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

I will also be using the gedit Text Editor.

You can use any text editor of your choice.

Knowledge of data structures in PERL will be an added advantage.

Please go through the relevant spoken tutorials on the spoken tutorial website.

These blocks get executed at various stages of a Perl program.

These blocks are:

1. BEGIN
2. END
3. UNITCHECK
4. CHECK
5. INIT

• 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

BEGIN {

# Piece of code to be executed at the time of compilation

}

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

gedit beginBlock.pl &and press Enter.

gedit beginBlock dot pl space ampersand

and press Enter.

#!/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";

Type the following piece of code as displayed on the screen.

Here, we have printed some text before and after the BEGIN blocks.

Putting a semicolon, will result in a syntax error, on execution of the program.

perl beginBlock.pl

perl beginBlock dot pl

and press Enter.

Inside First BEGIN block

Inside Second BEGIN block

Inside Third BEGIN block

First Line in PERL script

Last line in PERL script

• 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.

• The code written inside the BEGIN blocks gets 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.

• 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 declaration
• That is, in Last define First execute pattern.

END {

# Piece of code to be executed at the end of the PERL script

}

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

gedit endBlock.pl &and press Enter.

gedit endBlock dot pl space ampersand

and press Enter

#!/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";

Type the following piece of code as displayed on the screen.

• 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.

perl endBlock.pl

perl endBlock dot pl

and press Enter.

First Line in PERL script

Last line in PERL script

Inside Third END block

Inside Second END block

Inside First END block

• 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.

• 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.

These blocks are used rarely by developers and are a bit difficult to understand.

So, I will be just briefing you about these blocks.

• 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

whereas

INIT block runs in First In First Out manner.

UNITCHECK {

# Piece of code to be executed

}

UNITCHECK in capital letters space open curly bracket

Press Enter

Piece of code to be executed

Press Enter

Close curly bracket

CHECK {

# Piece of code to be executed

}

CHECK in capital letters space open curly bracket

Press Enter

Piece of code to be executed

Press Enter

Close curly bracket

INIT {

# Piece of code to be initialised

}

INIT in capital letters space open curly bracket

Press Enter

Piece of code to be initialised

Press Enter

Close curly bracket

In this tutorial, we have learnt -

• BEGIN and END blocks in detail and

• Introduction to UNITCHECK, CHECK and INIT blocks

• using sample programs

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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