Sort One Array from Another array order?

Question

var listOne = new string[] { "dog", "cat", "car", "apple"};
var listTwo = new string[] { "car", "apple"};

What I need is to order listOne by the order of items in listTwo (if present). So the new list would be in this order; "car", "apple", "dog", "cat"

I would like to do this in LINQ and have tried this;

var newList = from l1 in listOne
              join l2 in listTwo on l1 equals l2 in temp
              from nl temp.DefaultIfEmpty()
              select nl;

But it returns null so evidently my Linq-Fu is weak. Any advise is appreciated.

Can duplicates exist in listOne? If so, should they be preserved? — Ahmad Mageed
– Ahmad Mageed, Commented Mar 1, 2011 at 18:35

David Ruttka · Accepted Answer · 2011-03-01 18:39:24Z

4

You need all the items from listTwo that are in listOne followed by the remaining items from listOne?

var results = listTwo.Intersect(listOne).Union(listOne);
foreach (string r in results)
{
    Console.WriteLine(r);
}

edited Mar 1, 2011 at 18:39

answered Mar 1, 2011 at 18:29

David Ruttka

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7 Comments

Mark Sowul Over a year ago

Why do you need the second Except? It's a union; they won't appear twice.

David Ruttka Over a year ago

I had changed to Concat for a minute, but you're right. Union without the Except is probably more elegant. listTwo.Intersect(listOne).Union(listOne);

David Ruttka Over a year ago

@Lasse my original answer was a Union instead of a Concat. Mark is correct.

Ahmad Mageed Over a year ago

This could be further shortened to just listTwo.Union(listOne). Per MSDN: "This method excludes duplicates from the return set."

David Ruttka Over a year ago

@Ahmad if listTwo contained "orange" but listOne did not, your solution would include it but mine would not. The way I read it, listTwo should only be used to order the items of listOne and therefore orange should not be included.

|

Lasse V. Karlsen · Accepted Answer · 2011-03-01 18:26:58Z

3

Well, let me rephrase your sorting requirements:

First, all the items from the first list, that is also present in the second list, and return them in the order they appear in that second list, or: all the elements from the second list, if they appear in the first list
Then, all the items from the first list, that is not in that second list, and return them in the order they appear in that first list

This code would do the trick:

void Main()
{
    var listOne = new string[] { "dog", "cat", "car", "apple"};
    var listTwo = new string[] { "car", "apple"};

    var elements1 =
        from element in listTwo
        where listOne.Contains(element)
        select element;
    var elements2 =
        from element in listOne
        where !listTwo.Contains(element)
        select element;

    elements1.Concat(elements2).Dump();
}

You can also rewrite it without the LINQ syntax to make it a bit shorter:

void Main()
{
    var listOne = new string[] { "dog", "cat", "car", "apple"};
    var listTwo = new string[] { "car", "apple"};

    var elements = listTwo.Where(e => listOne.Contains(e))
        .Concat(listOne.Where(e => !listTwo.Contains(e)));

    elements.Dump();
}

Output (through LINQPad):

car 
apple 
dog 
cat

answered Mar 1, 2011 at 18:26

Lasse V. Karlsen

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2 Comments

digEmAll Over a year ago

Good solution, BTW having listOne {"A","A","B","C"} and listTwo{"A","C"} IMO should yield {"A","A","C","B"} not {"A","C","B"} as in your code...don't know if OP needs this though ;)

Mark Sowul Over a year ago

Incidentally Intersect and Except are far more efficient than Where..Contains

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