How to find all duplicate from a List string

Question

I have a List lt string gt  which has some words duplicated  I need to find all words which are duplicates   Any trick to get them all

User · Accepted Answer

In  NET framework 3 5 and above you can use Enumerable GroupBy which returns an enumerable of enumerables of duplicate keys  and then filter out any of the enumerables that have a Count of  lt  1  then select their keys to get back down to a single enumerable   var duplicateKeys   list GroupBy x   gt  x                           Where group   gt  group Count    gt  1                           Select group   gt  group Key

User · Answer

and without the LINQ   string   ss     1   1   1     var myList   new List lt string gt     var duplicates   new List lt string gt      foreach  var s in ss       if   myList Contains s         myList Add s      else       duplicates Add s         show list without duplicates  foreach  var s in myList     Console WriteLine s       show duplicates list foreach  var s in duplicates     Console WriteLine s

User · Answer

I m assuming each string in your list contains several words  let me know if that s incorrect   List lt string gt  list   File RealAllLines  foobar txt   ToList     var words   from line in list             from word in line Split new                                          StringSplitOptions RemoveEmptyEntries              select word   var duplicateWords   from w in words                      group w by w ToLower   into g                      where g Count    gt  1                      select new                                                 Word   g Key                           Count   g Count

User · Answer

lblrepeated Text            string value   txtInput Text      char   arr   value ToCharArray        char   crr new char 1              int count1   0              for  int i   0  i  lt  arr Length  i                  int count   0            char letter arr i           for  int j   0  j  lt  arr Length  j                          char letter3   arr j                   if  letter    letter3                                        count                                                            if  count1  lt  count                        Array Resize lt char gt  ref crr 0               int count2   0              for int l   0 l  lt  crr Length l                                  if  crr l     letter                      count2                                                    if  count2    0                                Array Resize lt char gt  ref crr  crr Length   1                   crr crr Length-1    letter                             count1   count                                   else if  count1    count                        int count2   0              for  int l   0  l  lt  crr Length  l                                  if  crr l     letter                      count2                                if  count2    0                                Array Resize lt char gt  ref crr  crr Length   1                   crr crr Length - 1    letter                             count1   count                        for  int k   0  k  lt  crr Length  k            lblrepeated Text   lblrepeated Text   crr k    count1 ToString

User · Answer

If you are using LINQ  you can use the following query   var duplicateItems   from x in list                      group x by x into grouped                      where grouped Count    gt  1                      select grouped Key    or  if you prefer it without the syntactic sugar   var duplicateItems   list GroupBy x   gt  x  Where x   gt  x Count    gt  1  Select x   gt  x Key     This groups all elements that are the same  and then filters to only those groups with more than one element  Finally it selects just the key from those groups as you don t need the count   If you re prefer not to use LINQ  you can use this extension method   public void SomeMethod       var duplicateItems   list GetDuplicates               public static IEnumerable lt T gt  GetDuplicates lt T gt  this IEnumerable lt T gt  source        HashSet lt T gt  itemsSeen   new HashSet lt T gt         HashSet lt T gt  itemsYielded   new HashSet lt T gt          foreach  T item in source            if   itemsSeen Add item                 if  itemsYielded Add item                     yield return item                                    This keeps track of items it has seen and yielded  If it hasn t seen an item before  it adds it to the list of seen items  otherwise it ignores it  If it hasn t yielded an item before  it yields it  otherwise it ignores it

User · Answer

I use a method like that to check duplicated entrys in a string   public static IEnumerable lt string gt  CheckForDuplicated IEnumerable lt string gt  listString        List lt string gt  duplicateKeys   new List lt string gt         List lt string gt  notDuplicateKeys   new List lt string gt         foreach  var text in listString                if  notDuplicateKeys Contains text                         duplicateKeys Add text                     else                       notDuplicateKeys Add text                       return duplicateKeys      Maybe it s not the most shorted or elegant way  but I think that is very readable

User · Answer

Using LINQ  ofcourse  The below code would give you dictionary of item as string  and the count of each item in your sourc list   var item2ItemCount   list GroupBy item   gt  item  ToDictionary x  gt x Key x  gt x Count

User · Answer

For what it s worth  here is my way   List lt string gt  list   new List lt string gt  new string      cat    Dog    parrot    dog    parrot    goat    parrot    horse    goat      Dictionary lt string  int gt  wordCount   new Dictionary lt string  int gt        count them all  list ForEach word   gt        string key   word ToLower        if   wordCount ContainsKey key           wordCount Add key  0       wordCount key            remove words appearing only once  wordCount Keys ToList   FindAll word   gt  wordCount word     1  ForEach key   gt  wordCount Remove key     Console WriteLine string Format  Found  0  duplicates in the list    wordCount Count    wordCount Keys ToList   ForEach key   gt  Console WriteLine string Format   0  appears  1  times   key  wordCount key

User · Answer

If you re looking for a more generic method   public static List lt U gt  FindDuplicates lt T  U gt  this List lt T gt  list  Func lt T  U gt  keySelector                return list GroupBy keySelector               Where group   gt  group Count    gt  1               Select group   gt  group Key  ToList            EDIT  Here s an example   public class Person       public string Name  get set       public int Age  get set      List lt Person gt  list   new List lt Person gt      new Person     Name    John   Age   22    new Person     Name    John   Age   30    new Person     Name    Jack   Age   30       var duplicateNames   list FindDuplicates p   gt  p Name   var duplicateAges   list FindDuplicates p   gt  p Age    foreach var dupName in duplicateNames        Console WriteLine dupName      Will print out John    foreach var dupAge in duplicateAges        Console WriteLine dupAge      Will print out 30

[c#] How to find all duplicate from a List<string>?

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