CODEWITHSUNDEEP
c#linqlistsorting
JGilmartin
JGilmartin

Reputation: 9298

Sort one list by another

I have 2 list objects, one is just a list of ints, the other is a list of objects but the objects has an ID property.

What i want to do is sort the list of objects by its ID in the same sort order as the list of ints.

Ive been playing around for a while now trying to get it working, so far no joy,

Here is what i have so far...

//**************************
//*** Randomize the list ***
//**************************
if (Session["SearchResultsOrder"] != null)
{
    // save the session as a int list
    List<int> IDList = new List<int>((List<int>)Session["SearchResultsOrder"]);
    // the saved list session exists, make sure the list is orded by this
    foreach(var i in IDList)
    {
        SearchData.ReturnedSearchedMembers.OrderBy(x => x.ID == i);
    }
}
else
{
    // before any sorts randomize the results - this mixes it up a bit as before it would order the results by member registration date                        
    List<Member> RandomList = new List<Member>(SearchData.ReturnedSearchedMembers);
    SearchData.ReturnedSearchedMembers = GloballyAvailableMethods.RandomizeGenericList<Member>(RandomList, RandomList.Count).ToList();

    // save the order of these results so they can be restored back during postback
    List<int> SearchResultsOrder = new List<int>();
    SearchData.ReturnedSearchedMembers.ForEach(x => SearchResultsOrder.Add(x.ID));
    Session["SearchResultsOrder"] = SearchResultsOrder;
}

The whole point of this is so when a user searches for members, initially they display in a random order, then if they click page 2, they remain in that order and the next 20 results display.

I have been reading about the ICompare i can use as a parameter in the Linq.OrderBy clause, but i can’t find any simple examples.

I’m hoping for an elegant, very simple LINQ style solution, well I can always hope.

Any help is most appreciated.

Upvotes: 75

Views: 40499

Answers (6)

Sanish Thomas
Sanish Thomas

Reputation: 11

docs = docs.OrderBy(d => docsIds.IndexOf(d.Id)).ToList();

Upvotes: -1

nawfal
nawfal

Reputation: 73163

Join is the best candidate if you want to match on the exact integer (if no match is found you get an empty sequence). If you want to merely get the sort order of the other list (and provided the number of elements in both lists are equal), you can use Zip.

var result = objects.Zip(ints, (o, i) => new { o, i})
                    .OrderBy(x => x.i)
                    .Select(x => x.o);

Pretty readable.

Upvotes: 9

Mark Sowul
Mark Sowul

Reputation: 10600

Not an answer to this exact question, but if you have two arrays, there is an overload of Array.Sort that takes the array to sort, and an array to use as the 'key'

https://msdn.microsoft.com/en-us/library/85y6y2d3.aspx

Array.Sort Method (Array, Array)
Sorts a pair of one-dimensional Array objects (one contains the keys and the other contains the corresponding items) based on the keys in the first Array using the IComparable implementation of each key.

Upvotes: 15

gregsdennis
gregsdennis

Reputation: 8428

Here is an extension method which encapsulates Simon D.'s response for lists of any type.

public static IEnumerable<TResult> SortBy<TResult, TKey>(this IEnumerable<TResult> sortItems,
                                                         IEnumerable<TKey> sortKeys,
                                                         Func<TResult, TKey> matchFunc)
{
    return sortKeys.Join(sortItems,
                         k => k,
                         matchFunc,
                         (k, i) => i);
}

Usage is something like:

var sorted = toSort.SortBy(sortKeys, i => i.Key);

Upvotes: 4

Simon D.
Simon D.

Reputation: 4481

Another LINQ-approach:

 var orderedByIDList = from i in ids 
                       join o in objectsWithIDs
                       on i equals o.ID
                       select o;

Upvotes: 122

Jimmy
Jimmy

Reputation: 91432

One way of doing it:

List<int>  order = ....;
List<Item> items = ....;

Dictionary<int,Item> d = items.ToDictionary(x => x.ID);

List<Item> ordered = order.Select(i => d[i]).ToList();

Upvotes: 26

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