I am teaching right now a friend, who wants a new job, and just yesterday thought about how to explain array of arrays or hash of hashes etc. Here is how I tried:

For a little bit forget about array (@) and hash (%) notation. Stick only to variable ($) or in other words, reference symbol and see how easy (I hope) it is.

Always when you want hash, use curly brackets {}, always if array - square brackets []. And on one nesting level treat everything as list of scalars, nothing more, nothing less.

$arrayref = [1, 3, 5, 7]; $hashref = {key1=>val1, key2=>val2, key3=>val3};

When you need some value from hash or array, do following:

# want array element - use square brackets $value = $$arrayref[2]; # want hash element - use curly brackets $othervalue = $$hashref{key2};

Lets build more complicated structure. The first rule: don't use any reference more than once while building another structure. Second rule: treat reference as normal scalar value.

# array of arrays # array - square brackets. Created three times, remember, every refere +nce can be used only once to build another structure. $arrayref1 = [1, 3, 5, 7]; $arrayref2 = [1, 3, 5, 7]; $arrayref3 = [1, 3, 5, 7]; # array of arrays - again: array = square brackets, and it is still on +ly list of scalars, nothing more $array_of_arrays = [$arrayref1, $arrayref2, $arrayref3]; # see how it looks (notice, that Dumper operates only on references. W +hat kind of brackets do you see in the dump?): use Data::Dumper; print Dumper $array_of_arrays; # now: get one element. We have three elements in hmm, lets say, 'oute +r' level, so the last one will be(array-square brackets): $$array_of_arrays[2]..... # and there are 4 values in 'inner' level, so lets take last one: $value = $$array_of_arrays[2][3];

Easy, isn't it? Now with hashes, similarly, only other brackets:

# hash of hashes # hash - curly brackets $hashref1 = {key1 => value1, key2 => value2, key3 => value3, key4 => v +alue4}; $hashref2 = {key1 => value1, key2 => value2, key3 => value3, key4 => v +alue4}; $hashref3 = {key1 => value1, key2 => value2, key3 => value3, key4 => v +alue4}; # hash of hashes, which brackets? :) $hash_of_hashes = {key11 => $hashref1, key22 => $hashref2, key33 => $h +ashref3}; # see how it looks: use Data::Dumper; print Dumper $hash_of_hashes; # get an element. Nothing surprising, starting from 'outer' level: $$hash_of_hashes{key22}......... # and end with 'inner' key $$hash_of_hashes{key22}{key4};

That's it. Easy? I think so. Lets mix the structures. The rules stay the same: use every reference only once, treat reference as a scalar, curly brackets = hash, square brackets = array.

$monk1 = 'Jack'; $monk2 = 'John'; $monk3 = 'Mark'; $monk4 = 'Rudolf'; #order of monks important - using array - square brackets $bank_left1 = [$monk1, '', $monk2]; $bank_left2 = []; $bank_left3 = []; $bank_left4 = []; # order of banks important - use array $left_nave = [$bank_left1, $bank_left2, $bank_left3, $bank_left4]; $bank_right1 = []; $bank_right2 = [$monk3]; $bank_right3 = []; $bank_right4 = []; $right_nave = [$bank_right1, $bank_right2, $bank_right3, $bank_right4] +; # existence of elements is important, order not - lets do hash $church = {'left nave'=>$left_nave, 'right nave'=>$right_nave, 'altar' +=>1, 'tabernacle'=>1}; # lets declare hash of arrays at once, order of plants not important, +but array is here more convenient. # See, instead of (key, value) pairs I use (key, reference-to-array). $garden = {'plants'=>['potato', 'carrot', 'apple tree'], 'monks'=>[$mo +nk4], 'rain'=>1}; # and finally whole monastery, not rich, but its our home :) $monastery = {'church'=>$church, 'garden'=>$garden}; # see how it looks and where monks reside: use Data::Dumper; print Dumper $monastery;

Now is time to go back to @ and %. Rules are simple: if you have reference $arrayref to an array and need to use it as array, just write @$arrayref. Similarly use $hashref as %hashref if some function needs hash. Two simple examples:

# using @ in join function $arrayref = $$monastery{'garden'}{'plants'}; print "Plants: ", join ", ", @{$arrayref}; # or @$arrayref # or just # print "Plants: ", join ", ", @{$$monastery{'garden'}{'plants'}}; # using % in keys function $hashref_churchelems = $$monastery{'church'}; print "\nChurch elements: ", join ", ", keys %{$hashref_churchelems}; # or just # print "\nChurch elements: ", join ", ", keys %{$$monastery{'church'} +}; print"\n"; # monk sitting in church -> in left nave -> in first bank -> as first print "Monk: ", $$monastery{'church'}{'left nave'}[0][0], "\n";

That's how structures look from ref point of view.

Replies are listed 'Best First'.
Re: Arrays and hashes only with references
by wade (Pilgrim) on Mar 14, 2008 at 17:16 UTC
    I code similarly to this but instead of:
    $value = $$arrayref[2];
    I prefer:
    $value = $arrayref->[2];
    The beauty of that notation is that it doesn't treat the outer level of a nest any differently.
Re: Arrays and hashes only with references
by Pancho (Pilgrim) on Mar 14, 2008 at 20:52 UTC

    Big ++ on this tutorial, since it is (a) brief and to the point, (b) step by step, (c) includes code to download and tinker with. Just what a noob (i.e. me) needs. Beggars can't be choosers but adding the alternate syntax (which Wade illustrated) would make it even better.

    Grizzley Keep 'em coming. Thanks

Re: Arrays and hashes only with references
by hsinclai (Deacon) on Sep 19, 2008 at 02:19 UTC
    ++ on your posting grizzley - this has gotten me over a huge hump !