Accessing a Dictionary Keys Key through a numeric index

Question

I m using a Dictionary lt string  int gt  where the int is a count of the key   Now  I need to access the last-inserted Key inside the Dictionary  but I do not know the name of it  The obvious attempt   int LastCount   mydict mydict keys mydict keys Count      does not work  because Dictionary Keys does not implement a   -indexer   I just wonder if there is any similar class  I thought about using a Stack  but that only stores a string  I could now create my own struct and then use a Stack lt MyStruct gt   but I wonder if there is another alternative  essentially a Dictionary that implements an   -indexer on the Keys

User · Answer

I agree with the second part of Patrick s answer  Even if in some tests it seems to keep insertion order  the documentation  and normal behavior for dictionaries and hashes  explicitly states the ordering is unspecified   You re just asking for trouble depending on the ordering of the keys  Add your own bookkeeping  as Patrick said  just a single variable for the last added key  to be sure  Also  don t be tempted by all the methods such as Last and Max on the dictionary as those are probably in relation to the key comparator  I m not sure about that

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Visual Studio s UserVoice gives a link to generic OrderedDictionary implementation by dotmore   But if you only need to get key value pairs by index and don t need to get values by keys  you may use one simple trick  Declare some generic class  I called it ListArray  as follows   class ListArray lt T gt    List lt T   gt        You may also declare it with constructors   class ListArray lt T gt    List lt T   gt        public ListArray     base           public ListArray int capacity    base capacity          For example  you read some key value pairs from a file and just want to store them in the order they were read so to get them later by index   ListArray lt string gt  settingsRead   new ListArray lt string gt     using  var sr   new StreamReader myFile         string line      while   line   sr ReadLine       null                string   keyValueStrings   line Split separator           for  int i   0  i  lt  keyValueStrings Length  i                keyValueStrings i    keyValueStrings i  Trim            settingsRead Add keyValueStrings              Later you get your key value strings simply by index string   myKeyValueStrings   settingsRead index     As you may have noticed  you can have not necessarily just pairs of key value in your ListArray  The item arrays may be of any length  like in jagged array

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A Dictionary is a Hash Table  so you have no idea the order of insertion   If you want to know the last inserted key I would suggest extending the Dictionary to include a LastKeyInserted value   E g    public MyDictionary lt K  T gt    IDictionary lt K  T gt        private IDictionary lt K  T gt   InnerDictionary       public K LastInsertedKey   get  set         public MyDictionary                  InnerDictionary   new Dictionary lt K  T gt                 region Implementation of IDictionary      public void Add KeyValuePair lt K  T gt  item                 InnerDictionary Add item           LastInsertedKey   item Key              public void Add K key  T value                 InnerDictionary Add key  value           LastInsertedKey   key                  rest of IDictionary methods       endregion      You will run into problems however when you use  Remove   so to overcome this you will have to keep an ordered list of the keys inserted

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In case you decide to use dangerous code that is subject to breakage  this extension function will fetch a key from a Dictionary lt K V gt  according to its internal indexing  which for Mono and  NET currently appears to be in the same order as you get by enumerating the Keys property    It is much preferable to use Linq   dict Keys ElementAt i   but that function will iterate O N   the following is O 1  but with a reflection performance penalty   using System  using System Collections Generic  using System Reflection   public static class Extensions       public static TKey KeyByIndex lt TKey TValue gt  this Dictionary lt TKey  TValue gt  dict  int idx                Type type   typeof Dictionary lt TKey  TValue gt            FieldInfo info   type GetField  entries   BindingFlags NonPublic   BindingFlags Instance           if  info    null                            NET             Object element     Array info GetValue dict   GetValue idx               return  TKey element GetType   GetField  key   BindingFlags Public   BindingFlags Instance  GetValue element                        Mono          info   type GetField  keySlots   BindingFlags NonPublic   BindingFlags Instance           return  TKey   Array info GetValue dict   GetValue idx

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Why don t you just extend the dictionary class to add in a last key inserted property  Something like the following maybe   public class ExtendedDictionary   Dictionary lt string  int gt        private int lastKeyInserted   -1       public int LastKeyInserted               get   return lastKeyInserted            set   lastKeyInserted   value               public void AddNew string s  int i                lastKeyInserted   i           base Add s  i

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I don t know if this would work because I m pretty sure that the keys aren t stored in the order they are added  but you could cast the KeysCollection to a List and then get the last key in the list    but it would be worth having a look   The only other thing I can think of is to store the keys in a lookup list and add the keys to the list before you add them to the dictionary    it s not pretty tho

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A dictionary may not be very intuitive for using index for reference but  you can have similar operations with an array of KeyValuePair   ex  KeyValuePair lt string  string gt    filters

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To expand on Daniels post and his comments regarding the key  since the key is embedded within the value anyway  you could resort to using a KeyValuePair lt TKey  TValue gt  as the value  The main reasoning for this is that  in general  the Key isn t necessarily directly derivable from the value   Then it d look like this   public sealed class CustomDictionary lt TKey  TValue gt      KeyedCollection lt TKey  KeyValuePair lt TKey  TValue gt  gt      protected override TKey GetKeyForItem KeyValuePair lt TKey  TValue gt  item          return item Key          To use this as in the previous example  you d do   CustomDictionary lt string  int gt  custDict   new CustomDictionary lt string  int gt      custDict Add new KeyValuePair lt string  int gt   key   7     int valueByIndex   custDict 0  Value  int valueByKey   custDict  key   Value  string keyByIndex   custDict 0  Key

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You can also use SortedList and its Generic counterpart  These two classes and in Andrew Peters answer mentioned OrderedDictionary are dictionary classes in which items can be accessed by index  position  as well as by key  How to use these classes you can find  SortedList Class   SortedList Generic Class

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I think you can do something like this  the syntax might be wrong  havent used C  in a while To get the last item  Dictionary lt string  int gt  KeyCollection keys   mydict keys  string lastKey   keys Last      or use Max instead of Last to get the max value  I dont know which one fits your code better

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As  Falanwe points out in a comment  doing something like this is incorrect   int LastCount   mydict Keys ElementAt mydict Count -1     You should not depend on the order of keys in a Dictionary  If you need ordering  you should use an OrderedDictionary  as suggested in this answer  The other answers on this page are interesting as well

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One alternative would be a KeyedCollection if the key is embedded in the value   Just create a basic implementation in a sealed class to use   So to replace Dictionary lt string  int gt   which isn t a very good example as there isn t a clear key for a int    private sealed class IntDictionary   KeyedCollection lt string  int gt        protected override string GetKeyForItem int item                   The example works better when the value contains the key  It falls down a bit for a dictionary of ints          return item ToString             KeyedCollection lt string  int gt  intCollection   new ClassThatContainsSealedImplementation IntDictionary     intCollection Add 7    int valueByIndex   intCollection 0

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The way you worded the question leads me to believe that the int in the Dictionary contains the item s  position  on the Dictionary  Judging from the assertion that the keys aren t stored in the order that they re added  if this is correct  that would mean that keys Count  or  Count - 1  if you re using zero-based  should still always be the number of the last-entered key   If that s correct  is there any reason you can t instead use Dictionary lt int  string gt  so that you can use mydict  mydict Keys Count

User · Answer

You can use an OrderedDictionary      Represents a collection of key value   pairs that are accessible by the key   or index

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You could always do this   string   temp   new string mydict count   mydict Keys CopyTo temp  0  int LastCount   mydict temp mydict count - 1     But I wouldn t recommend it  There s no guarantee that the last inserted key will be at the end of the array  The ordering for Keys on MSDN is unspecified  and subject to change  In my very brief test  it does seem to be in order of insertion  but you d be better off building in proper bookkeeping like a stack--as you suggest  though I don t see the need of a struct based on your other statements --or single variable cache if you just need to know the latest key

[c#] Accessing a Dictionary.Keys Key through a numeric index

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