 7 undef's$#stuff = 1; #@stuff now => (5, 18)

To receive the length of the array

$length = @array

To receive the first element of the array

($first)=@array


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Some Functions supported on Arrays

To copy one array to a new array

@array1 = @array2 To add an new value to beginning of the array

unshift(@array, newelement);

To add the value to the end of the array

push( @array, newelement); To remove first value of an array

shift(@array)

To remove the last element of the array

pop(@array)

To replace the specific element in an array $array[number]=$newelement;

To sort any array sort(@array)


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To reverse the elements in the array

reverse( @array)

Delete or replace elements with in the array can be made using splicefunction

splice(arrayname,position,numberof elements) If user wants to make a string from array elements with some delimiter

join function will be useful.

join (delimeter, arrayname)

The split function will be useful when user wants to create an array fromstring with constant delimeter

@browser=split(/delimeter/, $list)


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Hashes

Also known as associative arrays

Hash names start with a % A list of key/value pairs.

Unordered collection of values

Each element is linked to a key, which uniquely identifies that element.

Writing the value of a hash at an existing key overwrites the existingvalue.

Similar to arrays, but 'indices' can be any scalar value, not just integers.

both the keys and values can be any scalar value, including multipletypes in the same hash.

Any element from the hash can be retrieved, added, deleted using thekey

Used to maintain a list of corresponding values.


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Hash Example

Creating an empty Hash

%new_hash=();

Creating hash with elements

my %points_for = ('touchdown' => 6,'point after' => 1,'safety' => 2,'field goal' => 3

);

or

my %new_hash= (cpu' , intel',

printer' , cannon',

keyboard' , dell'

);


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Similar to arrays, access specific element by replacing % with $, andenclosing the key in { }

Accessing elements

Elements can be retrieved using key

$RetrievedElement = $Hash{Key};

Displaying hash elements

% new_hash= (

cpu' , intel',

printer' , cannon',

keyboard' , dell

)

print $new_hash{cpu};

print $new_hash{keyboard};

print %new_hash;


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Tip: If you feel the need to keep two lists of values, and accesscorresponding values in each list you want a hash.

Adding new elements

$Hash{ new key}= new element

Changing the existing elements

$Hash{key} = new element

Example

my %n=('two'=>'beta', 'one'=>'alpha');

if key is single word, can omit the quotes:

$n{two} = 2; #%n still has only 2 key/value pairs

$n{last} = 'omega';#%n now has 3 key/value pairs


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Deleting an element from Hash

delete ($Hash{$Key}) Reversing Hash

reverse (%Hash)

reverses keys and values

after reversal keys become values and vice versa Merging two Hashes

%NewHash = (%Hash1,%Hash2);

Using keys all keys of Hash can be retrieved


@keyarray = keys (%n); Using values all key values of Hash can be retrieved


@valuesarray = values (%n);


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Hashes will "flatten" if assigned to an array:

my %prof_of = (Perl => 'Lalli',CS1 => 'Hardwick',CS2 => 'Cutler');

my @profs_and_classes = %prof_of;

@profs_and_classes('CS1', 'Hardwick', 'CS2', 'Cutler', 'Perl', 'Lalli')


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Subroutines

Portion of the script that performs a specific task. Common repeatedtasks can be put under subroutine.

Function/subroutine same thing in perl Pre declaration not essential

If user needs to pre declare use the keyword sub

Example : sub ADD Any arguments passed to the subroutine are show up in the array @_.

Therefore, if you called a function with two arguments, those would bestored in $_[0] and $_[1].

The return value of a subroutine is the value of the last expressionevaluated. More explicitly, a return statement may be used to exit thesubroutine


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Sample examples

Example:

sub mySub{

print Oh!!! its really executed \n;

}

# Function call&hello;


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Arguments

Arguments passed as single list - the argument array @_

Example :&fun2(1, abc);

sub fun2{

print a1=$_[0], a2=$_[1]\n;

}

So Common technique to name arguments:Example:

sub another{

my($a, $b, $c) = @_;

# ...}

$a, $b, $c set to first 3 values of @_


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Regular expressions

A regular expression, often called a pattern in Perl, is atemplate that either matches or doesn't match a given string. Incredibly powerful feature of Perl Uses Match operator (=~ or !~ ) The =~ operator tests whether pattern is matched. !~ is similar to =~ operator, expect that it checks whether a

patter is not matched.


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^ Matches the beginning of input.

$ Matches the end of input. * Matches the preceding character zero or more times. For example,

"zo*" matches either "z" or "zoo".

+ Matches the preceding character one or more times. For example,

"zo+" matches "zoo" but not "z". ? Matches the preceding character zero or one time. For example,

"a?ve?" matches the "ve" in "never".

.Matches any single character except a newline character.

{n}n is a nonnegative integer. Matches exactly n times. For example,"o{2}" does not match the "o" in "Fob," but matches the first two o's in"foooood".{n,}n is a nonnegative integer. Matches at least n times.

For example, "o{2,}" does not match the "o" in "Fob" and matches allthe o's in "foooood". "o{1,}" is equivalent to "o+". "o{0,}" is equivalentto "o*".


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{n,m}m and n are nonnegative integers. Matches at least n and at mostm times. For example, "o{1,3}" matches the first three o's in "fooooood"."o{0,1}" is equivalent to "o?".

[xyz] A character set. Matches any one of the enclosed characters. Forexample, "[abc]" matches the "a" in "plain".

[ xyz] A negative character set. Matches any character not enclosed.For example, "[^abc]" matches the "p" in "plain".

\d Matches a digit character. Equivalent to [0-9].

\D Matches a non digit character. Equivalent to [^0-9].

\fMatches a form-feed character.

\n Matches a new line character.

\rMatches a carriage return character.


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\s Matches any white space including space, tab, form-feed, etc.Equivalent to "[ \f\n\r\t\v]".

\S Matches any nonwhite space character. Equivalent to "[^ \f\n\r\t\v]".

\t Matches a tab character.

\v Matches a vertical tab character. \w Matches any word character including underscore. Equivalent to

"[A-Za-z0-9_]".

\W Matches any non word character. Equivalent to "[^A-Za-z0-9_]".


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Sequence /foo/ # matches foo

Alternation /foo|bar/ # matches foo or bar; same as

/(foo)|(bar)/ Multipliers

/\d+/ # match a non-empty string of digits /(\+|\-)?\d+/ # match a signed integer /\s*/ # zero or more white-space characters /{\d}4\-{\d}2\-{\d}2/ # match the date format yyyy-

mm-dd

For more http://www.cs.tut.fi/~jkorpela/perl/regexp.html


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Example 1

$_ = 2006-09-25";If (/(\d\d\d\d)\-\d\d-(\d\d)/) {

$year = $1;

$day = $2;

}($year, $day) = /(\d\d\d\d)\-\d\d-(\d\d)/;

Example 2

$_ = "number_of_lines";

s/number_of_lines/numberOfLines/;

while () {next if /^\s*#/; # skip comment lines

s/^\s+//; # remove leading white space

s/\s+$//; # remove trailing white space}


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File Handling

Data that flow from the the program

needs to be stored in a permanent storage so that it can bereferred to later

In PERL files are given a name, a handle, basically another way ofsaying alias. All input and output with files is achieved throughfilehandling. Filehandles are also a means by one program maycommunicate with another program

A filehandle is nothing more than a nickname for the files

The die function exists with the help of this function we can shoot outerrors while handling files

Files are opened with open function and close with close function

Opening a File

open FILEHANDLE, FileName


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Data.txt)

>> File will be opened for appending if the the file name is prefixed

by >>

If the file doesnt exist new file will be created

open (FP,>>Data.txt)

Different file opening modes


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+>

+> will erase the existing contents and open the file for readand write

open (FP, +>Data.txt)

+

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