Difference between revisions of "PERL/C2/Blocks-in-Perl/English-timed"

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Revision as of 11:53, 14 May 2015

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
01:02 * END
01:03 * UNITCHECK
01:04 * CHECK
01:05 * INIT.
01:06 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 .
01:52 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 blocks 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.
04:40 Piece of code to be executed at the end of the PERL script,
04:45 press Enter.
04:46 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.
08:04 Piece of code to be executed,
08:07 press Enter.
08:08 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.
08:21 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.
08:37 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.

Contributors and Content Editors

Gaurav, PoojaMoolya, Sandhya.np14

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