PERL/C3/Referencing-and-Dereferencing/English-timed
From Script | Spoken-Tutorial
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| 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 a 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 | On 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. |
| 10:35 | The video at the following link summarizes the Spoken Tutorial project.Please download and watch it. |
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| 10:51 | Spoken Tutorial project is funded by NMEICT, MHRD, Government of India.
More information on this mission is available at this link. |
| 11:02 | This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching. |
