How to get only a subset of an ordered hash in Ruby 1.9?

Let's take this example:

d = {"a" => 1, "b开发者_StackOverflow社区" => 2, "c" => 3, "d" => 4}

Since hashes are now ordered, I might want to get data from a to b or from c to d. The problem is that I can't do d[0..1] or d[2..3].

I could however do:

irb > d.to_a[0..1]
=> [["a", 1], ["b", 2]] 

... but this feels messy and I don't want to cast my hash for an operation like this.

Is there a cleaner solution to handle this?

# Holy Grail
irb > d[0..1]
=> {"a" => 1, "b" => 2}

I can see how to program myself such a method, but there might be something native already done that I could use...?

---

Well you could do :

> a = {"a" => 1, "b" => 2, "c" => 3, "d" => 4}
> a.slice(*a.keys[0..1])
=> {"a" => 1, "b" => 1}

At least the hash is not cast, but it's still not very elegant in my opinion.

---

If you want to make comparisons on the key to select the subset, you can use Hash#select, it also works in 1.8.7 but returns an array of arrays instead (as per your example).

d.select {|k, v| k < 'c' } # => {"a"=>1, "b"=>2}
d.select {|k, v| k < 'c' } # 1.8.7 => [["a", 1], ["b", 2]]

You can also use ActiveSupport’s Hash extension that adds a slice method to Hash that can also work, provided you already know the keys you want.

require 'active_support/core_ext/hash/slice'
d.slice('a', 'b') # => {"a"=>1, "b"=>2}

---

Probably there's a better way to do it but this is an idea:

class Hash
    def slice_by_index(a, b = nil)  
        k = if a.is_a?(Range)
            keys[a]
        elsif b.nil?
            [keys[a]]
        else
            keys[a,b]
        end
        k.inject({}){|h, k| h[k] = self[k] ; h }
    end
end

h = {"a" => 1, "b" => 2, "c" => 3, "d" => 4}
p h.slice_by_index(1..3) #range
p h.slice_by_index(2) #single element
p h.slice_by_index(0,3) #start, lenght