Get index of a key value pair in a C dictionary based on the value

Question

I would like to know if some property or method exists that gets the index of a specific value   I found that dictionaries have the Contains   method which returns true if the value passed in exists  so this method almost implements what I need    I know that I can loop through all the value pairs and check the condition  but I ask because maybe there s an optimized way of doing this

User · Answer

You can find index by key values in dictionary   Dictionary lt string  string gt  myDictionary   new Dictionary lt string  string gt     myDictionary Add  a    x    myDictionary Add  b    y    int i   Array IndexOf myDictionary Keys ToArray     a    int j   Array IndexOf myDictionary Values ToArray     y

User · Answer

In your comment to max s answer  you say that what you really wanted to get is the key in  and not the index of  the KeyValuePair that contains a certain value  You could edit your question to make it more clear   It is worth pointing out  EricM has touched upon this in his answer  that a value might appear more than once in the dictionary  in which case one would have to think which key he would like to get  e g  the first that comes up  the last  all of them   If you are sure that each key has a unique value  you could have another dictionary  with the values from the first acting as keys and the previous keys acting as values  Otherwise  this second dictionary idea  suggested by Jon Skeet  will not work  as you would again have to think which of all the possible keys to use as value in the new dictionary   If you were asking about the index  though  EricM s answer would be OK  Then you could get the KeyValuePair in question by using   yourDictionary ElementAt theIndexYouFound     provided that you do not add remove things in yourDictionary   PS  I know it s been almost 7 years now  but what the heck  I thought it best to formulate my answer as addressing the OP  but of course by now one can say it is an answer for just about anyone else but the OP  Fully aware of that  thank you

User · Answer

Let s say you have a Dictionary called fooDictionary  fooDictionary Values ToList   IndexOf someValue     Values ToList    converts your dictionary values into a List of someValue objects   IndexOf someValue   searches your new List looking for the someValue object in question 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   This does not however account for the issue that there may be several matching  someValue  objects

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Consider using System Collections Specialized OrderedDictionary  though it is not generic  or implement your own  example    OrderedDictionary does not support IndexOf  but it s easy to implement   public static class OrderedDictionaryExtensions       public static int IndexOf this OrderedDictionary dictionary  object value                for int i   0  i  lt  dictionary Count    i                        if dictionary i     value  return i                    return -1

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If searching for a value  you will have to loop through all the data  But to minimize code involved  you can use LINQ  Example  Given Dictionary defined as following  Dictionary lt Int32  String gt  dict   You can use following code      Search for all keys with given value Int32   keys   dict Where kvp   gt  kvp Value Equals  quot SomeValue quot    Select kvp   gt  kvp Key  ToArray                Search for first key with given value Int32 key   dict First kvp   gt  kvp Value Equals  quot SomeValue quot    Key

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You can use LINQ to help you with this   Dictionary lt int  string gt  dict   new Dictionary lt int  string gt     dict Add 1   hi    dict Add 2   NotHi    dict Add 3   Bah     var item    from d in dict             where d Value     hi              select d Key  FirstOrDefault     Console WriteLine item     Prints 1

User · Answer

no   there is nothing similar IndexOf for Dictionary although you can make use of ContainsKey method to get whether a key belongs to dictionary or not

User · Answer

There s no such concept of an  index  within a dictionary - it s fundamentally unordered  Of course when you iterate over it you ll get the items in some order  but that order isn t guaranteed and can change over time  particularly if you add or remove entries    Obviously you can get the key from a KeyValuePair just by using the Key property  so that will let you use the indexer of the dictionary   var pair        var value   dictionary pair Key   Assert AreEqual value  pair Value     You haven t really said what you re trying to do  If you re trying to find some key which corresponds to a particular value  you could use   var key   dictionary Where pair   gt  pair Value    desiredValue                       Select pair   gt  pair Key                       FirstOrDefault      key will be null if the entry doesn t exist   This is assuming that the key type is a reference type    if it s a value type you ll need to do things slightly differently   Of course  if you really want to look up values by key  you should consider using another dictionary which maps the other way round in addition to your existing dictionary

[c#] Get index of a key/value pair in a C# dictionary based on the value

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