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A Clever Way to Use default_proc in a Hash

Let’s explore an interesting way to use the default_proc feature in a Ruby Hash. Suppose we have a RANKS hash to determine the badge a player receives in a game based on their points. If the points don’t match any of the defined ranges, the result should be :none. We’ll create a rank method to receive the points and iterate through the hash to find the appropriate badge.

RANKS = {
  1..10   => :rabbit,
  11..50  => :falcon,
  51..100 => :dog,
  101..   => :tiger
}
RANKS.default = :none

def rank(points)
  _, badge = RANKS.find { |range, _| range.include?(points) }
  badge || RANKS.default
end

Here’s what we get when we use the rank method:

puts rank(0)    # none
puts rank(20)   # falcon
puts rank(100)  # dog
puts rank(5000) # tiger

It would be convenient to retrieve the badge directly from the hash, for example, using RANKS[20] to get :falcon. However, since the keys are ranges, we can’t directly use a number as a key. This is where default_proc comes in handy.

The default_proc is called whenever we try to access an element in the hash using a non-existent key. Since the keys of RANKS are ranges, default_proc will be triggered when we attempt to use the number of points as the key, like RANKS[20].

RANKS = {
  1..10   => :rabbit,
  11..50  => :falcon,
  51..100 => :dog,
  101..   => :tiger
}

RANKS.default_proc = proc do |hash, key|
  _, badge = hash.find { |range, _| range.include?(key) }
  badge || :none
end

Now, we can retrieve the badge directly from the hash, using the points as a key, and let the hash handle the heavy lifting:

puts RANKS[0]    # none
puts RANKS[20]   # falcon
puts RANKS[100]  # dog
puts RANKS[5000] # tiger

Cheers!

Banner photo by Muriel Liu on Unsplash

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