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### Arrays and hashes only with references

by grizzley (Chaplain)
 on Mar 14, 2008 at 13:50 UTC Need Help??

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

Pancho
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 !

-Harold

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