Subroutines in Perl

Perl allows you to define your own functions, called subroutines. They are used for code reusability, so you don’t have to write the same code again and again. For example if you want to take input from user in several places of your program, then you can write the code in a subroutine and call the subroutine wherever you wanna take input. This way you do not have to write the same code again, this also improves code readability.

Advantages of subroutines:

1) Code re-usability
2) Improves code readability

Syntax:

defining a subroutine

The first thing you need to do is create a subroutine. sub keyword is used to define a subroutine in Perl program. You can choose any meaningful subroutine name.

sub subroutine_name
{
   statement(s);
   return;
}

calling a subroutine

A subroutine is called by using subroutine name prefixed with “&” character. You can pass arguments as well while calling the subroutine.

&subroutine_name; #calling without parameter
&subroutine_name(10);

Simple example of subroutine

Lets take a simple example to understand this:

#!/usr/bin/perl
my $msg;

# defining three subroutines
sub ask_user {
   printf "Please enter something: ";
}
sub get_input {
   $msg = <STDIN>;
   return $msg;
}
sub show_message {
   printf "You entered: $msg";
}
#calling subroutines
&ask_user;
&get_input;
&show_message;

Output:

Please enter something: Welcome to beginnersbook
You entered: Welcome to beginnersbook

Passing Parameters to subroutines

In the above example, we did not pass any parameter while calling the subroutine, however we can pass various parameters while calling a subroutine. All the parameters (often referred as arguments) are stored in special array (@_). Lets have a look at the example below to understand this:

#!/usr/bin/perl

# defining subroutine
sub printparams {
   printf "@_\n";
   return;
}
#calling subroutine
&printparams("This", "is", "my", "blog");

Output:

This is my blog

In the above example, we have printed all the parameters using special array (@_). However if you want you can display selected parameters using “$_[n]” where n is the index of parameter, for example $_[0] would refer first parameter and $_[1] would refer second parameter. Lets take an example:

#!/usr/bin/perl

# defining subroutine
sub printparams {
   printf "First Parameter: $_[0]\n";
   printf "Fourth Parameter: $_[3]\n";
   return;
}
#calling subroutine
&printparams("This", "is", "my", "blog");

Output:

First Parameter: This
Fourth Parameter: blog

Passing arrays to subroutine

In the above example, we have passed few string parameters while calling subroutine. We can also pass arrays to subroutine. Here is an example:

#!/usr/bin/perl

# defining subroutine
sub printparams {
   printf "First Parameter: $_[0]\n";
   printf "Third Parameter: $_[2]\n";
   printf "Fourth Parameter: $_[3]\n";
   printf "Sixth Parameter: $_[5]\n";
   return;
}

@array1 = ("This", "is", "text");
$num = 100;
@array2 = ("Welcome", "here");
#calling subroutine
&printparams(@array1, @array2, $num);

Output:

First Parameter: This
Third Parameter: text
Fourth Parameter: Welcome
Sixth Parameter: 100

Passing hashes to subroutine

#!/usr/bin/perl

sub DisplayMyHash{
   #copying passed hash to the hash
   my %hash = @_;

  for my $key (keys %hash) {
    print "Key is: $key and value is: $hash{$key}\n";
  }
}
%hash = ('Item1', 'Orange', 'Item2', 'Apple', 'Item3', 'Banana');

# Function call with hash parameter
DisplayMyHash(%hash);

Output:

Key is: Item1 and value is: Orange
Key is: Item2 and value is: Apple
Key is: Item3 and value is: Banana