Doubt on hash keys

velusamy has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: Doubt on hash keys by tilly (Archbishop) on Jan 16, 2009 at 04:55 UTC
Because the parens just create a flat list. What you wrote is the same as: `my %hash = ( 'hi' => 'hello', 'all' => 'bye', 'later' => 'gone', ); print $hash{'all'};` [download] If you want nested hash references you need to use {}. Like this: `my %hash = ( 'hi' => {'hello' => 'all'}, 'bye' => {'later' => 'gone'}, );` [download] For more on nested data structures read references quick reference.	[reply] [d/l] [select]
Re: Doubt on hash keys by ysth (Canon) on Jan 16, 2009 at 04:55 UTC
Because parentheses almost always only affect precedence - they don't create a list (or in this case, some kind of "sub list"), and => is just a kind of comma with some semi-magical auto-quoting properties. You are initializing the hash to ( 'hi', 'hello', 'all', 'bye', 'later', 'gone' ), where 'hello', 'bye', and 'gone' are the values for the keys (respectively) 'hi', 'all', and 'later'. It looks like you want to make the values for $hash{'hi'} and $hash{'bye'} be hashes. Since hash values can only be scalars, you need to explicitly make a hash reference: `my %hash = ( 'hi' => { 'hello' => 'all' }, 'bye' => { 'later' => 'gone' }, ); print $hash{'hi'}{'hello'}; # prints "all"` [download] -- Online Fortune Cookie Search Office Space merchandise	[reply] [d/l]
Re: Doubt on hash keys by kyle (Abbot) on Jan 16, 2009 at 04:58 UTC
What did you expect it to print? `use Data::Dumper; my %hash = ( 'hi' => ('hello' => 'all'), 'bye' => ('later' => 'gone') ); print Dumper \%hash; __END__ $VAR1 = { 'hi' => 'hello', 'later' => 'gone', 'all' => 'bye' };` [download] The "sublists" that you think you have get flattened into one list `qw( hi hello all bye later gone )`. Perhaps you want: `my %hash = ( 'hi' => {'hello' => 'all'}, 'bye' => {'later' => 'gone'} );` [download] See perlreftut and perlref.	[reply] [d/l] [select]
Re: Doubt on hash keys by jethro (Monsignor) on Jan 16, 2009 at 05:03 UTC
UPDATE: I took the "How" at face value instead of interpreting it as a 'Why'. Please disregard this answer. The simple answer would be: `print "bye";`. The more serious answer: How do you want to identify the "bye"?. Do you want to get the key for which the value is a pointer to a hash with the key 'later'? `my $result; foreach my $v (keys %hash) { if (exists $hash{$v}{'later'}) { $result= $v; }` [download] Or do you want the second value in the hash? That is not possible. There is no order in a hash, so there is no second value Or do you want to print all hash keys? But that would also print 'hi': `print join(' ',keys %hash);` [download] Or do you really just want to print the fixed string "bye" no matter what? See my first answer.	[reply] [d/l] [select]
Re: Doubt on hash keys by targetsmart (Curate) on Jan 16, 2009 at 10:47 UTC
What you wrote is same as my %hash = qw(hi hello all bye later gone); from man perlop The "=>" operator is a synonym for the comma, but forces any word (consisting entirely of word characters) to its left to be interpreted as a string (as of 5.001). This includes words that might otherwise be considered a constant or function call.	[reply]