1/11/2022

Data Structure in PERL : Associative Array and Reference


In previous article we have discussed about the Array . Now , in this article we will cover the Linked List or the HASH-Table i.e Associative Array in PERL.


Associative Array / Hash in PERL :

  • An associative array(hash) is an array of scalar values associated with the subscripts/index that are not 0, 1, 2, 3 ... 
  • Rather subscripts/index are any scalar values with no particular order. 
  • These subscripts are called "keys". Corresponding array elements are called "values".
  • A "value" is mapped to its corresponding "key".
  • Thus a hash is a table of uniquely mapped key-value pairs. Generically it is called as hash table or hash.
  • Hash variables always start with % (percent) symbol.

Creation of an Associative Array  :

The HASH can be created in below ways :-

1. Entire hash variables(key&value) are referenced using the “%” ;

my %hash = ("name", "Billy",

"age", 21 ,

"occupation", "student"

);

my %hash = ("name" => "Billy",

"age" => 21,

"occupation" => "student"

);

2. Individual hash variables (scalars) are mapped using $name{key} = value ;

$hash{"name"}= "Billy" ; # note we are using { } and not using  [ ]

$hash{"age"} = 21;

$hash{"occupation"} = "student";


Accessing Hash Key/Value after Hash Creation :

The key-value pair can be access in below ways :-

1. When you know the key ;

print "$hash{"name"}"; #prints Billy

print "$hash{"age"}"; #prints 21;

print "$hash{"occupation"}"; # Prints student


2. When you do not know the keys;

my @arr = keys(%hash); #returns list of keys

foreach my $key (@arr) {

     print “Key = $key has Value = $hash{$key}\n”;

}

3. Get the Key Value pair at once using each function ;

while (($key,$value) = each(%hash)) {

   print “Key = $key has Value = $value \n”;

}

Associative Array / Hash Functions :

There are few functions are there to operate exclusively on HASH :

  • keys - returns a list of the keys
  • my @keyArr = keys(%hash);
  • values - returns a list of the values
  • my @valArr = values(%hash);
  • each - returns a (key, value) pair in sequence
  • delete - removes a key-value pair from a hash
  • delete $hash{"key1"} ; 
  • %b = %a; # copies hash into other
  • %a = reverse %b; # swap keys and values, values should be unique


Array and Hash : Mutual Conversions :

We can mutually convert the array and hash in to each other as :-

  • my @arr = %hash; # converts a hash into a array
  • # Not necessarily in that order, but always with keys
  • # and values paired adjacently
  • my %newHash = @arr; # converts array back to a hash
  • my %arr = ("key1", val1 ,"key2", val1, "key3", val3); # creates a hash from list
  • my @arr{"key1","key2","key3"} = (val1,val2,val3); # same thing


References in PERL :

  • PERL allows you have "hard" references to any piece of data or code.
  • Any scalar may hold a hard reference. 
  • Arrays and hashes contain scalars.
  • Hence one can now easily build nested data structures as :
  • arrays of arrays, arrays of hashes , hashes of arrays, arrays of hashes of functions, and many other combinations..…
  • [ ITEMS ] makes a new, anonymous array, and returns a reference to that array.
  • { ITEMS } makes a new, anonymous hash, and returns a reference to that hash.


Backslash Operator & ref() function  : 

By using the backslash operator on a variable, subroutine, or value typically creates another reference to a variable.

Backslash Operator (\) is similar to the address-of-operator(&) in C.

$scalarref = \$foo; #reference to a scalar 

$arrayref  = \@ARGV; #reference to an Array

$hashref   = \%ENV; #reference to a Hash

$constref = \186_282.42; #reference to a constant

$coderef   = \&handler; #reference to a piece of code

$objRef = new Student("Name", "Surname" , "Roll"); #reference to a object 

There is a inbuilt function ref() which will check & return the reference type 

Usage : ref($refVar) #$refVar is the reference variable in concern

Reference Example 

Through this example , we will demonstrate the use of ref() function and see its output in various cases :

 

Upon execution this code gives the below output which is self explanatory :


With the below code-example we do the Anonymous Referencing & De-Referencing :

Request you to run this code at your end .

De-Referencing : Arrow Notation :

# Accessing A Value 

print "$ArrayRef->[0]";

print "$HashRef->{"KEY"}";

# Assigning A Value

$ArrayRef->[0] = "January";   # Array element

$HashRef->{"KEY"} = "VALUE";  # Hash element

Unnamed Associative Array/Hash :

With the below code example we will create and access unnamed hash :

Upon execution the code gives the below output :


The entire article is well narrated in the below video :