get dictionary key by value

Question

How do I get a Dictionary key by value in C    Dictionary lt string  string gt  types   new Dictionary lt string  string gt                    1    one                  2    two                  3    three        I want something like this   getByValueKey string value     getByValueKey  one   must be return  1    What is the best way do this  Maybe HashTable  SortedLists

User · Answer

You could do that    By looping through all the KeyValuePair lt TKey  TValue gt  s in the dictionary  which will be a sizable performance hit if you have a number of entries in the dictionary   Use two dictionaries  one for value-to-key mapping and one for key-to-value mapping  which would take up twice as much space in memory       Use Method 1 if performance is not a consideration  use Method 2 if memory is not a consideration   Also  all keys must be unique  but the values are not required to be unique   You may have more than one key with the specified value   Is there any reason you can t reverse the key-value relationship

User · Answer

Values not necessarily have to be unique so you have to do a lookup  You can do something like this   var myKey   types FirstOrDefault x   gt  x Value     one   Key    If values are unique and are inserted less frequently than read  then create an inverse dictionary where values are keys and keys are values

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I was in a situation where Linq binding was not available and had to expand lambda explicitly  It resulted in a simple function   public static T KeyByValue lt T  W gt  this Dictionary lt T  W gt  dict  W val        T key   default      foreach  KeyValuePair lt T  W gt  pair in dict                if  EqualityComparer lt W gt  Default Equals pair Value  val                         key   pair Key              break                      return key      Call it like follows   public static void Main         Dictionary lt string  string gt  dict   new Dictionary lt string  string gt                    1    one              2    two              3    three               string key   KeyByValue dict   two               Console WriteLine  Key      key       Works on  NET 2 0 and in other limited environments

User · Answer

maybe something like this   foreach  var keyvaluepair in dict        if Object ReferenceEquals keyvaluepair Value  searchedObject                   dict Remove keyvaluepair Key           break

User · Answer

Below Code only works if It contain Unique Value Data   public string getKey string Value        if  dictionary ContainsValue Value                 var ListValueData new List lt string gt             var ListKeyData   new List lt string gt              var Values   dictionary Values          var Keys   dictionary Keys           foreach  var item in Values                        ListValueData Add item                      var ValueIndex   ListValueData IndexOf Value           foreach  var item in Keys                        ListKeyData Add item                      return  ListKeyData ValueIndex              return string Empty

User · Answer

types Values ToList   IndexOf  one      Values ToList   converts your dictionary values into a List of objects  IndexOf  one   searches your new List looking for  one  and returns the Index which would match the index of the Key Value pair in the dictionary   This method does not care about the dictionary keys  it simply returns the index of the value that you are looking for   Keep in mind there may be more than one  one  value in your dictionary  And that is the reason there is no  get key  method

User · Answer

I have very simple way to do this  It worked out perfect for me    Dictionary lt string  string gt  types   new Dictionary lt string  string gt      types Add  1    one    types Add  2    two    types Add  3    three     Console WriteLine  Please type a key to show its value      string rLine   Console ReadLine     if types ContainsKey rLine         string value For Key   types rLine       Console WriteLine  Value for     rLine     is    value For Key

User · Answer

I have created a double-lookup class        lt summary gt      dictionary with double key lookup      lt  summary gt       lt typeparam name  T1  gt primary key lt  typeparam gt       lt typeparam name  T2  gt secondary key lt  typeparam gt       lt typeparam name  TValue  gt value type lt  typeparam gt  public class cDoubleKeyDictionary lt T1  T2  TValue gt        private struct Key2ValuePair           internal T2 key2          internal TValue value            private Dictionary lt T1  Key2ValuePair gt  d1   new Dictionary lt T1  Key2ValuePair gt         private Dictionary lt T2  T1 gt  d2   new Dictionary lt T2  T1 gt               lt summary gt          add item         not exacly like add  mote like Dictionary     overwriting existing values          lt  summary gt           lt param name  key1  gt  lt  param gt           lt param name  key2  gt  lt  param gt      public void Add T1 key1  T2 key2  TValue value            lock  d1                d1 key1    new Key2ValuePair                   key2   key2                  value   value                             d2 key2    key1                            lt summary gt          get key2 by key1          lt  summary gt           lt param name  key1  gt  lt  param gt           lt param name  key2  gt  lt  param gt           lt returns gt  lt  returns gt      public bool TryGetValue T1 key1  out TValue value            if  d1 TryGetValue key1  out Key2ValuePair kvp                 value   kvp value              return true            else               value   default              return false                            lt summary gt          get key1 by key2          lt  summary gt           lt param name  key2  gt  lt  param gt           lt param name  key1  gt  lt  param gt           lt remarks gt          2x O 1  operation          lt  remarks gt           lt returns gt  lt  returns gt      public bool TryGetValue2 T2 key2  out TValue value            if  d2 TryGetValue key2  out T1 key1                 return TryGetValue key1  out value             else               value   default              return false                            lt summary gt          get key1 by key2          lt  summary gt           lt param name  key2  gt  lt  param gt           lt param name  key1  gt  lt  param gt           lt remarks gt          2x O 1  operation          lt  remarks gt           lt returns gt  lt  returns gt      public bool TryGetKey1 T2 key2  out T1 key1            return d2 TryGetValue key2  out key1                   lt summary gt          get key1 by key2          lt  summary gt           lt param name  key2  gt  lt  param gt           lt param name  key1  gt  lt  param gt           lt remarks gt          2x O 1  operation          lt  remarks gt           lt returns gt  lt  returns gt      public bool TryGetKey2 T1 key1  out T2 key2            if  d1 TryGetValue key1  out Key2ValuePair kvp1                 key2   kvp1 key2              return true            else               key2   default              return false                            lt summary gt          remove item by key 1          lt  summary gt           lt param name  key1  gt  lt  param gt      public void Remove T1 key1            lock  d1                if  d1 TryGetValue key1  out Key2ValuePair kvp                     d1 Remove key1                   d2 Remove kvp key2                                           lt summary gt          remove item by key 2          lt  summary gt           lt param name  key2  gt  lt  param gt      public void Remove2 T2 key2            lock  d1                if  d2 TryGetValue key2  out T1 key1                     d1 Remove key1                   d2 Remove key2                                           lt summary gt          clear all items          lt  summary gt      public void Clear             lock  d1                d1 Clear                d2 Clear                              lt summary gt          enumerator on key1  so we can replace Dictionary by cDoubleKeyDictionary          lt  summary gt           lt param name  key1  gt  lt  param gt           lt returns gt  lt  returns gt      public TValue this T1 key1            get   gt  d1 key1  value                  lt summary gt          enumerator on key1  so we can replace Dictionary by cDoubleKeyDictionary          lt  summary gt           lt param name  key1  gt  lt  param gt           lt returns gt  lt  returns gt      public TValue this T1 key1  T2 key2            set               lock  d1                    d1 key1    new Key2ValuePair                       key2   key2                      value   value                                     d2 key2    key1

[c#] get dictionary key by value

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