=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.22 $, $Date: 2002/05/16 12:44:24 $)
+perlfaq4 - Data Manipulation ($Revision: 1.24 $, $Date: 2002/05/20 16:50:08 $)
=head1 DESCRIPTION
=head2 What's the difference between "delete" and "undef" with hashes?
-Hashes are pairs of scalars: the first is the key, the second is the
-value. The key will be coerced to a string, although the value can be
-any kind of scalar: string, number, or reference. If a key C<$key> is
-present in the array, C<exists($key)> will return true. The value for
-a given key can be C<undef>, in which case C<$array{$key}> will be
-C<undef> while C<$exists{$key}> will return true. This corresponds to
-(C<$key>, C<undef>) being in the hash.
+Hashes contain pairs of scalars: the first is the key, the
+second is the value. The key will be coerced to a string,
+although the value can be any kind of scalar: string,
+number, or reference. If a key $key is present in
+%hash, C<exists($hash{$key})> will return true. The value
+for a given key can be C<undef>, in which case
+C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
+will return true. This corresponds to (C<$key>, C<undef>)
+being in the hash.
-Pictures help... here's the C<%ary> table:
+Pictures help... here's the %hash table:
keys values
+------+------+
And these conditions hold
- $ary{'a'} is true
- $ary{'d'} is false
- defined $ary{'d'} is true
- defined $ary{'a'} is true
- exists $ary{'a'} is true (Perl5 only)
- grep ($_ eq 'a', keys %ary) is true
+ $hash{'a'} is true
+ $hash{'d'} is false
+ defined $hash{'d'} is true
+ defined $hash{'a'} is true
+ exists $hash{'a'} is true (Perl5 only)
+ grep ($_ eq 'a', keys %hash) is true
If you now say
- undef $ary{'a'}
+ undef $hash{'a'}
your table now reads:
and these conditions now hold; changes in caps:
- $ary{'a'} is FALSE
- $ary{'d'} is false
- defined $ary{'d'} is true
- defined $ary{'a'} is FALSE
- exists $ary{'a'} is true (Perl5 only)
- grep ($_ eq 'a', keys %ary) is true
+ $hash{'a'} is FALSE
+ $hash{'d'} is false
+ defined $hash{'d'} is true
+ defined $hash{'a'} is FALSE
+ exists $hash{'a'} is true (Perl5 only)
+ grep ($_ eq 'a', keys %hash) is true
Notice the last two: you have an undef value, but a defined key!
Now, consider this:
- delete $ary{'a'}
+ delete $hash{'a'}
your table now reads:
and these conditions now hold; changes in caps:
- $ary{'a'} is false
- $ary{'d'} is false
- defined $ary{'d'} is true
- defined $ary{'a'} is false
- exists $ary{'a'} is FALSE (Perl5 only)
- grep ($_ eq 'a', keys %ary) is FALSE
+ $hash{'a'} is false
+ $hash{'d'} is false
+ defined $hash{'d'} is true
+ defined $hash{'a'} is false
+ exists $hash{'a'} is FALSE (Perl5 only)
+ grep ($_ eq 'a', keys %hash) is FALSE
See, the whole entry is gone!
=head2 Why don't my tied hashes make the defined/exists distinction?
-They may or may not implement the EXISTS() and DEFINED() methods
-differently. For example, there isn't the concept of undef with hashes
-that are tied to DBM* files. This means the true/false tables above
-will give different results when used on such a hash. It also means
-that exists and defined do the same thing with a DBM* file, and what
-they end up doing is not what they do with ordinary hashes.
+This depends on the tied hash's implementation of EXISTS().
+For example, there isn't the concept of undef with hashes
+that are tied to DBM* files. It also means that exists() and
+defined() do the same thing with a DBM* file, and what they
+end up doing is not what they do with ordinary hashes.
=head2 How do I reset an each() operation part-way through?
if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
{ print "a C float\n" }
-Or you could check out the L<Data::Types|Data::Types> module on the CPAN,
-which exports functions that validate data types using these and other regular
-expressions.
+You can also use the L<Data::Types|Data::Types> module on
+the CPAN, which exports functions that validate data types
+using these and other regular expressions.
If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
function. Its semantics are somewhat cumbersome, so here's a C<getnum>
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