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.NET Forum / .NET Framework / New Users / October 2007Collections od KEYS only : BOOLEAN VS OBJECT, False Vs Null/Nothing
In a previous question I asked if there were collections similar to SortedList or Dictionary which would hold KEYS only, No values. It seems that the framework does not have such "reduced" collections, but one can use the standard one, adding as value a Null/Nothing value or a boolean (of creating a collection by inheritance). I am wondering what is most advisable to use as type for the values: Boolean or Object ? For instance: C# SortedList<MyKey, bool> MyListB = new SortedList<MyKey, bool>(); MyListB.add(new MyKey(), false); or SortedList<MyKey, object> MyListO = new SortedList<MyKey, object>(); MyListO.add(new MyKey(), null); VB dim MyListB as new SortedList(of MyKey, Boolean) MyListB.add(New MyKey, false) or dim MyListO as new SortedList(of MyKey, Object) MyListO.add(New MyKey, Nothing) I am wondering if using a value type (boolean) instead of a reference type would be more inefficient ? Your opinion ? -P > In a previous question I asked if there were collections > similar to SortedList or Dictionary which would hold KEYS only, No [quoted text clipped - 29 lines] > > -P It doesn't really matter, as a reference itself is a value type. The garbage collector, however, scans all references that exist when it's doing a garbage collection. As the references will all be null, it won't take long to ignore them, but if you don't use references the garbage collector doesn't have to be bothered with them at all.  SignatureGöran Andersson _____ http://www.guffa.com On 1 Ott, 12:53, G?ran Andersson <gu...@guffa.com> wrote: > > In a previous question I asked if there were collections > > similar to SortedList or Dictionary which would hold KEYS only, No [quoted text clipped - 42 lines] > > - Mostra testo tra virgolette - Ok, so as to the GC, you are saying, if I use boolean I will save it some little work. Right ? Actually these list can be pretty long, so it's not a bad idea to cut references. But what about memory use ? In any case both collection vould have a list of Values. Right ? Now, is it possible that all Nothing would waste less memory than a tons of FALSE values ? Or do the occupy the same ? And what about the Add(Key, Value) or other operations, would it make any difference to deal with a Value instead of a Reference ? -P >>> In a previous question I asked if there were collections >>> similar to SortedList or Dictionary which would hold KEYS only, No [quoted text clipped - 38 lines] > Ok, so as to the GC, you are saying, if I use boolean I will save it > some little work. Right ? Right. > Actually these list can be pretty long, so it's not a bad idea to cut > references. [quoted text clipped - 3 lines] > of Values. Right ? Now, is it possible that all Nothing would waste > less memory than a tons of FALSE values ? On a 32 bit system it will be the same. A reference is four bytes, and a boolean uses one byte but will require four bytes in the KeyValuePair structure. > Or do the occupy the same ? And what about the Add(Key, Value) or > other operations, would it make any difference > to deal with a Value instead of a Reference ? No difference at all. A reference is treated just as an integer, until the point where you actually dereference it to reach the object that it's pointing to. SignatureGöran Andersson _____ http://www.guffa.com > On a 32 bit system it will be the same. A reference is four bytes, and a > boolean uses one byte but will require four bytes in the KeyValuePair [quoted text clipped - 7 lines] > the point where you actually dereference it to reach the object that > it's pointing to. Thank you Goran. Now It's much more clear. I remain with a last doubt. Please, do not laugh at me :-)) I had "imagined" a key value pair like something like that: Key , Reference ---------> Object Now I was thinking that: Key, EmptyReference (4 bytes, for the empty reference) Key, Reference -----> BooleanValue (4bytes + 1 byte ) where the second would involve 1 Byte more (the boolean value). So it would be convenient to use an empty reference ! Why is this naive representation of mine wrong and instead the 2 above involve exactly the same memory occupation ? Is perhaps because in the second case there is no reference to a boolean and directly Key, BooleanValue ? (1 byte, for the value) But if this is so we would have a gain using boolean. I am confused !! :-) So what is actually the correct representation ? -P > -- > G?ran Andersson > _____http://www.guffa.com- Nascondi testo tra virgolette - > > - Mostra testo tra virgolette - > Thank you Goran. Now It's much more clear. > [quoted text clipped - 26 lines] > > -P The KeyValuePair structure will contain the value itself, not a reference to the value. When the value is a reference type, the reference is stored, but when the value is a value type, the value itself is stored. A boolean variable uses one byte, but in a structure the data will be padded to an even multiple of four bytes, so the boolean still requires four bytes in the structure. SignatureGöran Andersson _____ http://www.guffa.com On 2 Ott, 10:32, G?ran Andersson <gu...@guffa.com> wrote: > > Thank you Goran. Now It's much more clear. > [quoted text clipped - 41 lines] > > - Mostra testo tra virgolette - Ah, I understand now. Very clear and useful. Thanks G?ran -P By the way, just wanted to mention: If you want a collection of "keys" only, and not key/value pairs, why not just use the appropiate list class, such as: List<string> mykeys. Thanks, MB > In a previous question I asked if there were collections > similar to SortedList or Dictionary which would hold KEYS only, No [quoted text clipped - 29 lines] > > -P > By the way, just wanted to mention: If you want a collection of "keys" only, > and not key/value pairs, why not just use the appropiate list class, such > as: > > List<string> mykeys. Because checking whether a list contains an element or not is an O(n) operation instead of an O(1) operation. 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 > > By the way, just wanted to mention: If you want a collection of "keys" only, > > and not key/value pairs, why not just use the appropiate list class, such [quoted text clipped - 8 lines] > Jon Skeet - <sk...@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 Yes and also because we are talking here about sorted list and dictionaries. Ah, I see. Although one could do list.Sort() (once), followed by list.BinarySearch() to gain seom of the same advantages. >> > By the way, just wanted to mention: If you want a collection of "keys" >> > only, [quoted text clipped - 14 lines] > Yes and also because we are talking here about sorted list and > dictionaries. > Ah, I see. > > Although one could do list.Sort() (once), followed by list.BinarySearch() to > gain seom of the same advantages. Actually, I do not think you can replace, in most cases a sorted list with a list + Ordering without a huge performace dropdown. I mean in the cases where a sorted list is actually needed. For instance, you cannot be sorting after any add... -P > Ah, I see. > > Although one could do list.Sort() (once), followed by list.BinarySearch() to > gain seom of the same advantages. True. That would give O(log(N)) performance instead of O(N) performance - but with a decently chosen hashcode, a hashtable type of structure will be much quicker for large sets. 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
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