Get all subsets of a collection

Question

I am trying to create a method that will return all subsets of a set.

For example if I have the collection 10,20,30 I will like to get the following ou

Answer 1

I know that this question is a little old but i was looking for a answer and dont find any good here, so i want to share this solution that is an adaptation found in this blog: http://praseedp.blogspot.com.br/2010/02/subset-generation-in-c.html

I Only transform the class into a generic class:

public class SubSet
{
    private IList _list;
    private int _length;
    private int _max;
    private int _count;

    public SubSet(IList list)
    {
        if (list== null)
            throw new ArgumentNullException("lista");
        _list = list;
        _length = _list.Count;
        _count = 0;
        _max = (int)Math.Pow(2, _length);
    }


    public IList Next()
    {
        if (_count == _max)
        {
            return null;
        }
        uint rs = 0;

        IList l = new List();            

        while (rs < _length)
        {
            if ((_count & (1u << (int)rs)) > 0)
            {
                l.Add(_list[(int)rs]);                    
            }
            rs++;
        }
        _count++;
        return l;            
    }
}

To use this code you can do like something that:

        List lst = new List();

        lst.AddRange(new string[] {"A", "B", "C" });

        SubSet subs = new SubSet(lst);

        IList l = subs.Next();

        while (l != null)
        {

            DoSomething(l);
            l = subs.Next();
        }

Just remember: this code still be an O(2^n) and if you pass something like 20 elements in the list you will get 2^20= 1048576 subsets!

Edit: As Servy sugest i add an implementation with interator block to use with Linq an foreach, the new class is like this:

private class SubSet : IEnumerable>
    {
        private IList _list;
        private int _length;
        private int _max;
        private int _count;

        public SubSet(IEnumerable list)
        {
            if (list == null)
                throw new ArgumentNullException("list");
            _list = new List(list);
            _length = _list.Count;
            _count = 0;
            _max = (int)Math.Pow(2, _length);
        }

        public int Count
        {
            get { return _max; }
        }



        private IList Next()
        {
            if (_count == _max)
            {
                return null;
            }
            uint rs = 0;

            IList l = new List();

            while (rs < _length)
            {
                if ((_count & (1u << (int)rs)) > 0)
                {
                    l.Add(_list[(int)rs]);
                }
                rs++;
            }
            _count++;
            return l;
        }

        public IEnumerator> GetEnumerator()
        {
            IList subset;
            while ((subset = Next()) != null)
            {
                yield return subset;
            }
        }

        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }

and you now can use it like this:

        List lst = new List();

        lst.AddRange(new string[] {"A", "B", "C" });

        SubSet subs = new SubSet(lst);

        foreach(IList l in subs)
        {
            DoSomething(l);
        }

Thanks Servy for the advice.