Perl $_ vs. $_[]

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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.


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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