PERL/C3/Referencing-and-Dereferencing/English-timed

Time	Narration
00:01	Welcome to the Spoken Tutorial on Referencing and Dereferencing in Perl.
00:07	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.
00:22	For this tutorial, I am using: Ubuntu Linux 12.04 operating system Perl 5.14.2 gedit Text Editor
00:33	You can use any text editor of your choice.
00:37	You should have working knowledge of: Perl programming Array functions and Hash functions.
00:43	If not, then go through the relevant Perl tutorials on this website.
00:49	What is Reference?
00:51	A reference is a pointer or an address to a variable, array, hash or a subroutine.
00:58	It does not contain data directly.
01:01	Reference is an easy, compact scalar value.
01:05	Reference will improve the performance of Perl code when you pass or return large data-structures.
01:12	It saves memory as it passes a reference to a subroutine rather than passing a value.
01:18	Easy to manage complicated Perl data structures.
01:22	Let us learn how to create a reference.
01:25	We can create a reference for any variable, subroutine or value, by putting a backslash (\) in front of it.
01:33	A scalar variable is referenced by backslash and dollar sign ($) as shown here.
01:39	An array variable is referenced by backslash and at the rate(@) symbol.
01:45	A hash variable is referenced by backslash and percentage(%) symbol as shown in the example here.
01:53	what is dereference?
01:55	When a reference is dereferenced, the actual value is returned.
02:00	Dereference is done by enclosing the reference variable within curly brackets
02:06	and preceding the left curly bracket with a character denoting the type of reference it is.
02:12	Let us see how to dereference variables.
02:16	A scalar variable is dereferenced by dollar sign ($) and curly brackets.
02:21	An array variable is dereferenced by at the rate (@) symbol and curly brackets.
02:27	A hash variable is dereferenced by percentage(%) symbol and curly brackets.
02:33	Let us see a simple program for Scalar reference and dereference.
02:38	Let me open a sample program in 'gedit' Text editor.
02:43	Open the terminal and type: gedit scalarRef dot pl ampersand and press Enter.
02:50	Type the following code as displayed on the screen.
02:55	Let me explain the code.
02:57	First line declares a scalar variable '$a' and initialized to 10.
03:03	As mentioned earlier, a scalar variable is referenced by backslash and dollar sign ($).
03:10	This line will print memory address of the variable that is created as reference.
03:16	To print the actual value, the variable is dereferenced by curly brackets preceded by '$'.
03:23	Here ref() function will return the reference type such as scalar or array or hash.
03:30	Now, press Ctrl+S to save the file.
03:34	Let us execute the program.
03:36	Switch to the terminal and type: perl scalarRef dot pl and press Enter.
03:43	The output is displayed as shown.
03:46	First line shows the memory address where the value 10 is stored.
03:51	The second line returns the actual value 10.
03:55	Ref() function returns "SCALAR" as output.
03:59	Next, let us understand how to create a reference and dereference array by using a sample program.
04:07	I already have a sample program. Let me open it in 'gedit' Text editor.
04:13	In the terminal, type: gedit arrayRef dot pl ampersand and press Enter.
04:20	Type the following code as displayed on the screen, in the arrayRef dot pl file.
04:26	Let me explain the code now.
04:28	Here, in the first line, I have declared an array @color and initialized it with three values.
04:35	It is referenced with backslash @color which is the array name and assigned to $colorRef.
04:42	The print statement will print the reference value and the dereferenced value.
04:47	Now, press Ctrl+S to save the file.
04:51	Let us execute the program.
04:53	Switch back to the terminal and type: perl arrayRef dot pl and press Enter.
05:00	The output is displayed as shown here.
05:04	The first line shows the output of the memory address of the variable that is created as reference.
05:10	The second line shows the actual value that is dereferenced.
05:16	Next, we will see how to declare direct reference for an array.
05:21	Let’s come back to our program.
05:24	I have changed the existing program to show the direct reference for an array.
05:29	You can create a direct reference for an array by using square brackets [] as shown.
05:35	Use arrow operator (->) to dereference.
05:39	print statement will print "Green" as output.
05:43	Here, the print statement takes the value of index[1]. i.e 'Green' in our program.
05:50	Press Ctrl+S to save the file.
05:54	Switch back to the terminal and type: perl arrayRef dot pl and press Enter to execute.
06:03	I’ll show an example on how to use the direct hash reference in the same code file. So, let’s switch to gedit.
06:11	You can create a direct reference to hash by using curly brackets {} as shown here.
06:18	Use arrow operator (->) to dereference it. “Name” is the hash key.
06:24	On executing this block of code, both the print statements will print "Sunil" as output.
06:31	Next we will see how to add, remove, access elements to array reference with a sample program.
06:39	I already have a sample program. Let me open it in gedit Text editor.
06:45	Open the terminal and type: gedit arrayRefadd dot pl ampersand and press Enter.
06:54	'arrayRefadd.pl' file is now open in gedit. Type the code as shown here in your file.
07:02	The first line initializes an array.
07:06	We have referenced an array with backslash @numarray and assigned to $ref.
07:13	Now, we will see how to access a particular element from the array reference.
07:19	We need to use the array index in square brackets “[ ]” to access the particular value and an arrow operator (“->”) to dereference it.
07:28	The print statement will print the value of index [0].
07:32	Push() function adds elements at the last position of an array reference. In our case 5, 6, 7 are added to the end of the existing array 1, 2, 3, 4.
07:47	This print statement shows the output, after adding to the array reference.
07:53	Pop() function removes an element from the last position of an array reference.
07:58	In our example, 7 will be removed from the existing array reference.
08:03	print statement shows the output after deleting from the array reference.
08:08	Now, press Ctrl+S to save the file.
08:11	Let us execute the program.
08:14	Switch back to the terminal and type: perl arrayRefadd dot pl and press Enter.
08:22	The output is displayed as shown here.
08:26	Now, let us see another sample program to add, remove, and access elements of hash reference.
08:34	In the terminal, type: gedit hashRefadd dot pl ampersand and press Enter.
08:42	This will open the file 'hashRefadd.pl' in gedit.
08:47	Let me explain the sample program.
08:50	I have declared a direct hash reference that can be stored in a scalar variable $weektemp.
08:57	I have used curly brackets to represent the hash reference and the arrow operator to dereference.
09:04	This code stores the temperature values from Monday to Friday.
09:09	I am using the “keys” built-in function to loop through the keys of the hash.
09:15	print statement will print each element of the hash.
09:19	We can access the particular value of an element as shown here.
09:25	print statement will print the temperature on Monday.
09:29	Now, save the file.
09:32	Switch to terminal and type: perl hashRefadd dot pl and press Enter to see the output.
09:41	The hash keys and hash values are stored in a random order.
09:46	The displayed output is not related to the order in which they were added.
09:52	With this, we come to the end of this tutorial. Let us summarize.
09:57	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.
10:14	Here is an assignment for you. Add new keys “Saturday” and “Sunday” in hash weektemp, in our hashRefadd dot pl file.
10:24	Delete key “Saturday” at the end.
10:27	Print hash weektemp.
10:30	Save and execute the program. Now check the result.
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11:02	This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.

Contributors and Content Editors

PoojaMoolya, Sandhya.np14