linq filtering such that order remains intact

Question

How to filter a list such that filtering order remains intact in the output.

void Main()
{
    var states = new List<State>() { new State("CA", "California"), 
                                    new State("NY", "New York"), 
                                    new State("AL", "Alabama"),
                                    new State("NV", "Nevada")};
    var myOrder = new List<string>() {"AL", "NY"};

    var result = states.Where(s => myOrder.Exists(c => string.Equals(c, s.Code, StringComparison.OrdinalIgnoreCase)));
    foreach(var res in result)
    {
        Console.WriteLine(res.Code);
    }
    //this outputs - NY AL
}

public class State
{
    public string Code;
    public string Name;
    public State(string code, string name)
    {
        Code = code;
        Name = name;
    }
}

I need the output in the same order as given in myOrder list (expected output - AL NY).

Then look up the state based on myOrder instead of states. That way the ordering is based on the items in myOrder. — Peter Bons, May 26 '17 at 18:08

15ee8f99-57ff-4f92-890c-b56153 · Answer 1 · 2017-05-26T18:17:41.873

If I correctly understand what you want, a join would do that:

static void Main()
{
    //below is a database call, for sake of code simplicity, I have hardcoded few data samples.
    var states = new List<State>() { 
            new State("CA", "California"),
            new State("NY", "New York"),
            new State("AL", "Alabama"),
            new State("NV", "Nevada")
        };

    var myOrder = new List<string>() { "AL", "DE", "NY", "TX" };

    //  Join myOrder...
    var result = myOrder.Join(
        //  to states...
        states,
        //  ...on the whole myOrder item...
        o => o, 
        //  ...equals the Code field of the State
        s => s.Code,
        //  And the result of the join is just the State
        (o, s) => s);

    //  this outputs - AL NY 

    foreach (var res in result)
    {
        Console.WriteLine(res.Code);
    }
}

The easiest way to use the ordering from myOrder is just start there and don't change it. There are a number of ways to do this, but that'll work.

thanks @Ed Plunkett this is more elegant solution ! – DivideByzero May 26 '17 at 18:23 — DivideByzero, May 26 '17 at 18:23

degant · Accepted Answer · 2017-05-26T18:09:06.513

1

Add a condition to OrderBy correctly:

.OrderBy(s => myOrder.IndexOf(s.Code));

Your condition becomes:

var result = states
    .Where(s => myOrder.Exists(c => string.Equals(c, s.Code, StringComparison.OrdinalIgnoreCase)))
    .OrderBy(s => myOrder.IndexOf(s.Code));

Dotnet Fiddle Demo

edited May 26 '17 at 18:09

answered May 26 '17 at 18:06

degant

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1

This doesn't work in L2E. That's why EF questions should not be mixed with L2O examples "for sake of code simplicity" – Ivan Stoev May 26 '17 at 18:08
Thank you @degant, it worked. – DivideByzero May 26 '17 at 18:18
2

@Ivan Stoev, I have checked my code, the db call is returning an IEnumerbale<>, so this logic is an in memory operation. the entity framework tag doesnt really matter in this case. Bad on my part to mention the db call. – DivideByzero May 26 '17 at 18:21
@NandaKumarChintam Then you'd better remove the `entity-framework` tag and the *database call* comment. – Ivan Stoev May 26 '17 at 18:23
1

@Ivan Stoev, done, cheers ! – DivideByzero May 26 '17 at 18:27

linq filtering such that order remains intact

2 Answers2