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Exercise 40: Dictionaries, Oh Lovely Dictionaries

Now I have to hurt you with another container you can use, because once you learn this container a massive world of ultra-cool will be yours. It is the most useful container ever: the hash.

Ruby calls them "hashes", other languages call them, "dictionaries". I tend to use both names, but it doesn't matter. What does matter is what they do when compared to arrays. You see, an array lets you do this:

ruby-1.9.2-p180 :015 > things = ['a','b','c','d']
=> ["a", "b", "c", "d"]
ruby-1.9.2-p180 :016 > print things[1]
b=> nil
ruby-1.9.2-p180 :017 > things[1] = 'z'
=> "z"
ruby-1.9.2-p180 :018 > print things[1]
z=> nil
ruby-1.9.2-p180 :019 > print things
["a", "z", "c", "d"] => nil
ruby-1.9.2-p180 :020 >

You can use numbers to "index" into an array, meaning you can use numbers to find out what's in arrays. You should know this by now, but what a hash does is let you use anything, not just numbers. Yes, a hash associates one thing to another, no matter what it is. Take a look:

ruby-1.9.2-p180 :001 > stuff = {:name => "Rob", :age => 30,
:height => 5*12+10} => {:name => "Rob", :age => 30,
:height => 70}
ruby-1.9.2-p180 :002 > puts stuff[:name]
Rob => nil
ruby-1.9.2-p180 :003 > puts stuff[:age]
30 => nil
ruby-1.9.2-p180 :004 > puts stuff[:height]
70 => nil
ruby-1.9.2-p180 :005 > stuff[:city] = "New York"
=> "New York"
ruby-1.9.2-p180 :006 > puts stuff[:city]
New York => nil
ruby-1.9.2-p180 :007 >

You will see that instead of just numbers we're using symbols, which are just lightweight strings (think name tags) in Ruby, to say what we want from the stuff hash. We can also put new things into the hash with symbols. It doesn't have to be symbols though, we can also do this:

ruby-1.9.2-p180 :004 > stuff[1] = "Wow"
=> "Wow"
ruby-1.9.2-p180 :005 > stuff[2] = "Neato"
=> "Neato"
ruby-1.9.2-p180 :006 > puts stuff[1]
Wow => nil
ruby-1.9.2-p180 :007 > puts stuff[2]
Neato => nil
ruby-1.9.2-p180 :008 > puts stuff
{:name=>"Rob", :age=>30, :height=>70, :city=>"New York",
1=>"Wow", 2=>"Neato"} => nil
ruby-1.9.2-p180 :009 >

In this one I just used numbers. I could use anything. Well almost but just pretend you can use anything for now.

Of course, a hash that you can only put things in is pretty stupid, so here's how you delete things, with the delete function:

ruby-1.9.2-p180 :009 > stuff.delete(:city)
=> "New York"
ruby-1.9.2-p180 :010 > stuff.delete(1)
=> "Wow"
ruby-1.9.2-p180 :011 > stuff.delete(2)
=> "Neato"
ruby-1.9.2-p180 :012 > stuff
=> {:name=>"Rob", :age=>30, :height=>70}
ruby-1.9.2-p180 :013 >

We'll now do an exercise that you must study very carefully. I want you to type this exercise in and try to understand what's going on. It is a very interesting exercise that will hopefully make a big light turn on in your head very soon.

What You Should See

$ ruby ex40.rb
State? (ENTER to quit) > CA
San Francisco
State? (ENTER to quit) > FL
State? (ENTER to quit) > O
Not found.
State? (ENTER to quit) > OR
State? (ENTER to quit) > VT
Not found.
State? (ENTER to quit) >

Extra Credit

  1. Go find the Ruby documentation for hashes and try to do even more things to them.
  2. Find out what you can't do with hashes.
  3. Try doing a for-loop over them, and then try the each method of iterating through a hash.
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