CODEWITHSUNDEEP
c#.netlinqlazy-evaluationinfinite-sequence
Cristian Diaconescu
Cristian Diaconescu

Reputation: 35631

linq infinite list from given finite list

Given a finite list of elements, how can I create a (lazily-evaluated, thanks LINQ!) infinite list that just keeps iterating over my initial list?

If the initial list is {1, 2, 3}, I want the new list to return {1, 2, 3, 1, 2, 3, 1, ...}

Upvotes: 10

Views: 2604

Answers (4)

Ryan Brunner
Ryan Brunner

Reputation: 14851

yield return is a fairly handy operator for this, although it doesn't really require LINQ specifically.

IEnumerable<int> GetInfiniteSeries(IEnumerable<int> items) {
    while (true) {
       foreach (var item in items) { 
           yield return item;
       }
    }
}

Upvotes: 19

Matt Greer
Matt Greer

Reputation: 62027

Another option, implement IEnumerator<T>:

  public class InfiniteEnumerator<T> : IEnumerator<T>
    {
        private IList<T> _items;
        private int _index = -1;

        public InfiniteEnumerator(IList<T> items)
        {
            if (items == null)
            {
                throw new ArgumentNullException("items");
            }
            _items = items;
        }

        public T Current
        {
            get { return _items[_index]; }
        }

        public void Dispose()
        {

        }

        object System.Collections.IEnumerator.Current
        {
            get { return _items[_index]; }
        }

        public bool MoveNext()
        {
            if (_items.Count == 0)
            {
                return false;
            }

            _index = (_index + 1) % _items.Count;
            return true;
        }

        public void Reset()
        {
            _index = -1;
        }
    }

Upvotes: 3

Cristian Diaconescu
Cristian Diaconescu

Reputation: 35631

Here's how I've done it eventually: 

    public static IEnumerable<T> AdNauseam<T>(this IEnumerable<T> i_list)
    {
        using(var etor = i_list.GetEnumerator())
        {
            while(true)
            {
                while(etor.MoveNext())
                {
                    yield return etor.Current;
                }
                etor.Reset();
            }
        }
    }

Usage: 

var list = new[] {1, 2, 3}
var infinite = list.AdNauseam().Take(10);

The result:

{1, 2, 3, 1, 2, 3, 1, 2, 3, 1}

Upvotes: 3

James Curran
James Curran

Reputation: 103495

IEnumerable<T> Infinite(this IEnumerable<T> ienum)
{
    List<T> list = ienum.ToList();
    while (true)
       foreach(var t in list)
           yield return t;
}



foreach(int i in Enumerable.Range(1,3).Infinite())
      Console.WriteLine(i);

Upvotes: 6

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