Question

I’m doing something like this with a list ‘a’:

a.each_with_index |outer, i|
  a.each_with_index |inner, j|
    if(j > i)
      # do some operation with outer and inner
    end
  end
end

if the iterator is not going to use the same order, this won’t work. I don’t care what the order actually is, I just need for two .each_with_index iterators to use the same order.

I would assume that it would be a property of an array that it has a fixed order and I’m just being paranoid that the iterator wouldn’t use that order…

Answer 1

This depends on the specific Enumerable object you are operating on.

Arrays for example will always return elements in the same order. But other enumerable objects are not guaranteed to behave this way. A good example of this is the 1.8,7 base Hash. That is why many frameworks (most notably ActiveSupport) implement an OrderedHash.

One interesting side note: Even Hash will return objects in the same order if the hash has not changed between each calls. While many objects behave this way, relying on this subtlety is probably not a great idea.

So, no. The generic each will not always return objects in the same order.

P.S. Ruby 1.9’s hashes are now actually ordered http://www.igvita.com/2009/02/04/ruby-19-internals-ordered-hash