From: Stefan Lang <perfectly.normal.hacker@...>
Date: 2008-10-31T05:08:42+09:00
Subject: Re: Loops and Iterators Difference

2008/10/30 Soh Dubom <sohdubom@yahoo.com>:
> Hi. The way I understand, both loops and iterators are, 'loops', but the
> sutil difference is that a loop is used to just loop several times and
> an iterators loops several times and also access some attribute of the
> element being loop.
>
> I'm actually trying to find a better and clear explanation of the
> difference between loop and iterator.

An iterator is an object that allows us to traverse the
elements of a data structure. In Ruby we usually use
"internal" iterators, that means we don't need to explicitly
manage an iterator object:

    my_tree.each_node { |node| node.do_something }

Using an external iterator (in combination with a loop!) could look
like this:

    iterator = my_tree.each_node_iterator
    while iterator.has_next?
      node = iterator.next
      node.do_something
    end

In both cases, the iterator allows us to traverse the data
structure without knowledge about its internals. Which frees
us to change the implementation later without changing
client code.

A loop simply means repeating a sequence of statements,
endlessly in the simplest case:

    loop { do_something }

We could use a loop to traverse a data structure,
say a linked list:

    item = people
    while item
      item.object.do_something
      item = item.next
    end

But this code breaks when we decide to use e.g. an array
instead of a list to store people.

Stefan