Difference between revisions of "PERL/C3/Referencing-and-Dereferencing/English"

Learning objectives

• Scalar References
• Array References
• Hash References
• Dereferences and
• How to add, remove, access elements of array/hash references

System Requirements

• Ubuntu Linux 12.04 operating system
• Perl 5.14.2
• and the gedit Text Editor

You can use any text editor of your choice.

Pre-requisites

• Perl programming
• Array functions and
• Hash functions.

If not, then go through the relevant Perl tutorials on this website.

A reference is a pointer or an address to a variable, array, hash or a subroutine.

It does not contain data directly.

Reference is an easy, compact scalar value.

• Reference will improve the performance of Perl code when you pass or return large data structures.

• It saves memory as it passes a reference to a subroutine rather than passing a value.

• Easy to manage complicated Perl data structures.

Reference to a scalar variable: ($)

$fvalue = 22;

$fref = \$fvalue;

Reference to a array variable:(@)

my @color =('Red', 'Green', 'Yellow');

$colorRef = \@color;

Reference to a hash variable:(%)

%dept(

Name => “Sunil”,

Designation => “Manager”

);

$deptRef = \%dept;

A scalar variable is referenced by backslash and dollar sign ($) as shown here.

An array variable is referenced by backslash and at the rate(@) symbol.

A hash variable is referenced by backslash and percentage(%) symbol as shown in the example here.

• When a reference is dereferenced, the actual value is returned.

• Dereference is done by enclosing the reference variable within curly brackets.

• And preceding the left curly bracket with a character denoting the type of reference it is.

• A scalar variable is dereferenced by dollar sign ($) and curly brackets.

• An array variable is dereferenced by at the rate (@) symbol and curly brackets.

• A hash variable is dereferenced by percentage(%) symbol and curly brackets.

gedit scalarRef.pl &

and press Enter.

scalarRef.pl

$a =10;

# Now $aref has reference to $a scalar.

$aref = \$a;

# Print value available at the location stored in $aref.

print "Value of ‘aref’ is: ",$aref, "\n";

print "Value of a is : ", ${$aref}, "\n";

print "Reference type is : ", ref($aref), "\n";

First line declares a scalar variable '$a' and initialized to 10.

As mentioned earlier, a scalar variable is referenced by backslash and dollar sign ($)

This line will print memory address of the variable that is created as reference.

To print the actual value, the variable is dereferenced by curly brackets preceded by $.

Hereref() function will return the reference type such as scalar or array or hash.

perl scalarRef dot pl

perl scalarRef dot pl

and press Enter.

First line shows the memory address where the value 10 is stored.

The second line returns the actual value 10.

Ref() function returns 'Scalar' as output.

Let me open it in gedit Text editor.

gedit arrayRef.pl &

and press Enter.

arrayRef.pl

#!/usr/bin/perl

use strict;

use warnings;

my @color = ('Red', 'Green', 'Yellow');

my $colorRef =\@color;

print "Reference: $colorRef\n";

print "Dereferenced: @{$colorRef}\n";

Here, in the first line, I have declared an array @color and initialised it with three values.

It is referenced with backslash @color which is the array name and assigned to $colorRef.

The print statement will print the reference value and the dereferenced value.

perl arrayRef dot pl

perl arrayRef dot pl

and press Enter.

Reference: ARRAY(0x81e4638)

Dereferenced: Red Green Yellow

Highlight the second line

The second line shows the actual value that is dereferenced.

Let’s come back to our program.

I have changed the existing program to show the direct reference for an array.

my $colorRef = ['Red', 'Green', 'Yellow'];

Use arrow operator (->) to dereference.

print $colorRef ->[1];

Use arrow operator (->) to dereference.

print statement will print 'Green' as output.

Here the print statement takes the value of index[1]. i.e Green in our program.

perl arrayRef dot pl

and press Enter to execute.

my $deptRef = {

Name => “Sunil”, Designation=>“Manager”};

print ${$deptRef} {Name};

or

print $deptRef -> {Name};

Output: Sunil

Use arrow operator (->) to dereference it.

“Name” is the hash key.

On executing this block of code, both the print statements will print 'Sunil' as output.

Let me open it in gedit Text editor

gedit arrayRefadd .pl &

Type the code as shown here in your file.

#!/usr/bin/perl

use strict;

use warnings;

# initialize an array

my @numarray = (1, 2, 3, 4);

# get a reference to it

my $ref = \@numarray;

# Apply Accessing an element

print "First element:", $ref->[0], "\n";

# apply push (append a few elements)

push(@$ref, 5, 6, 7);

# or push @{$ref}, 5, 6, 7;

# print the array using the reference

print "After adding:" ,@{$ref},"\n";

# it displays 1 2 3 4 5 6 7

# apply pop (remove an element)

pop(@$ref);

# print the array using the reference

print "After deleting :",@{$ref},"\n";

We have referenced an array with backslash @numarray and assigned to $ref.

Now, we will see how to access a particular element from the array reference.

We need to use the array index in square brackets “[ ]” to access the particular value.

And an arrow operator (“->”) to dereference it.

The print statement will print the value of index[0]

Push function adds elements at the last position of an array reference.

In our case 5,6,7 is added to at the end of the existing array 1,2,3,4.

This print statement shows the output after adding to the array reference.

Pop function removes an element from the last position of an array reference

In our example, 7 will be removed from the existing array reference.

print statement shows the output after deleting from the array reference.

perl arrayRefadd dot pl

perl arrayRefadd dot pl

and press Enter.

First element:1

After adding:1234567

After deleting :123456

gedit hashRefadd.pl&

gedit hashRefadd dot pl ampersand

and press Enter.

Let me explain the sample program.

use strict;

use warnings;

my $weektemp = {

monday => 40,

tuesday => 38,

wednesday => 36,

thursday => 39,

friday => 41,

};

# Print all

foreach my $keys(keys %$weektemp) {

print "$keys: " . $weektemp->{$keys} . "\n";

}

Highlight

#Print Temp on monday

my $monday_temp=

print "First day temp is: " . ${$weektemp}{monday} . "\n";

I have used curly brackets to represent the hash reference and the arrow operator to dereference.

This code stores the temperature values from Monday to Friday.

I am using the “keys” built-in function to loop through the keys of the hash.

print statement will print each element of the hash.

We can access the particular value of an element as shown here.

print statement will print the temperature on Monday.

perl hashRefadd dot pl

and press Enter to see the output.

friday: 41

wednesday: 36

thursday: 39

tuesday: 38

monday: 40

The displayed output is not related to the order in which they were added.

Let us summarize.

Summary

• Scalar References
• Array References
• Hash References
• Dereferences and
• How to add, remove, access elements of array/hash references

• Scalar References
• Array References
• Hash References
• Dereferences and
• How to add, remove, access elements of array/hash references with examples.

Assignment

1. Add new keys “Saturday” and “Sunday” in hash weektemp in our hashRefadd dot pl file.
2. Delete key “Saturday” at the end.
3. Print hash weektemp.
4. Save and execute the program. Now check the result.

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