PERL/C2/Blocks-in-Perl/English

Title Of Script: BLOCKS in PERL

Author: Amol Brahmankar

Keywords: BLOCKS in perl video tutorial.

Visual Clue	Narration
Slide	Welcome to the spoken tutorial on BLOCKS in Perl
Slide: Learning Objectives	In this tutorial, we will learn about the various BLOCKS available in Perl
Slide: System Requirements	I am using Ubuntu Linux 12.04 operating system and Perl 5.14.2 I will also be using the gedit Text Editor. You can use any text editor of your choice.
Slide: Prerequisites	As a pre-requisite, you should have basic knowledge of Variables, Comments in Perl Knowledge of data structures in PERL will be an added advantage. Please go through the relevant spoken tutorials on the spoken tutorial website.
Slide	Perl provides 5 special blocks. These blocks get executed at various stages of a Perl program. These blocks are: BEGIN END UNITCHECK CHECK INIT
	Let us start with understanding the BEGIN block
Slide	* BEGIN block get executed at the time of compilation. So, any code written inside this block gets executed first during compilation. We can have several BEGIN blocks inside a Perl script. These blocks will get executed in the order of declaration. That is in the First define First execute pattern
Slide BEGIN { # Piece of code to be executed at the time of compilation }	The syntax for BEGIN block is as follows BEGIN in capital letters space open curly bracket Press Enter Piece of code to be executed at the time of compilation Press Enter Close curly bracket
	Now let us look at an example of BEGIN blocks.
Switch to the Terminal and type gedit beginBlock.pl and press Enter.	Open the Terminal and type gedit beginBlock dot pl space ampersand and press Enter
Gedit #!/usr/bin/perl print "First Line in PERL script\n"; BEGIN { print "Inside First BEGIN block\n"; } BEGIN { print "Inside Second BEGIN block\n"; } BEGIN { print "Inside Third BEGIN block\n"; } print "Last line in PERL script\n";	This will open the beginBlock dot pl file in gedit. Type the following piece of code as displayed on the screen.
Gedit	Let us look at what I have written inside the script. Here we have printed some text before and after the BEGIN blocks. Similarly, I have written one print statement in each BEGIN block. Please note, I have not given the semicolon after the BEGIN blocks. Putting a semicolon, will result in a syntax error on execution of the program.
Press Ctrl S	Now, press ctrl+s to save the file.
Switch to terminal perl beginBlock.pl	Then switch to the terminal and execute the script by typing, perl beginBlock dot pl and press Enter.
Highlight the output on the terminal Inside First BEGIN block Inside Second BEGIN block Inside Third BEGIN block First Line in PERL script Last line in PERL script	You will get the output as displayed on the terminal
Terminal	Notice that; The line written inside the first BEGIN block gets printed first and The first print statement in the script actually gets printed after the BEGIN block statements. BEGIN blocks gets executed in the order of their declaration.
Slide	From this example, it is evident that: The code written inside the BEGIN blocks get executed first. This is irrespective of the location of the BEGIN block inside PERL script. BEGIN blocks always get executed in the First In First Out manner So one of the use of this block is to include files inside a Perl script, before actual execution starts.
	Now, let us understand the END block
Slide	* END block get executed at the end of the PERL program Code written inside this block gets executed after PERL has finished executing the program We can have several END blocks inside a Perl script. These blocks will get executed in reverse order of definition. That is in Last define First execute pattern.
Slide END { # Piece of code to be executed at the end of the PERL script }	The syntax for END block is as follows END in capital letters space open curly bracket Press Enter Piece of code to be executed at the end of the PERL script Press Enter Close curly bracket
	Now let us look at an example of END blocks.
Switch to the Terminal and type gedit endBlock.pl and press Enter.	Open the Terminal and type gedit endBlock dot pl space ampersand and press Enter
Gedit #!/usr/bin/perl print "First Line in PERL script\n"; END { print "Inside First END block\n"; } END { print "Inside Second END block\n"; } END { print "Inside Third END block\n"; } print "Last line in PERL script\n";	This will open the endBlock dot pl file in gedit. Type the following piece of code as displayed on the screen.
Gedit	Let us look at what I have written inside this script. Here we have printed some text before and after the END blocks. Similarly, we have written one print statement in each END block. Please note, I have not given the semicolon after the END block. If we give the semicolon, there will be a syntax error on compilation.
Press Ctrl S	Now, press ctrl+s to save the file.
Switch to terminal perl endBlock.pl	Then switch to the terminal and execute the script by typing, perl endBlock dot pl and press Enter.
Highlight the output on the terminal First Line in PERL script Last line in PERL script Inside Third END block Inside Second END block Inside First END block	You will get the output as displayed on the terminal
Terminal	Notice that; The line written inside the END block is printed last. The last print statement in the script actually gets printed before the END block statements and END blocks gets executed in the reverse order of their declaration
Slide	From the example, it is evident that; The code written inside the END blocks get executed at the end This is irrespective of the location of the END block inside the PERL script and END blocks gets executed in the Last In First Out manner So, one use of END block is to destroy objects created in the program, before exiting.
Slide	Similarly PERL has UNITCHECK, CHECK and INIT blocks. These blocks are used rarely by developers and are a bit difficult to understand. So, I will be just brief you about these blocks.
Slide	UNITCHECK, CHECK and INIT blocks are useful; to catch the transition between compilation and execution phase of the main program and to perform some checks or initialisation after compilation and before execution
Slide	UNITCHECK and CHECK blocks runs in Last in First out manner whereas INIT block runs in First In First Out manner.
Slide UNITCHECK { # Piece of code to be executed }	The syntax for UNITCHECK block is as follows UNITCHECK in capital letters space open curly bracket Press Enter Piece of code to be executed Press Enter Close curly bracket
Slide CHECK { # Piece of code to be executed }	The syntax for CHECK block is as follows CHECK in capital letters space open curly bracket Press Enter Piece of code to be executed Press Enter Close curly bracket
Slide INIT { # Piece of code to be initialised }	The syntax for INIT block is as follows INIT in capital letters space open curly bracket Press Enter Piece of code to be initialised Press Enter Close curly bracket
	For better understanding, I recommend that you experiment with these blocks in your Perl scripts.
Slide: Summary	Let us summarize. In this tutorial, we have learnt - BEGIN and END blocks in details and Introduction to UNITCHECK, CHECK and INIT blocks using sample program
Slide: Assignment	Here is assignment for you - Type the below code inside a PERL script; #!/usr/bin/perl END { @array = (); print "Length of an array inside END block: ", $#array + 1, "\n"; } print "Length of an array: ", $#array + 1, "\n"; BEGIN { @array = (1, 2, 3); } execute the script and observe the output.
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	Hope you enjoyed this Perl tutorial. This is Amol Brahmankar signing off. Thanks for joining

Contributors and Content Editors

AmolBrahmankar, Nancyvarkey, PoojaMoolya