The opposite of Intersect

Question

Intersect can be used to find matches between two collections  like so      Assign two arrays  int   array1     1  2  3    int   array2     2  3  4       Call Intersect extension method  var intersect   array1 Intersect array2      Write intersection to screen  foreach  int value in intersect        Console WriteLine value      Output  2  3     However what I d like to achieve is the opposite  I d like to list items from one collection that are missing from the other      Assign two arrays  int   array1     1  2  3    int   array2     2  3  4       Call  NonIntersect  extension method  var intersect   array1 NonIntersect array2      I ve made up the NonIntersect method    Write intersection to screen  foreach  int value in intersect        Console WriteLine value      Output  4

User · Answer

array1 NonIntersect array2    Nonintersect such operator is not present in Linq you should do  except -  union -  except  a except b  union b Except a

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This code enumerates each sequence only once and uses Select x   gt  x  to hide the result to get a clean Linq-style extension method  Since it uses HashSet lt T gt  its runtime is O n   m  if the hashes are well distributed  Duplicate elements in either list are omitted   public static IEnumerable lt T gt  SymmetricExcept lt T gt  this IEnumerable lt T gt  seq1      IEnumerable lt T gt  seq2        HashSet lt T gt  hashSet   new HashSet lt T gt  seq1       hashSet SymmetricExceptWith seq2       return hashSet Select x   gt  x

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string left    411329 SOFT MAC GREEN   string right   SOFT MAC GREEN    string   l   left Split       string   r   right Split        string   distinctLeft   l Distinct   ToArray    string   distinctRight   r Distinct   ToArray     var commonWord   l Except r  StringComparer OrdinalIgnoreCase  string result   String Join     commonWord   result    411329

User · Answer

I think you might be looking for Except      The Except operator produces the set   difference between two sequences  It   will only return elements in the first   sequence that don t appear in the   second  You can optionally provide   your own equality comparison function    Check out this link  this link  or Google  for more information

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lt summary gt      Given two list  compare and extract differences     http   stackoverflow com questions 5620266 the-opposite-of-intersect      lt  summary gt  public class CompareList            lt summary gt          Returns list of items that are in initial but not in final list           lt  summary gt           lt param name  listA  gt  lt  param gt           lt param name  listB  gt  lt  param gt           lt returns gt  lt  returns gt      public static IEnumerable lt string gt  NonIntersect          List lt string gt  initial  List lt string gt  final                  subtracts the content of initial from final           assumes that final length  lt  initial length         return initial Except final                   lt summary gt          Returns the symmetric difference between the two list          http   en wikipedia org wiki Symmetric difference          lt  summary gt           lt param name  initial  gt  lt  param gt           lt param name  final  gt  lt  param gt           lt returns gt  lt  returns gt      public static IEnumerable lt string gt  SymmetricDifference          List lt string gt  initial  List lt string gt  final                IEnumerable lt string gt  setA   NonIntersect final  initial           IEnumerable lt string gt  setB   NonIntersect initial  final              sum and return the two set          return setA Concat setB

User · Answer

As stated  if you want to get 4 as the result  you can do like this   var nonintersect   array2 Except array1     If you want the real non-intersection  also both 1 and 4   then this should do the trick   var nonintersect   array1 Except array2  Union  array2 Except array1      This will not be the most performant solution  but for small lists it should work just fine

User · Answer

I m not 100  sure what your NonIntersect method is supposed to do  regarding set theory  - is it B   A  everything from B that does not occur in A   If yes  then you should be able to use the Except operation  B Except A

User · Answer

You can use   a Except b  Union b Except a      Or you can use   var difference   new HashSet a   difference SymmetricExceptWith b

[c#] The opposite of Intersect()

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