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