***** Positive Thinking, Happy Living

For a beginner, Perl can be a frightening language. This is largely due the numerous special variable used in perl. Once a programmer understands the special variables, he begins to appreciate the beauty and strength of Perl.

The special variable $_

$_ functions as a default variable. $_ needs to be initialized implicitly or explicitly before it can used.

$_ = "default value";

print;  # this will print the value of $_

Note that I did not specify which variable to print. When no variable is provided where a variable is expected, the $_ special variable is used. Here's another example:

while() 

  print $_;

This program will reprint whatever the user types. takes the input from the an external device such as a keyboard. This data must be assigned to a variable before it can be used. Since I did not specify a variable, the input is automatically stored in $_. If I remove the $_ from the second line, this program will still function properly since print; will print the value of $_ by default as seen in the previous example.

$_[], a subscript of @_ special variable

It is common for Perl beginners to confuse $_ with $_[]. In fact, $_[] is a subscript of @_ and it has nothing to do with the scalar variable $_.

@_ special variable contains a list of all the arguments passed to a subroutine. $_[argument number] is a way of accessing an individual argument.

sub printarguments 

  print join('-',@_);

  print "\nThe second argument was $_[1]";



printarguments('one','two','three');

Perl indexes begin from 0. Therefore $_[0] accesses the first argument, $_[1] accesses the second argument, and so on.

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Please feel free to publish this article, free of charge, as long as this resource box is visibly published. Copyright Nazim Rahman (c) http://www.molecularsciences.org

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