Can you use Linq to find relationships between elements in a collection?

Question

I have a List of Data, and I want to find the length of the longest sequence of elements in the list that are equal. This is easy to do with a loop over the collection, however I was wondering if there is a Linq query that can do this. Thanks for your help.

Paste the code with the loop, we have no idea what your data is like.. — Lews Therin, Feb 01 '13 at 21:45
It shouldn't matter, all that matters is that the element type supports equality comparison. Pretend it's a List of integers. — user200814, Feb 01 '13 at 21:48

score 1 · Answer 1 · answered Feb 01 '13 at 21:50

1

var longest = list.Select((x,i) => list.Skip(i).TakeWhile(c => c == x).Count()).Max();

answered Feb 01 '13 at 21:50

Eren Ersönmez

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Note that you do a lot of counting over and over again needlessly...This is O(n^2) when it could be O(n). If that matter to the OP... – Servy Feb 01 '13 at 21:51
Yes, looping could be significantly faster for large lists. – Eren Ersönmez Feb 01 '13 at 22:01

Servy · Answer 2 · 2013-02-01T22:02:55.680

Just iterate through the sequence incrementing a counter when an item is equal to the previous and resetting it if it's not.

public static int LongestSequence<T>(IEnumerable<T> source, IEqualityComparer<T> comparer)
{
    comparer = comparer ?? EqualityComparer<T>.Default;

    using (var iterator = source.GetEnumerator())
    {
        if (!iterator.MoveNext()) //empty sequence
            return 0;
        T previous = iterator.Current;
        int count = 1;
        int maxCount = 1;

        while (iterator.MoveNext())
        {
            if (comparer.Equals(iterator.Current, previous))
            {
                count++;
            }
            else
            {
                maxCount = Math.Max(maxCount, count);
                count = 1;
                previous = iterator.Current;
            }
        }

        return maxCount;
    }
}

Can you use Linq to find relationships between elements in a collection?

2 Answers2