以优雅的方式在枚举数结尾处停止？

Question

Is there a graceful way of realizing one is at the end of an enumerator?

For instance,

a = (1..10).to_a
e = a.each
e.next # should yield 1
while e.next
  # do something
end

Of course, e raises StopIteration when you get to the end of the enumerator.

Is there a nice way of breaking out of that while loop without a rescue? I know that I can just say e.each (or just not use the enumerator at all), but for my specific problem I want to do something special for the first few iterations and then something general for the last few.

I want a way of looking at the next value and getting nil instead of an error.

Any tips? There's probably something obvious that I'm missing...

NB. I don't have to use enumerators, but it seems like the easiest way to solve my problem. So if you have a non enumerator solution (for iterating through an enumerable), feel free to share. (on that note, maybe I should just use each or each_with_index and use the counter for the special cases...)

Answer 1

Rescuing from StopIteration is the way to do it. And the only way.*

When you realize that any value (eg nil , as you suggest) returned by next could be the next value of the enumerable, it becomes clear that it's not possible for it to return a special-case value, since that could then never be in the enumerable. This is why next must raise StopIteration when complete instead of doing something more "graceful".

_{* Assuming you must use Enumerator , as there's probably a better way to solve your real problem without doing so.}

Answer 2

What's wrong with using each_with_index like you suggest to have special behavior for the first N? This pattern shows up all the time:

a.each_with_index do |e, i|
  if (i <= n)
    # Do special stuff
  end
  # Do regular stuff
end

There's a lot of tools in the Enumerable toolbox you can make use of.