A Hash Example

We'll now do an exercise that you must study very carefully. I want you to type this code in and try to understand what's going on. Take note of when you put things in a hash, get them from a hash, and all the operations you use. Notice how this example is mapping states to their abbreviations and then the abbreviations to cities in the states. Remember, "mapping" or "associating" is the key concept in a hash.

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# create a mapping of state to abbreviation
states = {
  'Oregon' => 'OR',
  'Florida' => 'FL',
  'California' => 'CA',
  'New York' => 'NY',
  'Michigan' => 'MI'
}

# create a basic set of states and some cities in them
cities = {
  'CA' => 'San Francisco',
  'MI' => 'Detroit',
  'FL' => 'Jacksonville'
}

# add some more cities
cities['NY'] = 'New York'
cities['OR'] = 'Portland'

# puts out some cities
puts '-' * 10
puts "NY State has: #{cities['NY']}"
puts "OR State has: #{cities['OR']}"

# puts some states
puts '-' * 10
puts "Michigan's abbreviation is: #{states['Michigan']}"
puts "Florida's abbreviation is: #{states['Florida']}"

# do it by using the state then cities dict
puts '-' * 10
puts "Michigan has: #{cities[states['Michigan']]}"
puts "Florida has: #{cities[states['Florida']]}"

# puts every state abbreviation
puts '-' * 10
states.each do |state, abbrev|
  puts "#{state} is abbreviated #{abbrev}"
end

# puts every city in state
puts '-' * 10
cities.each do |abbrev, city|
  puts "#{abbrev} has the city #{city}"
end

# now do both at the same time
puts '-' * 10
states.each do |state, abbrev|
  city = cities[abbrev]
  puts "#{state} is abbreviated #{abbrev} and has city #{city}"
end

puts '-' * 10
# by default ruby says "nil" when something isn't in there
state = states['Texas']

if !state
  puts "Sorry, no Texas."
end

# default values using ||= with the nil result
city = cities['TX']
city ||= 'Does Not Exist'
puts "The city for the state 'TX' is: #{city}"

What You Should See

$ ruby ex39.rb
----------
NY State has: New York
OR State has: Portland
----------
Michigan's abbreviation is: MI
Florida's abbreviation is: FL
----------
Michigan has: Detroit
Florida has: Jacksonville
----------
Oregon is abbreviated OR
Florida is abbreviated FL
California is abbreviated CA
New York is abbreviated NY
Michigan is abbreviated MI
----------
CA has the city San Francisco
MI has the city Detroit
FL has the city Jacksonville
NY has the city New York
OR has the city Portland
----------
Oregon is abbreviated OR and has city Portland
Florida is abbreviated FL and has city Jacksonville
California is abbreviated CA and has city San Francisco
New York is abbreviated NY and has city New York
Michigan is abbreviated MI and has city Detroit
----------
Sorry, no Texas.
The city for the state 'TX' is: Does Not Exist

What Hashes Can Do

Hashes are another example of a data structure, and ,like arrays, they are one of the most commonly used data structures in programming. A hash is used to map or associate things you want to store to keys you need to get them. Again, programmers don't use a term like "hash" for something that doesn't work like an actual hash full of words, so let's use that as our real world example.

Let's say you want to find out what the word "Honorificabilitudinitatibus" means. Today you would simply copy-paste that word into a search engine and then find out the answer, and we could say a search engine is like a really huge super complex version of the Oxford English Dictionary (OED). Before search engines what you would do is this:

1. Go to your library and get "the dictionary". Let's say it's the OED.
2. You know "honorificabilitudinitatibus" starts with the letter 'H' so you look on the side of the book for the little tab that has 'H' on it.
3. Then you'd skim the pages until you are close to where "hon" started.
4. Then you'd skim a few more pages until you found "honorificabilitudinitatibus" or hit the beginning of the "hp" words and realize this word isn't in the OED.
5. Once you found the entry, you'd read the definition to figure out what it means.

This process is nearly exactly the way a hash works, and you are basically "mapping" the word "honorificabilitudinitatibus" to its definition. A hash in Ruby is just like a dictionary in the real world such as the OED.

Study Drills

1. Do this same kind of mapping with cities and states/regions in your country or some other country.
2. Find the Ruby documentation for hashes and try to do even more things to them.
3. Find out what you can't do with hashes. A big one is that they do not have order, so try playing with that.

Common Student Questions

What is the difference between an array and a hash?

A array is for an ordered array of items. A hash (or hash) is for matching some items (called "keys") to other items (called "values").

What would I use a hash for?

When you have to take one value and "look up" another value. In fact, you could call hashes "look up tables."

What would I use an array for?

Use an array for any sequence of things that need to be in order, and you only need to look them up by a numeric index.