Retrieving data from multidimensional hashes

Back2Coding has asked for the wisdom of the Perl Monks concerning the following question:

In my new consulting position, I've had to take up coding again as part of my Testing Analyst role. I'm also pretty new to Perl. I'm finding I love the language, but I'm occasionally confused by the multiple ways there are to do most things.

What I am looking for is the easiest/best way to know how many keys there are in the nth dimension of a multidimensional hash. (ie $hash{x}{y}).

For example, I'm using a 2D hash where I have the key to the first dimension, but I want a count of the number of "records" that exist for the 2nd dimension. And then return the value of a specific "record".

Comment on Retrieving data from multidimensional hashes

Replies are listed 'Best First'.
Re: Retrieving data from multidimensional hashes by friedo (Prior) on Aug 11, 2008 at 18:54 UTC
A couple of options, depending on what appeals to you: `my $num = keys %{ $hash{key} }; # or my $ref = $hash{key}; my $num = keys %$ref;` [download] The first one is short-hand for accessing the second-level hash directly, though I find it a tad ugly, especially if the structures get deeper than two levels. The second one grabs the reference for the second level hash and then calls `keys` on that, which is a bit more explicit.	[reply] [d/l] [select]
Re: Retrieving data from multidimensional hashes by ikegami (Patriarch) on Aug 11, 2008 at 19:13 UTC
If you can't handle multiple levels, do it level by level. `my $subhash = $hash{$x}; my $count = keys %$subhash;` [download]	[reply] [d/l]
Re^2: Retrieving data from multidimensional hashes by Back2Coding (Initiate) on Aug 11, 2008 at 19:20 UTC
ok, maybe it has been too long since I last wrote code ;) Whats the "my" all about in front of the variables on those lines? Is it just to say "this is a variable name"?	[reply]
Re^3: Retrieving data from multidimensional hashes by ikegami (Patriarch) on Aug 11, 2008 at 19:40 UTC
It's a scoping mechanism and a typo avoidance mechanism. Make sure to use `use strict;` in your programs and modules.	[reply] [d/l]
Re: Retrieving data from multidimensional hashes by eosbuddy (Scribe) on Aug 11, 2008 at 19:06 UTC
Please checkout this node: http://www.perlmonks.org/?node_id=626183 Also: `use strict; use warnings; use Data::Dumper; print Dumper(%hash);` [download] will print out the whole tree view of the hash.	[reply] [d/l]
Re^2: Retrieving data from multidimensional hashes by FunkyMonk (Bishop) on Aug 11, 2008 at 23:02 UTC
Please use PerlMonk's markup for links to PM nodes. If you'd used `[id://626183]`, it would have rendered as Traverse an unknown multi-dimensional hash See What shortcuts can I use for linking to other information? for other PM markup.	[reply] [d/l]