Nirmala Venkat: Created page with "'''Title of script''':''' Referencing and Dereferencing in Perl''' '''Author: Nirmala Venkat''' '''Keywords: Scalar References, Array References, Hash References, Dereferenc..."

2015-07-14T05:16:26Z

Created page with "'''Title of script''':''' Referencing and Dereferencing in Perl''' '''Author: Nirmala Venkat''' '''Keywords: Scalar References, Array References, Hash References, Dereferenc..."

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'''Title of script''':''' Referencing and Dereferencing in Perl'''

'''Author: Nirmala Venkat'''

'''Keywords: Scalar References, Array References, Hash References, Dereferences, video tutorial'''

{| style="border-spacing:0;"
| style="background-color:#c0c0c0;border:1pt solid #000000;padding:0.097cm;"| <center>'''Visual Cue'''</center>
| style="background-color:#c0c0c0;border:1pt solid #000000;padding:0.097cm;"| <center>'''Narration'''</center>

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 1:

| style="border:1pt solid #000000;padding:0.097cm;"| Welcome to the''' Spoken Tutorial''' on''' Referencing and Dereferencing in Perl.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 2:
Learning objectives
| style="border:1pt solid #000000;padding:0.097cm;"| In this tutorial we will learn about
* Scalar References
* Array References
* Hash References
* Dereferences and
* How to add, remove, access elements of array/hash references
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 3:

System Requirements
| style="border:1pt solid #000000;padding:0.097cm;"| For this tutorial, I am using
* '''Ubuntu Linux 12.04''' operating system
* '''Perl 5.14.2'''
* and the''' gedit''' Text Editor
You can use any text editor of your choice.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 4:
Pre-requisites
| style="border:1pt solid #000000;padding:0.097cm;"| You should have working knowledge of
* Perl Programming
* Array functions and
* Hash functions.

If not, then go through the relevant''' Perl''' spoken tutorials on this website.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 5:

| style="border:1pt solid #000000;padding:0.097cm;"| What is''' References'''?

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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 6:

| style="border:1pt solid #000000;padding:0.097cm;"| '''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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us learn how to create a '''reference'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 7:

'''Reference to a scalar variable: ($)'''

$fvalue = 22;

$fref = \$fvalue<nowiki>;</nowiki>

'''Reference to a array variable:(@)'''

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

$colorRef = \@color<nowiki>;</nowiki>

'''Reference to a hash variable:(%)'''

%dept(

Name <nowiki>=</nowiki>> “Sunil”,

Designation <nowiki>=</nowiki>> “Manager”

);

$deptRef = \%dept<nowiki>;</nowiki>

| style="border:1pt solid #000000;padding:0.097cm;"| We can create a''' reference''' for any variable, subroutine or value, by putting a backslash''' (\)''' in front of it.

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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 8:

| style="border:1pt solid #000000;padding:0.097cm;"| 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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us see how to''' dereference''' variables.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 9:

| style="border:1pt solid #000000;padding:0.097cm;"| 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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us see a simple program for''' Scalar reference''' and''' dereference.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to the file in''' gedit'''.
| style="border:1pt solid #000000;padding:0.097cm;"| Let me open a sample program in''' gedit Text editor.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to the Terminal and type

'''gedit scalarRef.pl &'''
| style="border:1pt solid #000000;padding:0.097cm;"| Open the terminal and type''' gedit scalarRef dot pl ampersand'''

and press Enter.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Point the cursor to

'''scalarRef.pl'''
| style="border:1pt solid #000000;padding:0.097cm;"| Type the following code as displayed on the screen in the '''scalarRef dot pl''' file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''<nowiki>#!/usr/bin/perl</nowiki>'''

'''$a =10;'''

'''<nowiki># Now $aref has reference to $a scalar.</nowiki>'''

'''$aref = \$a;'''

'''<nowiki># Print value available at the location stored in $aref.</nowiki>'''

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

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

'''print "Reference type is : ", ref($aref), "\n";'''
| style="border:1pt solid #000000;padding:0.097cm;"| Let me explain the code.

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 '''$'''.

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

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Press '''Ctrl+S'''
| style="border:1pt solid #000000;padding:0.097cm;"| Now, press''' Ctrl+S''' to save the file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us execute the program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to terminal

'''perl scalarRef dot pl'''
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to the '''terminal''' and type

'''perl scalarRef dot pl'''

and press''' Enter'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight

| style="border:1pt solid #000000;padding:0.097cm;"| The output is displayed as shown.

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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Next, let us understand how to create a''' reference''' and''' dereference''' array by using a sample program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to the file in''' gedit'''.
| style="border:1pt solid #000000;padding:0.097cm;"| I already have a sample program.

Let me open it in''' gedit Text editor.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to the Terminal and type

'''gedit arrayRef.pl &'''
| style="border:1pt solid #000000;padding:0.097cm;"| In the terminal, type''' gedit arrayRef dot pl ampersand'''

and press Enter.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Point the cursor

'''arrayRef.pl'''
| style="border:1pt solid #000000;padding:0.097cm;"| Type the following code as displayed on the screen in the '''arrayRef dot pl''' file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight in gedit

'''<nowiki>#!/</nowiki>usr/bin/perl'''

'''use strict<nowiki>;</nowiki>'''

'''use warnings<nowiki>;</nowiki>'''

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

'''my $colorRef =\@color<nowiki>;</nowiki>'''

'''print "Reference: $colorRef\n";'''

'''print "Dereferenced: @{$colorRef}\n";'''
| style="border:1pt solid #000000;padding:0.097cm;"| Let me explain the code now.

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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Now, press''' Ctrl+S''' to save the file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us execute the program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to terminal

'''perl arrayRef dot pl'''
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to the '''terminal''' and type

'''perl arrayRef dot pl'''

and press''' Enter'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight

Output
| style="border:1pt solid #000000;padding:0.097cm;"| The output is displayed as shown here

'''Reference: ARRAY(0x81e4638)'''

'''Dereferenced: Red Green Yellow'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight first line

Highlight the second line
| style="border:1pt solid #000000;padding:0.097cm;"| The first line shows the output of the memory address of the variable that is created as''' reference'''.

The second line shows the actual value that is''' dereferenced'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Next, we will see how to declare direct or anonymous''' reference''' for an '''array'''.

Let’s come back to our program.

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

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight

'''my $colorRef<nowiki> = ['</nowiki>Red', 'Green', 'Yellow'];'''

'''Use arrow operator (->) to dereference.'''

'''print $colorRef -<nowiki>>[1]; </nowiki>'''

| style="border:1pt solid #000000;padding:0.097cm;"| You can create a direct''' reference''' to an''' array''' by using square brackets<nowiki> [] </nowiki>as demonstrated.

Use arrow operator''' (->)''' to dereference.

Print statement will print ''''Green'''' as output.

Here the print statement takes the value of index<nowiki>[1]. </nowiki>i.e '''Green''' in our program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Now, press''' Ctrl+S''' to save the file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to the terminal and type

'''perl arrayRef dot pl'''

and press''' Enter''' to execute.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight the output
| style="border:1pt solid #000000;padding:0.097cm;"| <nowiki><<</nowiki>PAUSE>>

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| I’ll show an example on how to use the direct hash reference in the same code file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Open the gedit and paste the below code.
| style="border:1pt solid #000000;padding:0.097cm;"| So, let’s switch to '''gedit'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight in gedit

'''my $deptRef = {'''

'''Name => “Sunil”, Designation<nowiki>=>“</nowiki>Manager”};'''

'''print ${$deptRef} {Name};'''

'''or'''

'''print $deptRef -> {Name};'''

'''Output: Sunil'''

| style="border:1pt solid #000000;padding:0.097cm;"| You can create a direct''' reference''' to hash by using curly brackets {} as shown here.

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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Next we will see how to add, remove, access elements to '''array reference''' with a sample program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to the file in''' gedit'''
| style="border:1pt solid #000000;padding:0.097cm;"| I already have a sample program.

Let me open it in''' gedit Text editor'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to the Terminal and type

'''gedit arrayRefadd .pl &'''
| style="border:1pt solid #000000;padding:0.097cm;"| Open the''' terminal''' and type''' gedit arrayRefadd dot pl ampersand '''and press''' Enter'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| '''arrayRefadd.pl''' file is now open in''' gedit.'''

Type the code as shown here in your file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight in gedit

'''<nowiki>#!/</nowiki>usr/bin/perl'''

'''use strict<nowiki>;</nowiki>'''

'''use warnings<nowiki>;</nowiki>'''

'''<nowiki># </nowiki>initialize an array'''

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

'''<nowiki># </nowiki>get a reference to it'''

'''my $ref = \@numarray<nowiki>;</nowiki>'''

'''<nowiki># </nowiki>Apply Accessing an element'''

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

'''<nowiki># </nowiki>apply push (append a few elements)'''

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

'''<nowiki># </nowiki>or push @{$ref}, 5, 6, 7;'''

'''<nowiki># </nowiki>print the array using the reference'''

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

'''<nowiki># </nowiki>it displays 1 2 3 4 5 6 7'''

'''<nowiki># </nowiki>apply pop (remove an element)'''

'''pop(@$ref);'''

'''<nowiki># </nowiki>print the array using the reference'''

'''print "After deleting :",@{$ref},"\n";'''
| style="border:1pt solid #000000;padding:0.097cm;"| The first line initializes an array.

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<nowiki> “[ ]” </nowiki>to access the particular value.

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

The '''print''' statement will print the value of index<nowiki>[0]</nowiki>

'''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.

'''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.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Now, press''' Ctrl+S''' to save the file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us execute the program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to terminal

'''perl arrayRefadd dot pl'''
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to the '''terminal''' and type

'''perl arrayRefadd dot pl'''

and press''' Enter'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight the output:

'''First element:1'''

'''After adding:1234567'''

'''After deleting :123456'''
| style="border:1pt solid #000000;padding:0.097cm;"| The output is displayed as

'''First element:1'''

'''After adding:1234567'''

'''After deleting :123456'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Now let us see another sample program to add, remove, and access elements of''' hash reference.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''Swtich to the Terminal and type'''

'''gedit hashRefadd.pl&'''
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to the '''terminal''' and type

'''gedit hashRefadd dot pl ampersand'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''Point to the filename hashRefadd.pl in the Titlebar of gedit.'''
| style="border:1pt solid #000000;padding:0.097cm;"| This will open the file''' hashRefadd.pl''' in''' gedit'''.

Let me explain the sample program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''<nowiki>#!/</nowiki>usr/bin/perl'''

'''use strict<nowiki>;</nowiki>'''

'''use warnings<nowiki>;</nowiki>'''

'''my $weektemp = {'''

'''monday <nowiki>=> 40,</nowiki>'''

'''tuesday <nowiki>=> 38,</nowiki>'''

'''wednesday => 36,'''

'''thursday <nowiki>=> 39,</nowiki>'''

'''friday <nowiki>=> 41,</nowiki>'''

'''};'''

'''<nowiki># </nowiki>Print all'''

'''foreach my $keys(keys %$weektemp) {'''

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

'''}'''

'''Highlight'''

'''<nowiki>#</nowiki>Print Temp on monday'''

'''my $monday_temp<nowiki>=</nowiki>'''

'''print "First day temp is: " . ${$weektemp}{monday} . "\n";'''
| style="border:1pt solid #000000;padding:0.097cm;"| I have declared a direct''' hash reference''' that can be stored in a scalar variable''' $weektemp.'''

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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Now, save the file.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Terminal
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to '''terminal''' and type-

'''perl''' '''hashRefadd dot pl'''

and press''' Enter '''to see the output.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''Highlight output:'''

'''friday: 41'''

'''wednesday: 36'''

'''thursday: 39'''

'''tuesday: 38'''

'''monday: 40'''
| style="border-top:1pt solid #000000;border-bottom:0.75pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| The '''hash keys '''and '''hash values''' are stored in a random order.

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

Go through the code once again to understand why this is so.

|-
| style="border-top:1pt solid #000000;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:0.75pt solid #000000;padding:0.097cm;"|
| style="border:0.75pt solid #000000;padding:0.106cm;"| With this, we come to the end of this tutorial.

Let us summarize.

|-
| style="border-top:1pt solid #000000;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:0.75pt solid #000000;padding:0.097cm;"| Slide 10:

'''Summary'''

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

| style="border:0.75pt solid #000000;padding:0.106cm;"| In this tutorial, we learnt about:

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

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 11:

'''Assignment'''

| style="border-top:0.75pt solid #000000;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Here is an assignment for you.

# Add new keys “'''Saturday'''” and “'''Sunday'''” in hash '''weektemp''' in our''' hashRefadd dot pl file.'''
# Delete key “'''Saturday'''” at the end.
# Print hash '''weektemp.'''
# Save and execute the program. Now check the result.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 10:

About Spoken Tutorial project

| style="border-top:1pt solid #000000;border-bottom:0.75pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| The video at the following link summarises the Spoken Tutorial project.

Please download and watch it

|-
| style="border-top:1pt solid #000000;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:0.75pt solid #000000;padding:0.097cm;"| Slide 11:

Spoken Tutorial workshops

| style="border:0.75pt solid #000000;padding:0.106cm;"| We conduct workshops and give certificates for those who pass our online tests.

For more details, please write to us.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 12:

Acknowledgement

| style="border-top:0.75pt solid #000000;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Spoken Tutorial project is funded by NMEICT, MHRD, Government of India.

More information on this mission is available at

this link.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.

|}

PERL/C3/Referencing-and-Dereferencing/English - Revision history

Nancyvarkey at 04:03, 15 July 2015

Nirmala Venkat: Created page with "'''Title of script''':''' Referencing and Dereferencing in Perl''' '''Author: Nirmala Venkat''' '''Keywords: Scalar References, Array References, Hash References, Dereferenc..."