Alex Rivera | Logout

Find all intersecting data, not just the unique values

Asked 2010-02-01T21:00:44.537
14

I thought that I understood Intersect, but it turns out I was wrong. 

 List<int> list1 = new List<int>() { 1, 2, 3, 2, 3};
 List<int> list2 = new List<int>() { 2, 3, 4, 3, 4};

 list1.Intersect(list2) =>      2,3

 //But what I want is:
 // =>  2,3,2,3,2,3,3

I can figure a way like:

 var intersected = list1.Intersect(list2);
 var list3 = new List<int>();
 list3.AddRange(list1.Where(I => intersected.Contains(I)));
 list3.AddRange(list2.Where(I => intersected.Contains(I)));

Is there a easier way in LINQ to achieve this?

I do need to state that I do not care in which order the results are given.

2,2,2,3,3,3,3 would also be perfectly OK.

Problem is that I am using this on a very large collection, So I need efficiency.

We are talking about Objects, not ints. The ints were just for the easy example, but I realize this can make a difference.

c# linq intersection
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1 Answer

-1

I don't believe this is possible with the built-in APIs. But you could use the following to get the result you're looking for. 

IEnumerable<T> Intersect2<T>(this IEnumerable<T> left, IEnumerable<T> right) {
  var map = left.ToDictionary(x => x, y => false);
  foreach ( var item in right ) {
    if (map.ContainsKey(item) ) {
      map[item] = true;
    }
  }
  foreach ( var cur in left.Concat(right) ) {
    if ( map.ContainsKey(cur) ) {
      yield return cur;
    }
  }
}
answered 2010-02-01T21:19:56.427

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