Question

According to the ruby-doc: “Hashes enumerate their values in the order that the corresponding keys were inserted.”

Does this mean that hashes are indeed ordered like arrays, such that I can count on the indices of my_hash.keys and my_hash.values and my_hash.to_a always lining up?

For example is something like this a good or a bad idea?

my_hash = {"a"=>"alpha", "b"=>"beta", "c"=>"gamma"}

some_letters.each { |letter|
  if my_hash.has_key? letter then
    imp_index = my_hash.keys.find_index
    table.row[r].col[imp_index].value = my_hash.values[imp_index]
  end 
}

so… can I treat the implicit index of an item in the my_hash.keys as an explicit attribute?

edit: also would there be any time saved by caching the .keys and .values arrays before the loop?

Answer 1

Hashes enumerate their values in the order that the corresponding keys were inserted

Only in Ruby 1.9. In previous version hashes are unordered.

Does this mean that I can count on the indices of my_hash.keys and my_hash.values and my_hash.to_a always lining up?

Yes, as long as iterating twice on the same unmodified hash iterates in the same order. (Even if the hash is unordered.)