When using Take (...) in a List <T>, does the entire list return before applying Take (...)?

Question

I have Listelements like

    public class FriendList 
    {
        public List<string> friends { get; set; } // List of friends names
        public DateTime timestamp { get; set; } // date/time on the data file
    }

and I need a procedure that gets the top 2 ordered by timestamp(and then does some other things with them). So, I started writing

    public void CompareLastTwo ( )
    {
        if ( this._fhist.Count < 2 )
        {
            Console.WriteLine("Need at least two instances of Facebook profile data in the Data folder");
        }

        FriendList latest, secondLatest; 
        if ( this._fhist[0].timestamp > this._fhist[1].timestamp )
        {
            latest = this._fhist[0];
            secondLatest = this._fhist[1];
        }
        else
        {
            latest = this._fhist[1];
            secondLatest = this._fhist[0];
        }

        for ( int i = 2, n = this._fhist.Count; i < n; ++i )
        {
            if ( this._fhist[i].timestamp > latest.timestamp )
            {
                secondLatest = latest;
                latest = this._fhist[i];
            }
            else if ( this._fhist[i].timestamp > secondLatest.timestamp && this._fhist[i].timestamp <= latest.timestamp )
            {
                secondLatest = this._fhist[i];
            }
        }

        // ... 

    }

but then I realized, looking at How to get the first N list items in C #? which i can do

List<FriendList> latestTwoFriendLists = this._fhist.OrderBy(L => L.timestamp).Take(2);

which is more compact but effective ???? Or does the process of computing the right side of the equation get the entire ordered list up to Takeing first 2?

Answer 1

OrderBy , Take .

, LINQ O (n * log (n)), O (n), .

Answer 2

, OrderBy , . linq , . , , . , :

 public class FriendList
        {
            public List<string> friends { get; set; } // List of friends names
            public DateTime timestamp { get; set; } // date/time on the data file
        }

        public static Tuple<T, T> GetTwoBiggest<T>(IEnumerable<T> array, Comparison<T> comp)
        {
            var enumerator = array.GetEnumerator();

            if (!enumerator.MoveNext()) { throw new ArgumentException("We need collection with at least two items"); }
            T max1 = enumerator.Current;

            if (!enumerator.MoveNext()) { throw new ArgumentException("We need collection with at least two items"); }
            T max2 = enumerator.Current;

            if (comp(max1, max2) < 0)
            {
                T tmp = max1;
                max1 = max2;
                max2 = tmp;
            }

            while (enumerator.MoveNext())
            {
                T actual = enumerator.Current;
                if (comp(actual, max1) > 0)
                {
                    max2 = max1;
                    max1 = actual;
                }
                else if (comp(actual, max2) > 0)
                {
                    max2 = actual;
                }
            }

            return new Tuple<T, T>(max1, max2);
        }

        private void button6_Click(object sender, EventArgs e)
        {
            List<FriendList> list = new List<FriendList>()
            {
                new FriendList() { timestamp = new DateTime(2015,1,1) },
                new FriendList() { timestamp = new DateTime(2015,10,2) },
                new FriendList() { timestamp = new DateTime(2015,5,3) },
                new FriendList() { timestamp = new DateTime(2015,2,4) },
                new FriendList() { timestamp = new DateTime(2015,3,5) },
                new FriendList() { timestamp = new DateTime(2015,7,6) },
                new FriendList() { timestamp = new DateTime(2015,11,7) },
                new FriendList() { timestamp = new DateTime(2015,8,8) },
            };

            var twoBiggest = GetTwoBiggest(list, (a, b) => a.timestamp.CompareTo(b.timestamp));

            Console.WriteLine(twoBiggest.Item1.timestamp);
            Console.WriteLine(twoBiggest.Item2.timestamp);
        }