Question

Total Ruby noob. I’m going through Zed Shaw’s LRTHW, and I’m stuck on the Hash exercise. I can’t make any sense of this code, and can’t find anything on the web that looks like it.

cities = {'CA' => 'San Francisco', 'MI' => 'Detroit', 'FL' => 'Jacksonville'}

cities['NY'] = 'New York'
cities['OR'] = 'Portland'

def find_city (map, state)
  if map.include? state
    return map[state]
  else
    return "Not found."
  end
end

cities[:find] = method(:find_city)

while true
  print "State? (Enter to quit) "
  state = gets.chomp

  break if state.empty?

  puts cities[:find].call(cities, state)
end

Basically, I’m just stuck. I can’t make the connection between how I type in a state and it returns the city. Any dumbed down explanation would be greatly appreciated.

Answer 1

Here there are a hash (cities) with 3 pairs key => value

cities = {'CA' => 'San Francisco', 'MI' => 'Detroit', 'FL' => 'Jacksonville'}

You can access that values by the key:

puts cities['CA']  #=>  'San Francisco'

Now we add two new pairs.

cities['NY'] = 'New York'
cities['OR'] = 'Portland'

The whole hash will be:

p cities #=> {'CA' => 'San Francisco', 'MI' => 'Detroit', 'FL' => 'Jacksonville', 'NY' => 'New York', 'OR' => 'Portland'}

Now it’s defined a method called find_city. It takes a hash and a key.

def find_city (map, state)
  # if hash has the key, return its value. (There are better ways to do it.)
  if map.include? state
    return map[state]
  else
    return "Not found."
  end
end

Here there is the worst piece of Ruby code I’ve ever seen.

cities[:find] = method(:find_city)

Ok, that code gets the method find_city and turns it into a Method object, which could be assigned to a variable. BUT, instead of using a normal local variable, it is stored in a value of the cities hash!

That object is similar to a method, but it’s an object instead of a method, and it must be called with call (in Ruby 1.8.7).

I will use a variable called my_meth (it could be any name) to explain better.

# get the method find_city and turns it into an object assigned to my_meth
my_meth = method(:find_city)

while true
  print "State? (Enter to quit) "
  state = gets.chomp

  break if state.empty?

  # here we use find_city on the cities hash.
  puts my_meth.call(cities, state)
end

puts my_meth.class

But, instead to use a variable, the original code stored the Method object on the cities hash. So, cities will be:

cities[:find] = method(:find_city)
p cities #=> {'CA' => 'San Francisco', 'MI' => 'Detroit', 'FL' => 'Jacksonville', 'NY' => 'New York', 'OR' => 'Portland', :find => (the Method object)}

So, you can access the find_city thru cities[:find].call.