Ordering list by a specific order

Question

I have a List and I have the new order in which the List should have the items in int[] I want that the item in List should be re-ordered as per the items in int[]. Here is my code to do that:

   class Program
    {
        static void Main(string[] args)
        {
            List<Test> tests = new List<Test>() { 
                new Test(){ No = 201 },
                new Test(){ No = 101 },
                new Test(){ No = 300 },
                new Test(){ No = 401 },
                new Test(){ No = 500 },
                new Test(){ No = 601 }
            };


            int[] newOrder = new int[6] { 201, 401, 300, 101, 601, 500 };

            //after the opration the List should contain items in order 201, 401, 300, 101, 601, 500

            List<Test> newTests = new List<Test>();

            foreach(var order in newOrder)
            {
                var item = tests.SingleOrDefault(t => t.No == order);

                if (item != null)
                    newTests.Add(item);
            }


        }

    }

This works fine. But it creates a separate List and performs operation on it. Is there better way where I can use may be built in .Net operation for this or may be perform the operation on the same List without creating these Temp List etc?

Thank you.

Answer 1

You need to think about performance when running a sort like this.

If you expect only a handful of elements, then Pedro's solution is OK.

If you expect to have many elements (say, 100's or 1000's), then it's not a good idea to search through entire collection of tests for each element in newOrder. In this case, it would be helpful to use a Dictionary for all index/sort-order lookups. Try something like this:

List<Test> tests = new List<Test>() { 
    new Test(){ No = 101 },
    new Test(){ No = 201 },
    new Test(){ No = 300 },
    new Test(){ No = 401 },
    new Test(){ No = 500 },
    new Test(){ No = 601 }
};


int[] newOrder = new int[6] { 201, 401, 300, 101, 601, 500 };

// Create a Dictionary/hashtable so we don't have to search in newOrder repeatedly
// It will look like this: { {201,0}, {401,1}, {300,2}, {101,3}, {601,4}, {500,5} }
Dictionary<int, int> newOrderIndexedMap = Enumerable.Range(0, newOrder.Length - 1).ToDictionary(r => newOrder[r], r => r);

// Order using 1 CPU
var orderedTests = tests.OrderBy(test => newOrderIndexedMap[test.No]);
// Order using multi-threading
var orderedInParallelTests = tests.AsParallel().OrderBy(test => newOrderIndexedMap[test.No]);

// Order using 1 CPU, when it's possible that a match will not be found in newOrder
var orderedTestsSafe = tests.OrderBy(test => 
    {
        int index;
        bool foundIndex = newOrderIndexedMap.TryGetValue(test.No, out index);
        return foundIndex ? index : Int32.MaxValue;
    });

Note that both this answer and Pedro's assume that newOrder contains all values contained in the tests elements and vice versa.

Answer 2

var newTesties= newOrder.Select(o => tests.First(t => t.No == o));

Basically, I'm selecting every number 'o' in newOrder, and using that to pick up the corresponding test. You WILL end up with a new list though.