How to combine multiple lists with custom sequence

Question

Not sure if there's a algorithm to describe this problem but are there any elegant methods to combine the list in a custom sequence. For example:

List<string> list1 = new List<string>();
List<string> list2 = new List<string>();
List<string> list3 = new List<string>();

list1.Add("a");
list1.Add("b");
list1.Add("c");

list2.Add("d");
list2.Add("e");
list2.Add("f");

list3.Add("g");
list3.Add("h");
list3.Add("i");

List<string> combined = new List<string>();

I would like the contents of combined to contain a sequence as follows:

a //First record in list1
d //First record in list2
g //First record in list3
b //Second record in list1
e //Second record in list2
h //Second record in list3
c //Third record in list1 
f //Third record in list2 
i //Third record in list3

The number of records in each list may not be equal.

EDIT

When the number of records in each list may not be equal i mean:

List<string> list1 = new List<string>();
List<string> list2 = new List<string>();
List<string> list3 = new List<string>();

list1.Add("a");

list2.Add("b");
list2.Add("c");

list3.Add("d");
list3.Add("e");
list3.Add("f");

List<string> combined = new List<string>();

Expected results:

a //First record in list1
b //First record in list2
d //First record in list3
c //Second record in list2
e //Second record in list3
f //Third record in list3

@HenkHolterman: I really don't see the relation between easy and LINQ. A loop will be enought and still be easy. — Ignacio Soler Garcia, Mar 14 '13 at 14:49
For 2 lists the answer would have been `list1.Zip(list2)`. That is easier than a loop. — H H, Mar 14 '13 at 15:16

Tim Rogers · Accepted Answer · 2013-03-14T10:03:04.257

4

Not sure if there's a name. Merging? Splicing? But the code is easy.

var lists = new [] { list1, list2, list3 };
var combined = new List<string>(lists.Sum(l => l.Count));    

for (var i = 0; i < lists.Max(l => l.Count); i++)
{
   foreach (var list in lists)
   { 
      if (i < list.Count)
          combined.Add (list[i])
   }
}

edited Mar 14 '13 at 10:03

answered Mar 14 '13 at 09:46

Tim Rogers

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Close, but _The number of records in each list may not be equal._ – H H Mar 14 '13 at 09:55
What if list1.Count is higher than list2/list3.Count? – Oliver Mar 14 '13 at 09:55
Missed that requirement. This version works with any number of variable length lists. – Tim Rogers Mar 14 '13 at 10:03
Yes, sorry just edited my question. Let me test this code :) – atp03 Mar 14 '13 at 10:05
I think this code does it. Thanks! Is this known as splicing? – atp03 Mar 14 '13 at 10:10
In MoreLinq a similar method is called Zip. Splicing is more like splitting, the reverse. Call it TripleZip. – H H Mar 14 '13 at 10:12
I prefer my generic implementation :) – Oliver Mar 14 '13 at 10:12
Resharper suggests this if you prefer less lines: for (var i = 0; i < lists.Max(l => l.Count); i++) { combined.AddRange(from list in lists where i < list.Count select list[i]); } – atp03 Mar 14 '13 at 10:13

score 2 · Answer 2 · answered Mar 14 '13 at 09:50

int MaxCount = List1.Count; //Or whatever the highest list count.

            for (int i = 0; i < MaxCount; i++)
                {
                    if( list1.Count > i)
                    combined.Add(list1[i]);

                    if( list2.Count > i)
                    combined.Add(list2[i]);

                    if( list3.Count > i)
                    combined.Add(list3[i]);
                }

Oliver · Answer 3 · 2013-03-14T10:51:15.630

1

You could use this generic Merge<T> function:

    public static IEnumerable<T> Merge<T>(params List<T>[] lists)
    {
        var max = lists.Max(list => list.Count());

        for (int i = 0; i < max; i++)
        {
            foreach (var list in lists)
            {
                if (i < list.Count)
                {
                    yield return list[i];
                }
            }
        }
    }

Example usage:

var merged = Merge(list1, list2, list3).ToList();

edited Mar 14 '13 at 10:51

answered Mar 14 '13 at 09:43

Oliver

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No. Take a better look a the required result. – H H Mar 14 '13 at 09:48
This yields: a,b,c,d,e,f,g,h,i – atp03 Mar 14 '13 at 09:48
The new code will not deal with the remainders of the longer lists. . – H H Mar 14 '13 at 09:56

score 1 · Answer 4 · answered Mar 14 '13 at 09:55

Unfortunately there are no built in function for it. You should loop each list and add it to a new custom list.

Example:

List<string> result = new List<string>();
int listLength = Math.Max(list1.Count, Math.Max(list2.Count, list3.Count)); // get the largest list length
for(int index = 0; index < listLength; i++){
  if(list1.Count > index) result.Add(list1[index);
  if(list2.Count > index) result.Add(list3[index);
  if(list3.Count > index) result.Add(list3[index);
}

score 0 · Answer 5 · answered Mar 14 '13 at 10:00

A quick LINQPad example:

void Main()
{
    List<string> list1 = new List<string>();
    List<string> list2 = new List<string>();
    List<string> list3 = new List<string>();

    list1.Add("a");
    list1.Add("b");
    list1.Add("c");

    list2.Add("d");
    list2.Add("e");
    list2.Add("f");

    list3.Add("g");
    list3.Add("h");
    list3.Add("i");

    Merge(new[] { list1, list2, list3}, (c1, c2) => c1 + c2).SelectMany(s => s).Dump();
}

IEnumerable<T> Merge<T>(IEnumerable<IEnumerable<T>> sources, Func<T, T, T> combine)
{
    return sources.Aggregate((s1, s2) => s1.Zip(s2, combine));
}

The result is an IEnumerable<char> but this is simple enough to convert into a List<string> if necessary.

score -2 · Answer 6 · edited May 23 '17 at 12:09

-2

You can either use Concate() or AddRange, see this thread for more info about the difference between these methods

edited May 23 '17 at 12:09

Community

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1

answered Mar 14 '13 at 09:50

Diederik Koerselman

26
2

How to combine multiple lists with custom sequence

6 Answers6