Sorting a dictionary by keys in the order in an Arraylist

Question

I was asked the following question in an interview. How can I sort a Dictionary by the key, in the order which is in an array list.

So for example I have a dictionary as follows

Dictionary<string, string> stringDict = new Dictionary<string, string>();

stringDict.Add("1", "One");
stringDict.Add("7", "Seven");
stringDict.Add("6", "Six");
stringDict.Add("2", "Two");
stringDict.Add("3", "Three");
stringDict.Add("5", "Five");
stringDict.Add("4", "Four");

And an array list as follows

ArrayList stringArList = new ArrayList();

stringArList.Add("1");
stringArList.Add("2");
stringArList.Add("3");
stringArList.Add("5");
stringArList.Add("6");
stringArList.Add("7");
stringArList.Add("4");

How can I sort the dictionary in the order it is in the array list?

Answer 1

Well you can't sort a Dictionary per se, but you can extract the key-values pairs as a list and sort those:

IEnumerable<KeyValuePair<string, string>> pairs = 
    stringDict.OrderBy(kvp => stringArList.IndexOf(kvp.Key));

But there's not a way to "traverse" dictionary items in any particular order.

You could create a SortedDictionary and provide an IComparer<string>

var d = new SortedDictionary<string, string>(stringDict, 
                                        new PositionComparer(stringArList));

With the Comparer implementation as:

public class PositionComparer : IComparer<string>
{
   private ArrayList Keys {get; set;}

   public PositionComparer(ArrayList keys)
   {
       Keys = keys;
   }

   public int Compare(string s1, string s2)
   {
       return Keys.IndexOf(s1).CompareTo(Keys.IndexOf(s2));
   }
}

Answer 2

This will produce a list of the values sorted as required.

var sortedValues = stringDict.OrderBy(pair => stringArList.IndexOf(pair.Key))
                             .Select(pair => pair.Value)
                             .ToList();