Difference between revisions of "PERL/C3/Access-Modifiers-in-PERL/English"

Learning objectives

• Scope of variables
• Private variables
• Dynamically scoped variables
• Global variables

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.

Prerequisites

You should have basic knowledge of Perl Programming.

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

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.

• my means Private variables
• local means Dynamically scoped variables
• our means Global variables

my $fvalue;

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);

Let me open it in gedit Text editor.

scope-my.pl

Type the following code as displayed on the screen.

Let me explain the code now.

And assigned the value "Raghu" to it.

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

So, the print statement will print "Other" within this particular block.

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

perl scope-my dot pl >> press Enter

perl scope-my dot pl and press Enter.

Output

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.

Press Ctrl+S

Let us add my $fname = "John"; outside the blocks, before the last print statement.

Save the changes.

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.

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

local $fValue = 100;

local $fname = "Rakesh";

gedit scope-local dot pl ampersand and press Enter.

Type the following code as displayed on the screen.

Let me explain the code now.

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.

perl scope-local.pl and press Enter.

Output

Hello, Rakesh !

Hello, Welcome to Spoken tutorials!

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().

Here are some examples.

our $fvalue = 100;</nowiki>

our $fname ="Priya";

gedit scope-our dot pl ampersand and press Enter

Let me explain the sample program which I have written.

• 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.

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.

perl scope-our dot pl

and press Enter.

The output is as displayed on the terminal.

<Pause>

Summary

In this tutorial, we learnt:

• scope of variables
• declaration of private variables
• dynamically scoped variables and
• global variables with examples.
  

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

Assignment

Here is an assignment for you.

Write the code for the following assignment and execute it.

1. Declare a package as FirstModule
2. Declare a variable $age as our and assign the value 42
3. Declare another package as SecondModule
4. Declare a variable $ageword as our and assign the value "Forty-Two"
5. Declare a subroutine First()
6. Inside the subroutine, declare two variables with local and my keyword as below:

   local $age = 52;

   my $ageword ="Fifty-two";

7. Call another subroutine as Result()
8. Print the values of $age and $ageword inside this function.
9. End the subroutine
10. Declare the subroutine Result()
11. Again print the values of $age and $ageword
12. End the subroutine
13. Call the function First()
14. Print the Package First and Package Second as below:

    print "Package First : $FirstModule::age\n";

    print "Package Second : $SecondModule::ageword\n";

About the Spoken Tutorial Project

Please download and watch it.

About workshops

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Slide 14:

Acknowledgment

Spoken Tutorial project is funded by NMEICT, MHRD, Government of India.

More information on this mission is available at

this link.