You know how I normally rave about Ruby’s clear and natural syntax, how I prefer it over languages like C#, and generally say that Ruby is better than a baked loaf of dough cut into thin vertical strips.

But today… not so much.

Hash#select Returns… An Array?

Yes, Hash#select returns an array. I’m not sure why. I’m sure it seemed like a good idea at the time. But the other day I needed to get a “subset” of a Hash, and since Hash includes Enumerable, I thought I could do a select and get back the key-value pairs that I wanted. But, it returned a bunch of nested arrays instead.

vehicles = { :cars => 36, :boats => 8, :trains => 12, :planes => 21 }

vehicles.select { |k,v| v > 20 }
=> [[ :planes, 21], [ :cars, 36]]

Fortunately, it’s easy to turn this back into a hash. Hash overrides the class-level [ ] operator in an interesting way. If you pass it a flat list of values, it will construct a hash by assuming that the list is an alternating list of keys and values:

Hash[:apple, 'red', :banana, 'yellow']
=> { :apple=>"red", :banana=>"yellow"}

So back to the original problem: can we somehow turn [[:planes, 21], [:cars, 36]] into a flat list?

First, we “flatten” the array, which gets rid of all nested levels of the array:

a = [[:planes, 21], [:cars, 36]].flatten
=> [ :planes, 21, :cars, 36]

Close, but we still have an array, and we need a simple list of values instead.

Splat to the Rescue

We now use the Splat operator and use the Hash#[ ] class-level method to create our new hash:

h = Hash[*a]
=> { :planes=>21, :cars=>36}

Ta da!

Make It Part of Ruby

If you do this a lot, you could wrap this up in a method… but you can also choose to modify the Hash class directly:

class Hash
  alias :old_select :select

  def select(&block)
    a = old_select(&block)
    Hash[*a.flatten]
  end
end

Behold, our original code now does something beautiful:

vehicles.select { |k,v| v > 20 }
=> { :planes=>21, :cars=>36}

Wow, 7 lines of code and I’ve just made Ruby bend to my will. In fact, it turns out the next version of Ruby will support this syntax directly.

You know, there are some days when I just have to say, I think Ruby is better than sliced….