Tuples or arrays as Dictionary keys in C

Question

I am trying to make a Dictionary lookup table in C    I need to resolve a 3-tuple of values to one string   I tried using arrays as keys  but that did not work  and I don t know what else to do   At this point I am considering making a Dictionary of Dictionaries of Dictionaries  but that would probably not be very pretty to look at  though it is how I would do it in javascript

User · Answer

If you re on C  7  you should consider using value tuples as your composite key  Value tuples typically offer better performance than the traditional reference tuples  Tuple lt T1      gt   since value tuples are value types  structs   not reference types  so they avoid the memory allocation and garbage collection costs  Also  they offer conciser and more intuitive syntax  allowing for their fields to be named if you so wish  They also implement the IEquatable lt T gt  interface needed for the dictionary   var dict   new Dictionary lt  int PersonId  int LocationId  int SubjectId   string gt     dict Add  3  6  9    ABC    dict Add  PersonId  4  LocationId  9  SubjectId  10    XYZ    var personIds   dict Keys Select k   gt  k PersonId  Distinct   ToList

User · Answer

If your consuming code can make do with an IDictionary lt   interface  instead of Dictionary  my instinct would have been to use a SortedDictionary lt   with a custom array comparer  ie   class ArrayComparer lt T gt    IComparer lt IList lt T gt  gt      where T   IComparable lt T gt        public int Compare IList lt T gt  x  IList lt T gt  y                int compare   0          for  int n   0  n  lt  x Count  amp  amp  n  lt  y Count    n                        compare   x n  CompareTo y n                      return compare            And create thus  using int   just for concrete example s sake    var dictionary   new SortedDictionary lt int    string gt  new ArrayComparer lt int gt

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If you are on  NET 4 0 use a Tuple  lookup   new Dictionary lt Tuple lt TypeA  TypeB  TypeC gt   string gt      If not you can define a Tuple and use that as the key  The Tuple needs to override GetHashCode  Equals and IEquatable  struct Tuple lt T  U  W gt    IEquatable lt Tuple lt T U W gt  gt        readonly T first      readonly U second      readonly W third       public Tuple T first  U second  W third                this first   first          this second   second          this third   third             public T First   get   return first          public U Second   get   return second          public W Third   get   return third           public override int GetHashCode                 return first GetHashCode     second GetHashCode     third GetHashCode               public override bool Equals object obj                if  obj    null    GetType      obj GetType                          return false                    return Equals  Tuple lt T  U  W gt  obj              public bool Equals Tuple lt T  U  W gt  other                return other first Equals first   amp  amp  other second Equals second   amp  amp  other third Equals third

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Between tuple and nested dictionaries based approaches  it s almost always better to go for tuple based   From maintainability point of view     its much easier to implement a functionality that looks like   var myDict   new Dictionary lt Tuple lt TypeA  TypeB  TypeC gt   string gt       than  var myDict   new Dictionary lt TypeA  Dictionary lt TypeB  Dictionary lt TypeC  string gt  gt  gt       from the callee side  In the second case each addition  lookup  removal etc require action on more than one dictionary   Furthermore  if your composite key require one more  or less  field in future  you will need to change code a significant lot in the second case  nested dictionary  since you have to add further nested dictionaries and subsequent checks    From performance perspective  the best conclusion you can reach is by measuring it yourself  But there are a few theoretical limitations which you can consider beforehand    In the nested dictionary case  having an additional dictionary for every keys  outer and inner  will have some memory overhead  more than what creating a tuple would have   In the nested dictionary case  every basic action like addition  updation  lookup  removal etc need to be carried out in two dictionaries  Now there is a case where nested dictionary approach can be faster  i e   when the data being looked up is absent  since the intermediate dictionaries can bypass the full hash code computation  amp  comparison  but then again it should be timed to be sure  In presence of data  it should be slower since lookups should be performed twice  or thrice depending on nesting   Regarding tuple approach   NET tuples are not the most performant when they re meant to be used as keys in sets since its Equals and GetHashCode implementation causes boxing for value types    I would go with tuple based dictionary  but if I want more performance  I would use my own tuple with better implementation     On a side note  few cosmetics can make the dictionary cool    Indexer style calls can be a lot cleaner and intuitive  For eg   string foo   dict a  b  c     lookup dict a  b  c          update insertion   So expose necessary indexers in your dictionary class which internally handles the insertions and lookups   Also  implement a suitable IEnumerable interface and provide an Add TypeA  TypeB  TypeC  string  method which would give you collection initializer syntax  like   new MultiKeyDictionary lt TypeA  TypeB  TypeC  string gt            a  b  c  null

User · Answer

If for some reason you really want to avoid creating your own Tuple class  or using on built into  NET 4 0  there is one other approach possible  you can combine the three key values together into a single value   For example  if the three values are integer types together not taking more than 64 bits  you could combine them into a ulong   Worst-case you can always use a string  as long as you make sure the three components in it are delimited with some character or sequence that does not occur inside the components of the key  for example  with three numbers you could try   string Format   0   1   2    key1  key2  key3    There is obviously some composition overhead in this approach  but depending on what you are using it for this may be trivial enough not to care about it

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The good  clean  fast  easy and readable ways is    generate equality members  Equals   and GetHashCode    method for the current type  Tools like ReSharper not only creates the methods  but also generates the necessary code for an equality check and or for calculating hash code  The generated code will be more optimal than Tuple realization  just make a simple key class derived from a tuple    add something similar like this   public sealed class myKey   Tuple lt TypeA  TypeB  TypeC gt        public myKey TypeA dataA  TypeB dataB  TypeC dataC    base  dataA  dataB  dataC           public TypeA DataA   gt  Item1        public TypeB DataB   gt  Item2       public TypeC DataC   gt  Item3      So you can use it with dictionary   var myDictinaryData   new Dictionary lt myKey  string gt           new myKey 1  2  3    data123         new myKey 4  5  6    data456         new myKey 7  8  9    data789         You also can use it in contracts as a key for join or groupings in linq going this way you never ever mistype order of Item1  Item2  Item3      you no need to remember or look into to code to understand where to go to get something no need to override IStructuralEquatable  IStructuralComparable  IComparable  ITuple they all alredy here

User · Answer

Here is the  NET tuple for reference    Serializable   public class Tuple lt T1  T2  T3 gt    IStructuralEquatable  IStructuralComparable  IComparable  ITuple        private readonly T1 m Item1       private readonly T2 m Item2      private readonly T3 m Item3        public T1 Item1   get   return m Item1          public T2 Item2   get   return m Item2          public T3 Item3   get   return m Item3            public Tuple T1 item1  T2 item2  T3 item3             m Item1   item1           m Item2   item2          m Item3   item3              public override Boolean Equals Object obj            return   IStructuralEquatable  this  Equals obj  EqualityComparer lt Object gt  Default                Boolean IStructuralEquatable Equals Object other  IEqualityComparer comparer             if  other    null  return false           Tuple lt T1  T2  T3 gt  objTuple   other as Tuple lt T1  T2  T3 gt            if  objTuple    null                return false                      return comparer Equals m Item1  objTuple m Item1   amp  amp  comparer Equals m Item2  objTuple m Item2   amp  amp  comparer Equals m Item3  objTuple m Item3               Int32 IComparable CompareTo Object obj            return   IStructuralComparable  this  CompareTo obj  Comparer lt Object gt  Default              Int32 IStructuralComparable CompareTo Object other  IComparer comparer            if  other    null  return 1            Tuple lt T1  T2  T3 gt  objTuple   other as Tuple lt T1  T2  T3 gt            if  objTuple    null                throw new ArgumentException Environment GetResourceString  ArgumentException TupleIncorrectType   this GetType   ToString      other                       int c   0           c   comparer Compare m Item1  objTuple m Item1             if  c    0  return c            c   comparer Compare m Item2  objTuple m Item2            if  c    0  return c            return comparer Compare m Item3  objTuple m Item3                public override int GetHashCode              return   IStructuralEquatable  this  GetHashCode EqualityComparer lt Object gt  Default              Int32 IStructuralEquatable GetHashCode IEqualityComparer comparer             return Tuple CombineHashCodes comparer GetHashCode m Item1   comparer GetHashCode m Item2   comparer GetHashCode m Item3                Int32 ITuple GetHashCode IEqualityComparer comparer            return   IStructuralEquatable  this  GetHashCode comparer              public override string ToString             StringBuilder sb   new StringBuilder            sb Append                return   ITuple this  ToString sb               string ITuple ToString StringBuilder sb            sb Append m Item1            sb Append                sb Append m Item2           sb Append                sb Append m Item3            sb Append               return sb ToString                 int ITuple Size            get               return 3

User · Answer

I would override your Tuple with a proper GetHashCode  and just use it as the key   As long as you overload the proper methods  you should see decent performance

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