Selecting Unique Elements From a List in C#

Question

How do I select the unique elements from the list {0, 1, 2, 2, 2, 3, 4, 4, 5} so that I get {0, 1, 3, 5}, effectively removing the repeated elements {2, 4}?

Answer 1

var nums = new int{ 0...4,4,5};
var distinct = nums.Distinct();

make sure you're using Linq and .NET framework 3.5.

Answer 2

var numbers = new[] { 0, 1, 2, 2, 2, 3, 4, 4, 5 };

var uniqueNumbers =
    from n in numbers
    group n by n into nGroup
    where nGroup.Count() == 1
    select nGroup.Key;

Answer 3

If Linq isn't available to you because you have to support legacy code that can't be upgraded, then declare a Dictionary, where the first int is the number and the second int is the number of occurences. Loop through your List, loading up your Dictionary. When you're done, loop through your Dictionary selecting only those elements where the number of occurences is 1.

Answer 4

C# 2.0 solution:

 static IEnumerable<T> GetUniques<T>(IEnumerable<T> things)
 {
  Dictionary<T, int> counts = new Dictionary<T, int>();
  foreach (T item in things)
  {
   int count;
   if (counts.TryGetValue(item, out count))
    counts[item] = ++count;
   else
    counts.Add(item, 1);
  }
  foreach (KeyValuePair<T, int> kvp in counts)
  {
   if (kvp.Value == 1)
    yield return kvp.Key;
  }
 }

Answer 5

I believe Matt meant to say:

 static IEnumerable<T> GetUniques<T>(IEnumerable<T> things)
 {
     Dictionary<T, bool> uniques = new Dictionary<T, bool>();
     foreach (T item in things)
     {
         if (!(uniques.ContainsKey(item)))
         {
             uniques.Add(item, true);
         }
     }
     return uniques.Keys;
 }

Answer 6

With lambda..

var all = new[] {0,1,1,2,3,4,4,4,5,6,7,8,8}.ToList();
var unique = all.GroupBy(i => i).Where(i => i.Count() == 1).Select(i=>i.Key);