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.NET Forum / Languages / C# / September 2007Iterator vs enumerator
I'm reading about Iterators and the article I'm reading states that an enumerator must load all of the objects into memory, which is obviously a big waste if there are a large number of objects and you only need a few of them. On the other hand the iterator does not need to load more than 1 object hence uses less memory. But can't you do the same thing with an enumerator just by returning the next object when Current is called? If that's the case what's the advantage of an iterator? Thanks, Michael > I'm reading about Iterators and the article I'm reading states that an > enumerator must load all of the objects into memory, which is obviously a [quoted text clipped - 4 lines] > > If that's the case what's the advantage of an iterator? What is it describing as the difference between "enumerator" and "iterator"? I regard them as equivalent terms - I prefer "iterator" because it avoids confusion with an enumeration.  SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet If replying to the group, please do not mail me too > I'm reading about Iterators and the article I'm reading states that an > enumerator must load all of the objects into memory, which is obviously a [quoted text clipped - 7 lines] > Thanks, > Michael An Enumerator is an iterator. Here, the C# example of an iterator uses an Enumerator: http://en.wikipedia.org/wiki/Iterator When you are talking about differences between an Iterator and an Enumerator, you are talking about differences in specific implementations of iterators. For this discussion to go anywhere, you have to specify what exact implementations you are talking about. SignatureGöran Andersson _____ http://www.guffa.com > An Enumerator is an iterator. > [quoted text clipped - 3 lines] > When you are talking about differences between an Iterator and an > Enumerator, you are talking about differences in specific I think I get it, what they call an iterator is just a shortcut to avoid creating your IEnumerator class. Basically the article was wrong in saying it will save a lot of memory. > implementations of iterators. For this discussion to go anywhere, you have > to specify what exact implementations you are talking about. The article didn't specify an particular implementation, it was just a fake class with no code given, something like this: foreach(Entry e in PhoneBook.GetNumbersFor("New York")) { } The GetNumbersFor method was not provided. Michael >> An Enumerator is an iterator. >> [quoted text clipped - 6 lines] > I think I get it, what they call an iterator is just a shortcut to avoid > creating your IEnumerator class. Actually an enumerator is generally a struct, not a class. Typically it uses 16 bytes of stack space, which definitely isn't much to whine about. > Basically the article was wrong in saying > it will save a lot of memory. It might be correct for a specific implementation of an enumerator, but not for enumerators in general. >> implementations of iterators. For this discussion to go anywhere, you have >> to specify what exact implementations you are talking about. [quoted text clipped - 7 lines] > > The GetNumbersFor method was not provided. If the GetNumbersFor method would create a collection, the foreach statement will create an enumerator for that collection. The enumerator for a collection of course requires that the collection is present in memory, but that requirement is only for that specific implementation of an enumerator, not all enumerators. SignatureGöran Andersson _____ http://www.guffa.com >> Basically the article was wrong in saying it will save a lot of memory. > > It might be correct for a specific implementation of an enumerator, but > not for enumerators in general. I see what you mean. In the article there is nothing to indicate that the specific implementation loads a large number of objects and nothing at the end of the article to indicate that they solved it either. But I understand what they were trying to say. Here's the article: http://msdn2.microsoft.com/en-au/vcsharp/bb264519.aspx > If the GetNumbersFor method would create a collection, the foreach > statement will create an enumerator for that collection. The enumerator > for a collection of course requires that the collection is present in > memory, but that requirement is only for that specific implementation of > an enumerator, not all enumerators. That's true but the code at the end of the article appears to do the same thing. Michael > > It might be correct for a specific implementation of an enumerator, but > > not for enumerators in general. [quoted text clipped - 5 lines] > > http://msdn2.microsoft.com/en-au/vcsharp/bb264519.aspx The core of the article isn't comparing enumerators vs iterators - it's comparing a collection where all the elements are in memory at a time (such as List and Dictionary) with a more "streaming" approach where you only load and process one item at a time. > > If the GetNumbersFor method would create a collection, the foreach > > statement will create an enumerator for that collection. The enumerator [quoted text clipped - 4 lines] > That's true but the code at the end of the article appears to do the same > thing. No. The code at the end of the article never creates a collection. What exactly do you mean? SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet If replying to the group, please do not mail me too > The core of the article isn't comparing enumerators vs iterators - it's > comparing a collection where all the elements are in memory at a time > (such as List and Dictionary) with a more "streaming" approach where > you only load and process one item at a time. The link to the article is titled "Custom Iterators" and claims to talk about Custom Iterators and the yield statement. >> That's true but the code at the end of the article appears to do the same >> thing. > > No. The code at the end of the article never creates a collection. What > exactly do you mean? Both the code at the start and end of the article has the line IEnumerable<PhoneBookEntry> newYorkNumbers = PhoneBook.FindListFor("New York"); Isn't this creating a collection in both cases? Michael > > The core of the article isn't comparing enumerators vs iterators - it's > > comparing a collection where all the elements are in memory at a time [quoted text clipped - 3 lines] > The link to the article is titled "Custom Iterators" and claims to talk > about Custom Iterators and the yield statement. Absolutely. That's not the same as "Comparing iterators and enumerators" though - it's *implementing* iterators, and comparing iterators with collections which have been completely loaded before you start iterating. > >> That's true but the code at the end of the article appears to do the same > >> thing. [quoted text clipped - 8 lines] > > Isn't this creating a collection in both cases? No. It's fetching an IEnumerable<PhoneBookEntry>. That doesn't necessarily fetch all the data in one go. It could be using a DataReader, for example. In particular, the article states: <quote> For this discussion, let's assume you've changed PhoneBook.FindListFor() to be an enumerator method as well. </quote> Now, that doesn't prohibit it from being loaded in memory to start with - but it could be reading a file line by line, and only returning the next line when the caller asks for it. Jon Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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