Question

In Ruby 1.9 a Hash is sorted on the basis of order of insertion.
Why the Ruby koans’s assertion on test_hash_is_unordered method returns true?

To me, the method’s title is quite misleading… maybe it refers to the fact that Ruby will recognize 2 equal hashes that were created with different keys order insetions.

But, theorically, this kind of assertion:

hash1 = { :one => "uno", :two => "dos" }
hash2 = { :two => "dos", :one => "uno" }

assert_equal ___, hash1 == hash2

Should return false. Or not?

Answer 1

From the fine manual:

hsh == other_hash → true or false

Equality—Two hashes are equal if they each contain the same number of keys and if each key-value pair is equal to (according to Object#==) the corresponding elements in the other hash.

So two Hashes are considered equal if they have the same key/value pairs regardless of order.

The examples in the documentation even contain this:

h2 = { 7 => 35, "c" => 2, "a" => 1 }
h3 = { "a" => 1, "c" => 2, 7 => 35 }
h2 == h3   #=> true

Yes, the test_hash_is_unordered title is somewhat misleading as order isn’t specifically being testing, only order with respect to equality is being demonstrated.