Basic Perl Programming

Basic Perl Programming

Introduction:

• Perl - Practical Extraction and Report Language

• Using interpreter to run on the system (windows/unix/linux perl interpreter)

• Very powerful for handling and manipulate text

• Usually with *.pl file/script extension

1

Getting Perl

• In Linux/Unix OS it has become a standard package installed

• For Windows OS get it from:– http://www.activestate.com/– http://strawberryperl.com/

2

My first Perl program (Say hello to Perl)

#! /usr/bin/perl

# print a words "Say hello to Perl"

print "Say hello to Perl.\n";


3

Basic Standard Input/Output

#! /usr/bin/perl

print "\nPlease enter your name: ";

$user_name = <STDIN>;

print "\nHello ", $user_name,

"Please say hello to Perl\n";


4

Variable (Scalar)

• must be preceded with ‘$’ sign• no specific data types (int, float, char)

#! /usr/bin/perl

$food = "2-Piece Chicken";

$drink = "Pepsi";

print "Food = $food\n";

print "Drink = $drink\n\n";

$food = 4.59;

$drink = 1.7;

print "Food price= $food\n";

print "Drink price= $drink\n";

print "Total price= ", $drink + $food, "\n";


5

Exercise 1:

Write Perl script to ask student to enter name, e-mail, and course. The script will later print out all the information entered.


6

Variable (Array)

• preceded with ‘@’ sign

• several ways to define array:

@food = (“”);

$food[0] = “2-Piece Chickens”;

$food[1] = “Kentucky Nuggets (6 Pieces)”;

$food[2] = “Deli Burger”;

@drink = (“Pepsi”, “Ice Lemon Tea”, “Orange Juice”);


7

Variable (Array)

• syntax to get an array element:

$array_variable_name[index_value];

• retrieve the number of item and length of index of array variable:

$items = @drink # $ items will get the value of 3

$length = $#drink # $length will get the value of 2


8


Variable (Array)

• define and assign a value of array variable using split function

$f_price = “4.59 3.99 2.00”;

@food_price = split(/ /, $f_price);

@drink_price = split(/:/, “1.70:2.20:2.20”);

9


Exercise 2:

Modify the script from exercise 1 so it can handle more than single student information.

10

Basic Perl ProgrammingVariable (Hash)

• a series pairs of keys and values

• Preceded with “%” sign

• The syntax to define hash:

%hash_var = (key_1 => “value_1”, … , key_n => “value_n”

);

11

Basic Perl ProgrammingVariable (Hash)

• The other possible way to assign keys and values to hash:

%hash_var = undef;

$hash_var{key_1} = “value_1”;…$hash_var{key_n} = “value_n”;

12


Variable (Hash)

• Example:

%food = ("");

$food{fruit} = "Apple";

$food{animal} = "Beef";

%drink =(natural=>"Mineral Water",

carbonate=>"Coca-Cola");

13

Variable (Hash)

• syntax to get hash element:

$hash_var_name{key};

• get all keywords from hash and store it in an array

@array_var_name = keys(%hash_var_name);


14


Variable (Hash)

• Access hash keys name:

@keys = keys(%hash_var);

print "\$keys[0] = $keys[0] \n";print "\$keys[1] = $keys[1] \n";print "\$keys[.] = $keys[.] \n";print "\$keys[n] = $keys[n] \n";

15


Variable (Hash)

• Access and assign value to specific hash key:

print “$keys[0]= ” , $hash_var{$keys[0]}, “\n”;

$hash_var{$keys[0]} = “other value”;

print “$keys[0]= ” , $hash_var{$keys[0]}, “\n”;

16


Copying,Referencing & De-referencing Variable

• Copying variable:

$my_var = “hello”;$other_var = $my_var;

print “$my_var : $other_var \n”;

$other_var = “olleh”;

print “$my_var : $other_var \n”;

17


• Referencing & de-referencing variable:

$my_var = “hello”;$other_var = \$my_var;

print “$my_var : $$other_var \n”;

$$other_var = “olleh”;

print “$my_var : $$other_var \n”;

18


• Explanations:

$other_var = \$my_var; -> this will caused $other_var

store the address refer by

$my_var

$$other_var; -> this is the way we assign/access the value

stored by the address

$$other_var = “olleh”; -> this will caused address refer by both $my_var & $other_var had a new value (“olleh”)

19


• Referencing & de-referencing array & hash:

@my_array = (“One”, “Two”);%my_hash = (1 => “One”, 2 => “Two”);

$array_ref = \@my_array;$hash_ref = \%my_hash;

#de-referencing array & hash${$array_ref}[0] or $array_ref->[0]; ${$hash_ref}{2} or $hash_ref->{2};

#copy array & hash from reference @other_array = @{$array_ref}; %other_hash = %{$hash_ref};

20

Loop and control statement


if (…){} Executes when a specified condition is true

if (…) {} else {} Chooses between two alternatives

if (…){} elsif() {} else {} Chooses between more than two alternatives

for (…) {} Repeats a group of statements a specified number of times

foreach $var_name1 (@var_name2) Special loop to access all elements in array variable

while(…) {} Repeats a group of statements a specified number of times

21

Comparison Operators


Integer-Comparison Operators

Operator Description

< Less than

> Greater than

== Equal to

<= Less than or equal to

>= Greater than or equal to

!= Not equal to

22


Comparison Operators

String-Comparison Operators

Operator Description

lt Less than

gt Greater than

eq Equal to

le Less than or equal to

ge Greater than or equal to

ne Not equal to

23


Exercise 3:

Extend the script from exercise 2 so it can handle other new input (student’s CPA). Based on the CPA value determine either they get 1st, 2nd, or 3rd class following the given conditions below:

24

CPA Class

3.7 and above 1st

2.0 – 3.69 2nd

Below than 2.0 3rd

Pattern matching and string manipulation

The syntax for pattern matching:$string_variable =~ /pattern_to_match/opt;

Example:$str = “Hello World”; if ($str =~ /lo/) {

print “Found it.\n”;} else {

print “Not found.\n”;}


25



The syntax for pattern replacement:$string_variable =~ s/pattern_to_replace/replacement/opts;

Example:$str = “Hello World”;$str =~ s/l/m/g;

26



Options (opts) for pattern matching & replacement

Option Description

g Change all occurrences of the pattern

i Ignore case in pattern

e Evaluate replacement string as expression (only for substitution operator)

m Treat string to be matched as multiple lines

o Evaluate only once

s Treat string to be matched as single line

x Ignore white space in pattern

27



Regular expression in the pattern (pattern_to_replace) Meta-characters Char Meaning Char Meaning

^ beginning of string ? match 0 or 1 times

$ end of string | alternative

. any character except newline () grouping

* match 0 or more times [] set of characters

+ match 1 or more times {} repetition modifier

To present these meta-characters as a character itself use the ‘\’ character before each meta-characters. Examples: \^, \$, …

http://www.cs.tut.fi/~jkorpela/perl/regexp.html 28



Regular expression in the pattern (pattern_to_replace)

How to know if the string hold by the scalar $plate_no is a valid Johor’s car plate number?

- All letters must be capital start with ‘J’

^J

- The trailing letters not more than 2 and not including the ‘I’, ‘O’, and ‘Z’ letters

^J[A-H,J-N,P-Y]{1,2}

- Ending with numbers not starting with ‘0’ and not more than 4 in total

^J[A-H,J-N,P-Y]{1,2}[1-9]{1}[0-9]{0,3}

29



String manipulation functions

Function Description

index (string, substring)

Identify the location of a substring in a string.

rindex (string, substring)

Similar to the index function but starts searching from the right end of the string.

length (string) Returns the number of characters contained in a character string.

substr (string, num_of_skip_char, length)

Returns a part of a character string.

lc(string) Converts a string to lowercase.

uc(string) Converts a string to uppercase. 30



String concatenation:

#! /usr/bin/perl

$string1 = “The first string ”;$string2 = “The second string”; $my_string = $string1 . “ and ” . $string2 ;

print “$my_string \n”;

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File Input/Output

Read Operation:

if(open(MYFILE, "<directory/file_name")){

@array_var = <MYFILE>;

. . .

. . .

. . .

close(MYFILE);

}


32

File Input/Output

Write Operation:

if(open(MYFILE, “>directory/file_name")) {

. . .

. . .

. . .

print MYFILE ($var_store_file_content);

close(MYFILE);

}


33


Exercise 4:

Modify the script from exercise 3 so the output would also be available as an HTML file. Use the template below as a basic structure of the HTML file content.

<html>

<body>

<table>

<tr><td>Name</td><td>CPA</td><td>Class</td></tr>

<tr><td>_name_</td><td>_cpa_</td><td>_class_</td></tr>

</table>

</body>

</html>34

Basic Perl ProgrammingSubroutine/Function

Create a subroutine in Perl script :

sub subroutine_name {

. . .

. . .

. . .

}

Calling a subroutine:

&subroutine_name; 35

Basic Perl ProgrammingSubroutine

Skeleton of perl script that using a subroutine:

#! /usr/bin/perl

. . .

. . .

&subroutine_name;

. . .

. . .


. . .

. . .

} 36


Example:#! /usr/bin/perl

print "Perl script welcoming the users.\n\n";

&welcome_user;

sub welcomeUser {

print "Your name please: ";

$name = <STDIN>;

chop($name);

print "\nHello $name, welcome to Perl world.\n\n";

} 37


Pass and return values to/from subroutine

• make a subroutine more flexible and useful

• based on three guidelines :

1. The subroutine can accept more than one scalar variable and only one array variable

2. If the subroutine accept both scalar and array variables the array variable must appear last in the arguments

3. The first two guidelines above can be ignored by passing array/hash as a reference 38



The Skeleton:


$scalar_var_name_1 = shift @_;

. . .

$scalar_var_name_n = shift @_;

@array_var_name = @_;

. . .

. . .

return (value);

} 39



Calling a subroutine that can accept and return values:

$var_name = &subroutine_name(scalar1, . . ., scalarN, array);

40



The story so far:

• To return a value the command: return (value); is used inside the subroutine where value can be either scalar, array or result of an expression

• When values passed to the subroutine, all the values are stored in the special array variable named @_

• The subroutine must extract all the values from this special array variable and pass it to the appropriate variables following the guidelines given previously 41



The story so far:

• To return a value the command: return (value); is used inside the subroutine where value can be either scalar, array or result of an expression

• When values passed to the subroutine, all the values are stored in the special array variable named as: @_

• The subroutine then must extract all the values from this special array variable and pass it to the appropriate variables using the two guidelines above 42



The story so far:

• The command: shift @_; will extract and shift the first item in @_

• The command: @array_var_name = @_; will assigns all the values of @_ to @array_var_name

• Example:

• Variable @_ contains the values: (“A”, “B”, “C”) • The command $var_name = shift @_ will extract the first item of @_

which is “A”, pass it to $var_name and left the @_ with the values: (“B”, “C”)

• The next command @var_name2 = @_ will make the variable @var_name2 contains the values: (“B”, “C”)

43


Exercise 5:

Modify the script from exercise 4 so the core tasks are written modularly using subroutines/functions.

44


Writing subroutines as a libraries

• The technique for writing a subroutine before caused the subroutine can only be used by the script who implementing them

• Writing subroutines as a libraries provide more flexible subroutine that can be used many times by other scripts

45



• Skeleton for writing subroutine as a libraries:

package library_name;

subroutines goes here . . .

1;

• The scripts that are intended to be a libraries must be saved as library_name.pl

• The: 1; at the end of libraries script is a mandatory so the perl interpreter can call and run the libraries correctly 46



• Calling and use subroutine from the libraries:

#! /usr/bin/perl

require (“library_name.pl");

. . .

$val = &library_name’subroutine_name1;

&library_name’subroutine_name2(. . .);

. . .

47


Variable scope:

#! /usr/bin/perl

$name = "Your name";

print "\$name: $name\n";

&change_name;

print "\Sname: $name\n";

sub change_name { my $name = "My name";}

#! /usr/bin/perl

$name = "Your name";

print "\$name: $name\n";

&change_name;

print "\Sname: $name\n";

sub change_name { $name = "My name";}

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Basic Perl Programming

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