c# – How to avoid multiple IEnumerable enum?

Question:

There is a method that returns a sequence consisting of pairs of adjacent elements. For example, given the sequence {1,2,3}, the method should return two pairs: (1,2) and (2,3).

The solution that immediately came to mind:

return items 
    .Take(items.Count() - 1) 
    .Zip(items.Skip(1), Tuple.Create);

But in this case, IEnumerable is enumerated more than once. How can you solve the problem without multiple enums (keeping the method lazy)?

Answer:

For example, yes. Let's declare an extension method that produces consecutive pairs:

public static class EnumerableExtensions
{
    public static IEnumerable<R> Pairwise<T, R>(
            this IEnumerable<T> sequence, Func<T, T, R> selector)
    {
        using (var it = sequence.GetEnumerator())
        {
            if (!it.MoveNext())
                yield break;
            T prev = it.Current;
            while (it.MoveNext())
                yield return selector(prev, prev = it.Current);
        }
    }
}

And we use:

return items.Pairwise(Tuple.Create);