Tie::Hash has these:
sub FIRSTKEY { my $a = scalar keys %{$_[0]}; each %{$_[0]} }
sub NEXTKEY { each %{$_[0]} }
NEXTKEY takes two arguments, one of which is the last key but that arg is never referenced?
The various Tie docs don't shed any light on this other than this in perltie:
my $a = keys %{$self->{LIST}}; # reset each() iterator
looking at the doc for each doesn't add to this.
What's going on?
A hash is a set of key/value pairs. Hash variables are preceded by a percent (%) sign. To refer to a single element of a hash, you will use the hash variable name preceded by a "$" sign and followed by the "key" associated with the value in curly brackets..
A hash is a set of key-value pairs. Perl stores elements of a hash such that it searches for the values based on its keys. Hash variables start with a '%' sign. Perl requires the keys of a hash to be strings, whereas the values can be any scalars. These values can either be a number, string or reference.
You only need to worry about the second argument to NEXTKEY
if you care about which key was accessed last. By default, hashes don't care about the order, so it is not used.
As for the second part, the keys
function in scalar context returns the number of items in the hash. Any call to keys resets the iterator used by keys
and each
because it exhausts the iterator.
A call to keys
is really a call to FIRSTKEY
and calls to NEXTKEY
until there are no more items left in that haven't been returned.
A call to each
is a call to FIRSTKEY
(if FIRSTKEY
hasn't been called yet) or a call to NEXTKEY
(if FIRSTKEY
has been called).
#!/usr/bin/perl
use strict;
use warnings;
my $i = 0;
tie my %h, "HASH::Sorted", map { $_ => $i++ } "a" .. "g";
for my $key (keys %h) {
print "$key => $h{$key}\n";
}
print "\n";
my $first = each %h;
print "first $first => $h{$first}\n";
my ($second_key, $second_value) = each %h;
print "second $second_key => $second_value\n";
print "\nall of them again:\n";
for my $key (keys %h) {
print "$key => $h{$key}\n";
}
package HASH::Sorted;
sub TIEHASH {
my $class = shift;
return bless { _hash => { @_ } }, $class;
}
sub FETCH {
my ($self, $key) = @_;
return $self->{_hash}{$key};
}
sub STORE {
my ($self, $key, $value) = @_;
return $self->{_hash}{$key} = $value;
}
sub DELETE {
my ($self, $key) = @_;
return delete $self->{_hash}{$key};
}
sub CLEAR {
my $self = shift;
%{$self->{_hash}} = ();
}
sub EXISTS {
my ($self, $key) = @_;
return exists $self->{_hash}{$key};
}
sub FIRSTKEY {
my $self = shift;
#build iterator
$self->{_list} = [ sort keys %{$self->{_hash}} ];
return $self->NEXTKEY;
}
sub NEXTKEY {
my $self = shift;
return shift @{$self->{_list}};
}
sub SCALAR {
my $self = shift;
return scalar %{$self->{_hash}};
}
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