PERL/C2/Blocks-in-Perl/English-timed
From Script | Spoken-Tutorial
| Time | Narration |
| 00:01 | Welcome to the spoken tutorial on Blocks in Perl. |
| 00:06 | In this tutorial, we will learn about the various blocks available in Perl. |
| 00:13 | I am using Ubuntu Linux 12.04 operating system and Perl 5.14.2. |
| 00:21 | I will also be using the gedit Text Editor. |
| 00:26 | You can use any text editor of your choice. |
| 00:31 | As a pre-requisite, you should have basic knowledge of variables, comments in Perl. |
| 00:38 | Knowledge of data structures in PERL will be an added advantage. |
| 00:44 | Please go through the relevant spoken tutorials on the Spoken Tutorial website. |
| 00:50 | Perl provides 5 special blocks. |
| 00:53 | These blocks get executed at various stages of a Perl program. |
| 00:59 | These blocks are: |
| 01:01 | BEGIN , END |
| 01:03 | UNITCHECK, CHECK |
| 01:05 | INIT, Let us start with understanding the BEGIN block. |
| 01:10 | BEGIN block gets executed at the time of compilation. |
| 01:15 | So, any code written inside this block gets executed first during compilation. |
| 01:22 | We can have several BEGIN blocks inside a Perl script. |
| 01:26 | These blocks will get executed in the order of declaration. |
| 01:31 | That is, in the First define First execute pattern. |
| 01:35 | The syntax for BEGIN block is as follows: |
| 01:40 | BEGIN in capital letters space open curly bracket |
| 01:45 | Press Enter. |
| 01:47 | Piece of code to be executed at the time of compilation, |
| 01:51 | Press Enter .Close curly bracket. |
| 01:55 | Now, let us look at an example of BEGIN block. |
| 01:59 | Open the Terminal and type: |
| 02:02 | gedit beginBlock dot pl space ampersand |
| 02:08 | and press Enter. |
| 02:10 | This will open the beginBlock dot pl file in gedit. |
| 02:15 | Type the following piece of code as displayed on the screen. |
| 02:20 | Let us look at what I have written inside the script. |
| 02:24 | Here, we have printed some text before and after BEGIN blocks. |
| 02:31 | Similarly, I have written one print statement in each BEGIN block. |
| 02:37 | Please note, I have not given the semicolon after the BEGIN blocks. |
| 02:42 | Putting a semicolon will result in a syntax error on execution of the program. |
| 02:49 | Now, press Ctrl+s to save the file. |
| 02:53 | Then switch to terminal and execute the script by typing: |
| 02:58 | perl beginBlock dot pl |
| 03:01 | and press Enter. |
| 03:04 | You will get the output as displayed on the terminal. |
| 03:09 | Notice that the line written inside the first BEGIN block gets printed first and |
| 03:16 | the first print statement in the script actually gets printed after the BEGIN block statements. |
| 03:25 | BEGIN blocks get executed in the order of their declaration. |
| 03:31 | From this example, it is evident that |
| 03:34 | the code written inside the BEGIN block gets executed first. |
| 03:40 | This is irrespective of the location of the BEGIN block inside PERL script. |
| 03:46 | BEGIN blocks always get executed in the First In First Out manner. |
| 03:52 | So, one of the use of this block is to include files inside a Perl script, before actual execution starts. |
| 04:01 | Now, let us understand the END block. |
| 04:04 | END block gets executed at the end of PERL program. |
| 04:09 | Code written inside this block gets executed after PERL has finished executing the program. |
| 04:17 | We can have several END blocks inside a Perl script. |
| 04:21 | These blocks will get executed in reverse order of declaration. |
| 04:26 | That is, in Last define First execute pattern. |
| 04:30 | The syntax for END block is as follows: |
| 04:35 | END in capital letters open curly bracket |
| 04:39 | press Enter. Piece of code to be executed at the end of the PERL script, |
| 04:45 | press Enter.Close curly bracket. |
| 04:49 | Now, let us look at an example of END blocks. |
| 04:53 | Open the Terminal and type: |
| 04:56 | gedit endBlock dot pl space ampersand |
| 05:00 | and press Enter. |
| 05:03 | This will open the endBlock dot pl file in gedit. |
| 05:08 | Type the following piece of code as displayed on the screen. |
| 05:13 | Let us look at what I have written inside this script. |
| 05:17 | Here, we have printed some text before and after END blocks. |
| 05:23 | Similarly, we have written one print statement in each END block. |
| 05:29 | Please note, I have not given the semicolon after the END block. |
| 05:34 | If we give the semicolon, there will be a syntax error on compilation. |
| 05:41 | Now, press Ctrl+s to save the file. |
| 05:45 | Then switch to terminal and execute the script by typing: |
| 05:50 | perl endBlock dot pl |
| 05:53 | and press Enter. |
| 05:55 | You will get the output as displayed on the terminal. |
| 06:00 | Notice that : The line written inside the END block is printed last. |
| 06:06 | The last print statement in the script actually gets printed before the END block statements and |
| 06:13 | END blocks get executed in the reverse order of their declaration. |
| 06:20 | From the example, it is evident that |
| 06:23 | the code written inside the END blocks get executed at the end. |
| 06:29 | This is irrespective of the location of the END block inside the PERL script and |
| 06:36 | END blocks get executed in the Last In First Out manner. |
| 06:41 | So, one use of END block is to destroy objects created in the program before exiting. |
| 06:49 | Similarly, PERL has UNITCHECK, CHECK and INIT blocks. |
| 06:55 | These blocks are used rarely by developers and are a bit difficult to understand. |
| 07:02 | So, I will be just briefing you about these blocks. |
| 07:06 | UNITCHECK, CHECK and INIT blocks are useful: |
| 07:10 | to catch the transition between compilation and execution phase of the main program and |
| 07:18 | to perform some checks or initialization, after compilation and before execution. |
| 07:24 | UNITCHECK and CHECK blocks run in Last in First out manner |
| 07:31 | whereas INIT block runs in First In First Out manner. |
| 07:37 | The syntax for UNITCHECK block is as follows: |
| 07:41 | UNITCHECK in capital letters space open curly bracket |
| 07:46 | press Enter. |
| 07:48 | Piece of code to be executed, |
| 07:50 | press Enter. |
| 07:52 | Close curly bracket. |
| 07:54 | The syntax for CHECK block is as follows: |
| 07:58 | CHECK in capital letters space open curly bracket |
| 08:03 | press Enter. Piece of code to be executed, |
| 08:07 | press Enter.Close curly bracket. |
| 08:11 | The syntax for INIT block is as follows: |
| 08:15 | INIT in capital letters space open curly bracket |
| 08:20 | press Enter.Piece of code to be initialized, |
| 08:24 | press Enter . |
| 08:26 | Close curly bracket. |
| 08:28 | For better understanding, I recommend that you experiment with these blocks in your Perl scripts. |
| 08:36 | Let us summarize. In this tutorial, we have learnt: |
| 08:40 | BEGIN and END blocks in detail and |
| 08:44 | introduction to UNITCHECK, CHECK and INIT blocks |
| 08:48 | using sample programs. |
| 08:52 | Here is an assignment for you: |
| 08:54 | Type the below code inside a PERL script; |
| 08:58 | execute the script and observe the output. |
| 09:02 | Watch the video available at the following link. |
| 09:06 | It summarizes the Spoken Tutorial project. |
| 09:09 | If you do not have good bandwidth, you can download and watch it. |
| 09:14 | The Spoken Tutorial project team: Conducts workshops using spoken tutorials. |
| 09:20 | Gives certificates to those who pass an online test. |
| 09:24 | For more details, please write to: contact at spoken hyphen tutorial dot org. |
| 09:32 | "Spoken Tutorial" project is a part of the "Talk to a Teacher" project. |
| 09:37 | It is supported by the National Mission on Education through ICT, MHRD, Government of India. |
| 09:45 | More information on this mission is available at: spoken hyphen tutorial dot org slash NMEICT hyphen Intro. |
| 09:57 | Hope you enjoyed this Perl tutorial. |
| 10:00 | This is Amol, signing off. |
| 10:02 | Thanks for joining. |
