remove contiguous duplicates in the sequence of objects

Question

suppose we have the list of objects

List<int> list = new List<int>() { 1,1,1,2,3,3,3,2,2,2,1,1 };

what is the most elegant way to get the following result list?

{1,2,3,2,1}

Assuming your list contains primitive types you can use [Distinct()](https://msdn.microsoft.com/en-us/library/vstudio/bb348436(v=vs.100).aspx). e.g: `list.Distinct().ToList();`. If your list contains complex types you'll need to pass it an `IEqualityComparer` which your type implements — DGibbs, Oct 20 '15 at 15:59
@sstan please read my question bettera and remove "marked as duplicate" or provide a valid answer — Alex, Oct 20 '15 at 16:07
Your desired outcome `{1,2,3,2,1}` doesn't match your requirements `remove duplicates in the sequence of objects` which would give you a result of `1, 2, 3` based on your dataset. — DGibbs, Oct 20 '15 at 16:12
You can use something like var requiredList = list.Where((value, index) => index == 0 || value != list.ElementAt(index - 1)); — arunlalam, Oct 20 '15 at 16:24

score 6 · Answer 1 · answered Oct 20 '15 at 17:21

I like the idea of an extension method:

public static IEnumerable<T> RemoveContiguousDuplicates<T>(this IEnumerable<T> items) where T: IEquatable<T>
{
    bool init = false;
    T prev = default(T);

    foreach (T item in items)
    {
        if (!init)
            init = true;
        else if (prev.Equals(item))
            continue;

        prev = item;
        yield return item;
    }
}

And then, of course, to use it:

var singles = list.RemoveContiguousDuplicates().ToList();

Salah Akbari · Answer 2 · 2015-10-20T20:14:11.927

2

Try this:

List<int> newList = new List<int>();
foreach (var item in list.Where(c => newList.Count == 0 || newList.Last() != c))
{
    newList.Add(item); // 1,2,3,2,1 will add to newList
}

edited Oct 20 '15 at 20:14

answered Oct 20 '15 at 15:53

Salah Akbari

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remove contiguous duplicates in the sequence of objects

2 Answers2