PERL/C2/Blocks-in-Perl/English-timed

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 get 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 blocks.
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 gets 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 get 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 gets 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 gets 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 initialisation, after compilation and before execution
07:24	UNITCHECK and CHECK blocks runs 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 initialised
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 assignment for you -
08:54	Type the below code inside a PERL script;
08:58	Execute the script and observe the output.
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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