Sorting lists in c#

Question

I have two lists in C#.

public List<MyClass> objectList = new List<MyClass>(); // it is filled with MyClass objects

public List<int> numberList = new List<int>(); // it is filled with numbers

The index of numbers in the numberList correspond to object index in the objectList: For example: objectList[0] = o1 and numberList[0] = 3 ; objectList[1] = o2 and numberList[1] = 5 ...

objectList:             |o1 | o2 | o3 | o4 | o5 | ...
numberList:              3     5    6    1    4 ...

I want to sort the numbers in the numberList in ascending order and I want for the objetcs in objectList to move with them: After sorting:

objectList:             |o4 | o1 | o5 | o2 | o3 | ...
numberList:              1     3    4    5    6 ...

In practical use I need this for implementing the Hill climbing algorithm on a N queen problem. In the objectList I store positions of all the queens on the board and in the numberList I store the calculated heuristics for the positions. Then I want to sort the numberList so I get the position with the lowest heuristic value. The goal is to move to the position with the lowest heuristic value.

Better to combine them into the same list using a wrapping object :/ — user1228
– user1228, Commented Mar 7, 2017 at 17:05
If your values are connected, why are you keeping them in separate lists? In any event, what's your question? What isn't working? — David
– David, Commented Mar 7, 2017 at 17:06
There is an Array.Sort overload that will do this for you. So you could create temporary arrays from your lists, do the sort, and then copy the array contents back. To my knowledge, there is no corresponding functionality in the List<T> API, or in LINQ. — Jim Mischel
– Jim Mischel, Commented Mar 7, 2017 at 18:10
Why was this question closed? It's perfectly clear what he's asking to do, and it's not at all an uncommon operation. — Jim Mischel
– Jim Mischel, Commented Mar 7, 2017 at 18:16

Eric Lippert · Accepted Answer · 2017-03-08 17:00:20Z

Transform your object list into a sequence of items paired with their indices:

var pairs = objectList.Select(item, index) => new { item, index };

Now you have something you can use to do an ordering:

var orderedPairs = pairs.OrderBy(pair => numberList[pair.index]);

Now you have an ordered list of pairs. Turn that back into an ordered list of items:

var ordered = orderedPairs.Select(pair => pair.item);

and turn it into a list:

var orderedList = ordered.ToList();

Note that your original lists are not altered. This creates a new list that is in the order you want.

Of course you can do it all in one expression if you like:

objectList = objectList
  .Select((item, index) => new { item, index } )
  .OrderBy(pair => numberList[pair.index])
  .Select(pair => pair.item)
  .ToList();

Now, all that said: it sounds like you're doing too much work here because you've chosen the wrong data structure. It sounds to me like your problem needs a min heap implementation of a priority queue, not a pair of lists. Is there some reason why you're not using a priority queue?

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