This section describes what is the 'undef' value and what is an undefined variable. A scalar, array or hash variable without any value assigned is an undefined variable, which is interpreted as 0, '', or FALSE depending on the context.
Perl also supports a special value called "undef", which is used to indicate the status of an undefined variable.
The "undef" value and undefined variables can be used in a Perl script under these rules:
A scalar variable without a scalar value assigned is an undefined variable.
A array variable without a list value assigned, or assigned with an empty list, is an undefined variable.
A hash variable without a list value assigned, or assigned with an empty list, is an undefined variable.
The undef() function removes the value from a scalar variable, an array variable or a hash variable, and makes it an undefined variable.
An undefined variable is equal to the undef value in a comparison operation.
An undefined variable will be interpreted as 0 in an operation, if a numerical value is expected.
An undefined variable will be interpreted as "" in an operation, if a string value is expected.
An undefined variable will be interpreted as FALSE in an operation, if a Boolean value is expected.
An undefined variable will be interpreted as an empty list, if a list value is expected.
Here is a Perl tutorial script on how to use undef value and undefined variables:
#- UndefinedVariable.pl
#- Copyright (c) 1995 by Dr. Herong Yang, http://www.herongyang.com/
#
#- Undefined scalar variable
$price;
$string = '('.$price.')'; # Interpreted as ""
print($string, "\n");
$price = $price + 3.99; # Interpreted as 0
print($price, "\n");
undef($price); # Undefined again
print("undef", "\n") if ($price == undef);
# Interpreted as undef
print("priceless!", "\n") if (! $price);
# Interpreted as FALSE
#- Undefined array variable
@links; # Undefined
@links = (@links, "herongyang.com", "perl.org", "cpan.org");
print(@links, "\n"); # Interpreted as empty list
undef(@links); # Undefined again
print("undef", "\n") if (@links == undef);
# Interpreted as undef
print("empty array!", "\n") if (! @links);
# Interpreted as FALSE
#- Undefined hash variable
%siteRanks; # Undefined
%siteRanks = (%siteRanks, "herongyang.com", 4,
"perl.org", 6, "google.com", 9);
print(%siteRanks, "\n"); # Interpreted as empty list
undef(%siteRanks); # Undefined again
print("undef", "\n") if (%siteRanks == undef);
# Interpreted as undef
print("empty hash!", "\n") if (! %siteRanks);
# Interpreted as FALSE
