Note that you have a two different issues here:
has a scalar (either a stand-alone scalar, or
a value in an array or hash) has ever been assigned
to; or, for hashes in particular, whether a key/value
pair has ever been created.
Perl has two keywords that answer these questions:
defined and exists,
respectively. If you actually want to assign or
create those values, a good idiom is to use
unless:
$a = "default" unless defined $a;
$h{key} = "value" unless exists $h{key};
If you know that valid values do not include the
values that perl considers false (undef, empty string,
and 0), you can generally just test the value
directly. As an added amusement, you can use the
||= operator to "assign unless true":
$a = "default" unless $a;
$h{key} ||= "value";
Finally, if you want to extract a value or a default
without modifying the original, the tinary arithmetic
conditional operator is a nice fit:
my $a_val = defined $a ? $a : "default";
my $h_key_val = exists $h{key} ? $h{key} : "value";
Again, if you know the values are never "false" in the
perl sense, you can dispense with the specific tests
and use the || operator:
my $a_val = $a || "default";
my $h_key_val = $h{key} || "value";
Finally, there is a useful idiom for assigning default
values into hashes, overriding only keys that are
explicitly mentioned. This takes advantage of the fact
that, when assigning into a hash from a list, later
key/value pairs overwrite the same key seen earlier:
sub do_stuff
{
# get args from the call
my ( %raw_args ) = @_;
# build up hash of default arg values
my %default_args = ( foo => 'bar', baz => 'quux' );
# now join the two, giving precedence to %raw_args
my %args = ( %default_args, %raw_args );
}
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