C# Two lists to one list, LinQ

Question

C# Two lists to one list, LinQ

Markus Freitag 3,101

Hello,
I have two lists and need a list as result. Is there a way to solve this with LinQ?
If so, how?

My attempts, it would be better maybe to load it into a dictionary, but how?

HashSet<Tuple<string, bool>> hsResultList = new HashSet<Tuple<string, bool>>();  
  
 for (int side = 0; side <= 2; side++)   // Side 1 and Side 2  
 {  
  
 foreach (var sB in CurrentPanel.ListSides[(int)side - 1].Boards )  
 {  
 var qryAlreadyInside = hsResultList.Where(x => x.Item1.Contains(sB.Index)).FirstOrDefault();  
 if ( qryAlreadyInside  == null )  
 hsResultList.Add(new Tuple<string, bool>(sB.Index, sB.Badmark));

If the status is false in one list, the entire status must be false in the merged dictionary.

var dicSide1 = CurrentPanel.ListSides[0].ListSingleBoards.ToDictionary(x => x.Index, v => v.State);  
var dicSide2 = CurrentPanel.ListSides[1].ListSingleBoards.ToDictionary(x => x.Index, v => v.State);  
var dicResultSide1Side2Badmarks = dicSide1.Union(dicSide2.Where(k => !dicSide1.ContainsKey(k.Key))).ToDictionary(k => k.Key, v => v.Value);  
  
int badBoards = dicResultSide1Side2Badmarks.Where(x => x.Value == false).Count();  
int goodBoards = dicResultSide1Side2Badmarks.Where(x => x.Value == true).Count();

Answer 1

Timon Yang-MSFT 9,511

You can also consider this method, merge them, and then group them to determine whether there is false in each group:

        public static void Main()  
        {  
            List<A> list1 = new List<A>() {  
            new A(){ Index = 1, State = false },  
            new A(){ Index = 2, State = false },  
            new A(){ Index = 3, State = true },  
            new A(){ Index = 4, State = false },  
            new A(){ Index = 5, State = false },  
        };  
            List<A> list2 = new List<A>() {  
            new A(){ Index = 1, State = false },  
            new A(){ Index = 2, State = true },  
            new A(){ Index = 3, State = true },  
            new A(){ Index = 4, State = false },  
            new A(){ Index = 5, State = false },  
            new A(){ Index = 6, State = true },  
            new A(){ Index = 7, State = false },  
        };  
            var list = list1.Concat(list2);  
  
            var re = from a in list  
                     group a by a.Index into g  
                     select new  
                     {  
                         Key = g.Key,  
                         Value = g.Where(x => x.State == false).Count() > 0 ? false : true  
                     };  
            Dictionary<int, bool> dict1 = re.ToDictionary(x => x.Key, y => y.Value);  
        }

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Markus Freitag 3,101

Thanks too, looks good.

G9xSCu

I have all possibilities here. Do you know if I can set a breakpoint at the dotnetfiddle?

Can I write the query like this?
//var qry = list.GroupBy(x => x.Index).Select(y => y.Key, z =>

            var list = list1.Concat(list2);

            var re = from a in list
                     group a by a.Index into g
                     select new
                     {
                         Key = g.Key,
                         Value = g.Where(x => x.State == true).Count() > 0 ? true : false
                     };
            //var qry = list.GroupBy(x => x.Index).Select(y => y.Key, z => z.v)))

            Dictionary<int, bool> dict1 = re.ToDictionary(x => x.Key, y => y.Value);

Timon Yang-MSFT 9,511 Reputation points

2020-12-04T09:22:22.387+00:00

@Markus Freitag
The code can be written like this:
var re1 = list.GroupBy(a => a.Index).Select((g, index) => new { g.Key, Value = g.Where(x => x.State == false).Count() > 0 ? false : true });
I noticed that you modified Value = xxx in the code. Now as long as there is true in the group, the corresponding Value in the result is true, which seems to be contrary to your original description.
Did the requirements change, or was it accidentally wrong?
By the way, I don't know how to add a breakpoint in that website, I haven't used it.
Markus Freitag 3,101 Reputation points

2020-12-04T09:34:09.073+00:00

Did the requirements change, or was it accidentally wrong?

I wanted to test in case the requirement of the check should be changed. So I have all possibilities. Thanks a lot.

Answer 2

Cheong00 3,426

My take on this with LINQ: https://dotnetfiddle.net/RlLMHU

Markus Freitag 3,101 Reputation points

2020-12-03T11:22:05.217+00:00

Ok, thanks, but looks complicated. Is there a solution with Dictionary? Can you explain anything else about it?
Cheong00 3,426 Reputation points

2020-12-03T11:41:47.637+00:00

LINQ syntax do not offer "full join" directly, so you have to do left join then right join and use .Union() to combine them.

Modified a bit to change to Dictionary<int, A>: https://dotnetfiddle.net/TexkOH
Markus Freitag 3,101 Reputation points

2020-12-03T12:08:19.517+00:00
OK, your solution is to create a left and a right result list first. Then a merged result list with union. Right?

Sample aj8MEk

OK, if I need to check with true, I can go this way, with | instead &, right?

var left = (from l1 in dicSide1 join l2 in dicSide2 on l1.Key equals l2.Key into j from l2 in j.DefaultIfEmpty() select new Dictionary<string, bool>() { Keys = l1.Key, Value = l1.Value | (l2.Key == null ? false : l2.Value) } );

Can we use for all a dictionary?
Cheong00 3,426 Reputation points

2020-12-04T01:35:54.757+00:00

Correct.

For the second question, you can use forall on it. See example here.

Also check JaliyaUdagedara's answer, his method is way simpler than mine by simply decide which list is longer than use it as "left" and do it once only. You can use this method if you can ensure both list does not have skipping number as index.

Afterwards you can use my code to do the select directly, and place the result into dictionary.
Cheong00 3,426 Reputation points

2020-12-04T02:42:21.117+00:00

If you don't have any reason to use LINQ, I think use Dictionary with "the good old way" to do it would be the most efficient.

https://dotnetfiddle.net/QWdJV6

Answer 3

I'd do something like this:

using System;
using System.Collections.Generic;
using System.Linq;

namespace scratch
{
    class Program
    {
        static void Main(string[] args)
        {
            var listOne = new List<bool> { true, false, true, false, true, false, true, true };
            var listTwo = new List<bool> { false, true, true };
            var result = Merge(listOne, listTwo);
        }

        static List<bool> Merge(List<bool> one, List<bool> two)
        {
            var longer = one.Count > two.Count ? one : two;
            var shorter = one.Count > two.Count ? two : one;
            return longer
                .Select((item, ix) => ix >= shorter.Count ? item : item && shorter[ix])
                .ToList();
        }
    }
}

The merge method takes in two lists. First it finds the longer of the two, and does a logical AND on the two values or just takes the remaining values.

I don't think there is much reason to load these into a Dictionary first since you can always get values out of a arrays by index in constant time.

An extra concise version might look like this:

static List<bool> Merge2(List<bool> one, List<bool> two)
{
    return Enumerable.Range(0, Math.Max(one.Count, two.Count))
        .Select(ix =>
            (ix >= one.Count || one[ix])
            &&
            (ix >= two.Count || two[ix]))
        .ToList();
}

It does the same as above - but is a one-liner.

Answer 4

May be something simple,

var list1 = new List<A>()
{
    new A(){ Index = 1, State = false },
    new A(){ Index = 2, State = false },
    new A(){ Index = 3, State = true },
    new A(){ Index = 4, State = false },
    new A(){ Index = 5, State = false },
};

var list2 = new List<A>()
{
    new A(){ Index = 1, State = false },
    new A(){ Index = 2, State = true },
    new A(){ Index = 3, State = false },
    new A(){ Index = 4, State = false },
    new A(){ Index = 5, State = false },
    new A(){ Index = 6, State = true },
    new A(){ Index = 7, State = false },
};

List<A> leftList = list2;
List<A> rightList = list1;
if (list1.Count > list2.Count)
{
    leftList = list1;
    rightList = list2;
}

IEnumerable<A> finalList = leftList
    .Select(x =>
    {
        var matchingState = rightList.SingleOrDefault(y => y.Index == x.Index)?.State;
        var finalState = matchingState == null ? x.State : matchingState.Value && x.State;

        return new A
        {
            Index = x.Index,
            State = finalState
        };
    });

Output:

1 False
2 False
3 False
4 False
5 False
6 True
7 False

Answer 5

Another option:

using System;
using System.Collections.Generic;
using System.Linq;
using Newtonsoft.Json;

public class Program
{
    public static void Main()
    {
        var first = new List<Item>{new Item(1,false), new Item(2, false), new Item(3,true), new Item(4,false), new Item(5,false)};
        var second = new List<Item>{new Item(1,false), new Item(2,true), new Item(3,false), new Item(4,false), new Item(5,false), new Item(6,true), new Item(7,false)};

        // Pure LINQ
        var result = second.Zip(first, (firstItem, secondItem) => new Item(firstItem.Index,  firstItem.State && secondItem.State)).Concat(second.Skip(first.Count));

        Console.WriteLine(JsonConvert.SerializeObject(result));
    }

    private record Item(int Index, bool State);
}

C# Two lists to one list, LinQ

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