PERL/C3/Access-Modifiers-in-PERL/English-timed

Time	Narration
00:01	Welcome to the Spoken Tutorial on Access Modifiers in PERL
00:07	In this tutorial we will learn about Scope of variables Private variables Dynamically scoped variables Global variables
00:19	For this tutorial, I am using Ubuntu Linux 12.04 operating system Perl 5.14.2 and the gedit Text Editor
00:32	You can use any text editor of your choice.
00:36	You should have basic knowledge of Perl Programming.
00:40	If not, then go through the relevant Perl spoken tutorials on the spoken tutorial website.
00:47	Let us start with the introduction to the Scope of variables.
00:51	The scope of a variable is the region of code within which a variable can be accessed.
00:58	In other words, it refers to the visibility of variables.
01:03	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
01:20	Variables declared with my keyword will lose scope outside the block in which they are declared.
01:28	You can declare a variable without giving it a value, like this: my $fvaluesemicolon
01:37	You can also declare a variable by assigning a value to it, as:
01:43	my $fValue = 1 semicolon
01:48	my $fname = within double quotes Rahul semicolon
01:55	The syntax to declare several variables with the same my statement is as follows:
02:02	my open bracket $fname comma $lname comma $age close bracket semicolon
02:12	Let us understand private variables using a sample program.
02:17	I already have a sample program. Let me open it in gedit Text editor.
02:24	Open the terminal and type gedit scope hyphen my dot pl ampersand and press Enter
02:34	Scope-my dot pl file is now open in gedit.
02:39	Type the following code as displayed on the screen. Let me explain the code now.
02:46	Here, I have declared a private variable $fname with my keyword.
02:52	And assigned the value "Raghu" to it.
02:56	Within this block, the print statement prints the value in the fname variable, i.e. "Raghu".
03:04	In the next block, I have assigned the value "Other" to the same private variable $fname.
03:11	So, the print statement will print "Other" within this particular block.
03:17	The last print statement in this program, will not print any output.
03:23	This is because outside the scope of the blocks defined earlier, fname has no value assigned to it.
03:32	Now, press Ctrl+S to save the file.
03:37	Let us now execute the program.
03:40	Switch back to the terminal and type perl scope hyphen my dot pl and press Enter.
03:49	The output is displayed as Block 1: Raghu Block 2: Other Outside Block: There is no output
03:59	So, the scope of the my variable is accessed only within a particular block of code.
04:06	Now let us change the existing program a little.
04:10	Let us add my $fname = "John" semicolon outside the blocks, before the last print statement. Save the changes
04:23	Switch back to the terminal and execute as before.
04:28	Analyse the output that is displayed.
04:32	Hope you are able to understand the scope of using my variable within a block and outside a block.
04:41	Next we will see about dynamically scoped variable in Perl.
04:47	Local keyword gives a temporary scope to a global variable.
04:52	The variable is visible to any function called from the original block.
04:58	You can declare a local variable as, local $fValue = 100 semicolon local $fname” = within double quotes Rakesh semicolon
05:13	Let us understand this using a sample program.
05:17	Open the terminal and type gedit scope-local dot pl ampersand and press Enter.
05:27	This will open scope-local dot pl file in gedit.
05:33	Type the following code as displayed on the screen. Let me explain the code now.
05:40	Here, in the first line we have declared a variable $fname and initialised it.
05:47	Inside the function Welcome, we have declared a local variable by the same name, $fname.
05:54	Notice the local keyword before the variable name.
05:59	And we have assigned the value "Rakesh" to this variable.
06:03	So, basically, inside function Welcome(), $fname is modified as a new temporary local variable. Then, the function Hello is being called.
06:15	Here is the function definition of Hello.
06:18	At the end of the program, we are calling both the functions Welcome and Hello.
06:25	Now press Ctrl + S to save the program.
06:29	Let us execute the program.
06:31	Switch back to the terminal and type, perl scope-local.pl and press Enter.
06:41	The output is displayed as Hello, Rakesh ! Hello, Welcome to Spoken tutorials!
06:48	Let us understand the output.
06:51	When the function Welcome() is called, the function Hello() within it, accesses the local variable.
06:59	Within Welcome(), $fname has the value "Rakesh".
07:04	After this, the function Hello() accesses the variable $fname once again.
07:11	But this time, it is the variable $fname which was initialized to "Welcome to spoken tutorials".
07:19	It does not access the local variable $fname within the function Welcome().
07:25	Which means that, the local variable restores the scope, after leaving the block Welcome().
07:32	Next, we will see about global variables in Perl.
07:38	A global variable can be accessed anywhere in the program.
07:43	Global variables are declared with our keyword.
07:47	Here are some examples. our $fvalue = 100 semicolon </nowiki> our $fname ="Priya" semicolon
08:01	Now let us look at a working example of global variables.
08:06	Switch back to the terminal and type gedit scope-our dot pl ampersand and press Enter
08:16	This will open the file scope-our.pl in gedit.
08:22	Let me explain the sample program which I have written.
08:27	I have declared package main and a global variable as our $i and I have initialised it to 100;
08:37	Notice the package First declaration.
08:40	A package is a collection of code, which has its own namespace.
08:46	Namespace prevents variable name collisions between packages.
08:51	We will see more about package and namespace in future tutorials.
08:56	Within package First, the global variable "i" holds the value 10.
09:02	In package Second, the global variable "i" is assigned the value 20.
09:08	The main package uses both package First variable and the package Second variable.
09:15	In my program, I have declared the same variable "i" in all the packages.
09:21	The package variable is referred by package name::variable name
09:29	In our example it is $First::i, $Second::i
09:39	We have multiple packages within one file, and the global variable will be accessed by all the packages.
09:47	Now, save the file and execute the program.
09:51	So, switch to terminal and type- perl scope-our dot pl and press Enter.
09:59	The output is as displayed on the terminal.
10:03	Analyze the output by yourself to understand how the assignment to the variable i was done.
10:11	This brings us to the end of this tutorial. Let us summarise.
10:16	In this tutorial, we learnt: scope of variables declaration of private variables dynamically scoped variables and global variables with examples.
10:29	It is preferred to use my than local as the compilation is faster.
10:35	Here is an assignment for you.
10:37	Write the code for the following assignment and execute it.
10:42	Declare a package as FirstModule
10:46	Declare a variable $age as our and assign the value 42
10:52	Declare another package as SecondModule
10:56	Declare a variable $ageword as our and assign the value within double quotes "Forty-Two"
11:05	Declare a subroutine First()
11:08	Inside the subroutine, declare two variables with local and my keyword as below:
11:16	local' $age = 52 semicolon
11:20	my $ageword = within double quotes Fifty-two;
11:27	Call another subroutine as Result()
11:31	Print the values of $age and $ageword inside this function.
11:37	End the subroutine
11:39	Declare the subroutine Result()
11:42	Again print the values of $age and $ageword
11:47	End the subroutine
11:49	Call the function First()
11:51	Print the Package First and Package Second as below:
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12:31	This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.

Contributors and Content Editors

PoojaMoolya, Sandhya.np14