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what does undef means

hatem_from_mesr
on
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Last Modified: 2012-05-07
can any one help me to know what does undef substitute for in the next fragment of code

sub delete_item
{
my ($dbh, $id) = @_;
    return ($dbh->do (qq{ DELETE FROM todo WHERE id = ? },
                undef,
                $id));
}
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ozo
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Commented:
perldoc -f undef
       undef EXPR
       undef   Undefines the value of EXPR, which must be an lvalue.  Use only
               on a scalar value, an array (using "@"), a hash (using "%"), a
               subroutine (using "&"), or a typeglob (using "*").  (Saying
               "undef $hash{$key}" will probably not do what you expect on
               most predefined variables or DBM list values, so don't do that;
               see delete.)  Always returns the undefined value.  You can omit
               the EXPR, in which case nothing is undefined, but you still get
               an undefined value that you could, for instance, return from a
               subroutine, assign to a variable or pass as a parameter.
               Examples:

                   undef $foo;
                   undef $bar{'blurfl'};      # Compare to: delete $bar{'blurfl'};
                   undef @ary;
                   undef %hash;
                   undef &mysub;
                   undef *xyz;       # destroys $xyz, @xyz, %xyz, &xyz, etc.
                   return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
                   select undef, undef, undef, 0.25;
                   ($a, $b, undef, $c) = &foo;       # Ignore third value returned

               Note that this is a unary operator, not a list operator.
Suhas .Senior QA Manager
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Commented:
perldoc undef
http://perldoc.perl.org/functions/undef.html

In general, any value that is not defined is called as undef.

A good example:
http://www.linuxtopia.org/Perl_Programming/pickingUpPerl_29.html
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