Nancyvarkey at 01:59, 30 May 2015

2015-05-30T01:59:38Z

Nirmala Venkat: Created page with "'''Title of script''':''' Access Modifiers in PERL''' '''Author: Nirmala Venkat''' '''Keywords: Scope of variables, Dynamically scoped variable, global variable, gedit, vide..."

2015-05-19T11:14:51Z

Created page with "'''Title of script''':''' Access Modifiers in PERL''' '''Author: Nirmala Venkat''' '''Keywords: Scope of variables, Dynamically scoped variable, global variable, gedit, vide..."

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'''Title of script''':''' Access Modifiers in PERL'''

'''Author: Nirmala Venkat'''

'''Keywords: Scope of variables, Dynamically scoped variable, global variable, gedit, 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''' Access Modifiers 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

* Scope of variables
* Private variables
* Dynamically scoped variables
* Global variables

|-
| 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:

Prerequisites

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

You should have basic knowledge of '''Perl''' Programming.

If not, then go through the relevant '''Perl '''spoken tutorials on the '''spoken tutorial '''website.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 5:

| style="border:1pt solid #000000;padding:0.097cm;"| Let us start with the introduction to the''' Scope of variables'''.

The''' scope''' of a variable is the region of code within which a variable can be accessed.

In other words, it refers to the visibility of variables.

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

| style="border:1pt solid #000000;padding:0.097cm;"| First, we will discuss about''' my, local''' and''' our''' modifiers in '''Perl.'''

* '''my '''means '''Private variables'''
* '''local''' means '''Dynamically scoped variables'''
* '''our '''means '''Global variables'''

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

| style="border:1pt solid #000000;padding:0.097cm;"| '''Variables '''declared with''' my''' keyword will lose scope outside the '''block''' in which they are declared.

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

| style="border:1pt solid #000000;padding:0.097cm;"| You can declare a '''variable''' without giving it a value, like this:

'''my''' $fvalue<nowiki>;</nowiki>

You can also declare a '''variable''' by assigning a value to it, as:

'''my''' $fValue = 1;

'''my''' $fname = "Rahul";

The syntax to declare several '''variables '''with the same''' my''' statement is as follows:

'''my''' ($fname, $lname, $age);

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us understand '''private variables''' 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 scope-my.pl &'''
| style="border:1pt solid #000000;padding:0.097cm;"| Open the '''terminal '''and type''' gedit scope-my dot pl ampersand '''and press '''Enter'''

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

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

Type the following code as displayed on the screen.

Let me explain the code now.

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

| style="border:1pt solid #000000;padding:0.097cm;"| Here, I have declared a '''private variable $fname''' with''' my''' keyword.

And assigned the value “'''Raghu'''” to it.

Within this block, the '''print''' '''statement''' prints the value in the '''fname variable''', i.e. “'''Raghu'''”.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| In the next block, I have assigned the value “'''Other'''” to the same '''private variable $fname.'''

So, the '''print statement '''will print “'''Other'''” within this particular block.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| The last '''print statement '''in this program, will not print any output.

This is because outside the scope of the blocks defined earlier, '''fname '''has no value assigned to it.

|-
| 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 now execute the program.

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

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

'''perl scope-my 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

'''Block 1: Raghu'''

'''Block 2: Other'''

'''Outside Block: '''( There is no output)

So, the scope of the '''my''' '''variable '''is accessed only within a particular block of code.

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

| style="border:1pt solid #000000;padding:0.097cm;"| Now let us change the existing program a little.

Let us add''' my $fname<nowiki>= "</nowiki>John";''' outside the blocks, before the last '''print statement'''.

Save the changes.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| On the terminal
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to the '''terminal '''and execute as before.

Analyse the output that is displayed.

Hope you are able to understand the scope of using''' my''' '''variable '''within a block and outside a block.

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

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"|
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Next we will see about '''dynamically scoped variable''' in '''Perl'''.

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

| style="border:1pt solid #000000;padding:0.097cm;"| '''Local''' '''keyword''' gives a temporary scope to a '''global''' '''variable'''.

The '''variable''' is visible to any '''function''' called from the original block.

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

| style="border:1pt solid #000000;padding:0.097cm;"| You can declare a '''local variable '''as,

'''local''' $fValue = 100;

'''local''' $fname = "Rakesh";

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us understand this using a sample program.

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

'''gedit scope-local dot pl''' '''ampersand''' and press '''Enter'''.

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

| style="border:1pt solid #000000;padding:0.097cm;"| This will open''' scope-local dot pl''' file in''' gedit'''.

Type the following code as displayed on the screen.

Let me explain the code now.

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

| style="border:1pt solid #000000;padding:0.097cm;"| Here, in the first line we have declared a '''variable $fname '''and initialised it.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Inside the '''function Welcome, '''we have declared a '''local variable '''by the same name,''' $fname.'''

Notice the '''local''' '''keyword''' before the '''variable '''name.

And we have assigned the value “'''Rakesh'''” to this '''variable.'''

So, basically, inside function '''Welcome(), $fname''' is modified as a new temporary '''local''' '''variable.'''

Then, the '''function Hello '''is being '''called'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Here is the '''function definition '''of '''Hello.'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| At the end of the program, we are calling both the '''functions Welcome '''and '''Hello'''.

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

|-
| 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

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

'''perl scope-local.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

'''Hello, Rakesh !'''

'''Hello, Welcome to Spoken tutorials!'''

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

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to program
| style="border:1pt solid #000000;padding:0.097cm;"| When the function '''Welcome()''' is called, the '''function''' '''Hello()''' within it, accesses the '''local variable'''.

Within '''Welcome(), $fname''' has the value “'''Rakesh'''”.

After this, the '''function''' '''Hello() '''accesses the '''variable $fname '''once again'''.'''

But this time, it is the '''variable $fname '''which was initialized to''' “Welcome to spoken tutorials”.'''

It does not access the '''local variable $fname '''within the function '''Welcome().'''

Which means that, the '''local variable '''restores the '''scope''', after leaving the block '''Welcome()'''.

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

|-
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"|
| style="border-top:none;border-bottom:1pt solid #000000;border-left:1pt solid #000000;border-right:1pt solid #000000;padding:0.097cm;"| Next we will see about '''global variables''' in '''Perl'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 10:

| style="border:1pt solid #000000;padding:0.097cm;"| A '''global variable '''can be accessed anywhere in the program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 10:

| style="border:1pt solid #000000;padding:0.097cm;"| '''Global variables '''are declared with''' our''' '''keyword'''.

Here are some examples.

'''our''' $fvalue<nowiki>=100;</nowiki>

'''our''' $fname =”Priya”;

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Now let us look at a working example of '''global variables'''.

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

'''gedit scope-our dot pl ampersand and '''press''' Enter'''

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

Let me explain the sample program which I have written.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight in gedit
| style="border:1pt solid #000000;padding:0.097cm;"| I have declared '''package main '''and''' '''a '''global variable''' as''' our $i '''and I have initialised it to '''100;'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Notice the''' package First''' declaration.

A '''package''' is a collection of code, which has its own '''namespace'''.

'''Namespace''' prevents '''variable name collisions''' between '''packages'''.

We will see more about '''package''' and '''namespace''' in future tutorials.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Within '''package First,''' the '''global variable''' “'''i'''” holds the value 10.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| In '''package Second,''' the '''global variable “i” '''is assigned the value 20.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| The main '''package '''uses both '''package First variable '''and the '''package Second variable'''.

In my program, I have declared the same variable “'''i'''” in all the '''packages'''.

The '''package variable''' is referred by''' '''

'''package name::variable name'''

In our example it is''' $First::i , $Second::i'''

We have multiple '''packages '''within one file, and the '''global variable '''will be accessed by all the '''packages'''.

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

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

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

'''perl scope-our dot pl'''

and press '''Enter'''.

The output is as displayed on the '''terminal'''.

<nowiki><Pause></nowiki>

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Analyze the output by yourself to understand how the assignment to the '''variable i '''was done.

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

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

Summary

| style="border:1pt solid #000000;padding:0.097cm;"| This brings us to the end of this tutorial. Let us summarise.

In this tutorial, we learnt:

* scope of variables
* declaration of private variables
* dynamically scoped variables and
* global variables with examples.<br/>

It is preferred to use''' my''' than''' local''' as the compilation is faster.

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

Assignment

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

Here is an assignment for you.

Write the code for the following assignment and execute it.

# Declare a package as '''FirstModule'''
# Declare a variable '''$age''' as '''our''' and assign the value '''42'''
# Declare another package as '''SecondModule'''
# Declare a variable '''$ageword''' as '''our''' and assign the value '''“Forty-Two”'''
# Declare''' '''a subroutine''' First()'''
# Inside the subroutine declare two variables with '''local''' and '''my''' keyword as below:
#:'''local''' '''$age<nowiki>=52</nowiki>'''<nowiki>;</nowiki>
#:'''my''' '''$ageword<nowiki>="</nowiki>Fifty-two";'''
# Call another subroutine as '''Result()'''
# Print the values of '''$age''' and '''$ageword''' inside this function.
# End the subroutine
# Declare the subroutine '''Result()'''
# Again print the values of '''$age''' and '''$ageword'''
# End the subroutine
# Call the function '''First()'''
# Print the Package First and package Second as below:
#:'''print "Package First : $FirstModule::age\n";'''
#:'''print "Package Second : $SecondModule::ageword\n";'''

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

About the Spoken Tutorial Project

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

Please download and watch it.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 13:

About workshops
| style="border:1pt solid #000000;padding:0.097cm;"| The Spoken Tutorial Project Team conducts workshops and gives certificates for those who pass an online test.

For more details, please write to us.

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

Slide 14:

Acknowledgment

| style="border: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/Access-Modifiers-in-PERL/English - Revision history

Nancyvarkey at 01:59, 30 May 2015

Nirmala Venkat: Created page with "'''Title of script''':''' Access Modifiers in PERL''' '''Author: Nirmala Venkat''' '''Keywords: Scope of variables, Dynamically scoped variable, global variable, gedit, vide..."