Combine entries from two lists by position using LINQ

Question

Say I have two lists with following entries

List<int> a = new List<int> { 1, 2, 5, 10 };
List<int> b = new List<int> { 6, 20, 3 };

I want to create another List c where its entries are items inserted by position from two lists. So List c would contain the following entries:

List<int> c = {1, 6, 2, 20, 5, 3, 10}

Is there a way to do it in .NET using LINQ? I was looking at .Zip() LINQ extension, but wasn't sure how to use it in this case.

Thanks in advance!

Answer 1

To do it using LINQ, you can use this piece of LINQPad example code:

void Main()
{
    List<int> a = new List<int> { 1, 2, 5, 10 };
    List<int> b = new List<int> { 6, 20, 3 };

    var result = Enumerable.Zip(a, b, (aElement, bElement) => new[] { aElement, bElement })
        .SelectMany(ab => ab)
        .Concat(a.Skip(Math.Min(a.Count, b.Count)))
        .Concat(b.Skip(Math.Min(a.Count, b.Count)));

    result.Dump();
}

Output:

This will:

• Zip the two lists together (which will stop when either runs out of elements)
• Producing an array containing the two elements (one from a, another from b)
• Using SelectMany to "flatten" this out to one sequence of values
• Concatenate in the remainder from either list (only one or neither of the two calls to Concat should add any elements)

---

Now, having said that, personally I would've used this:

public static IEnumerable<T> Intertwine<T>(this IEnumerable<T> a, IEnumerable<T> b)
{
    using (var enumerator1 = a.GetEnumerator())
    using (var enumerator2 = b.GetEnumerator())
    {
        bool more1 = enumerator1.MoveNext();
        bool more2 = enumerator2.MoveNext();

        while (more1 && more2)
        {
            yield return enumerator1.Current;
            yield return enumerator2.Current;

            more1 = enumerator1.MoveNext();
            more2 = enumerator2.MoveNext();
        }

        while (more1)
        {
            yield return enumerator1.Current;
            more1 = enumerator1.MoveNext();
        }

        while (more2)
        {
            yield return enumerator2.Current;
            more2 = enumerator2.MoveNext();
        }
    }
}

Reasons:

• It doesn't enumerate a nor b more than once
• I'm skeptical about the performance of Skip
• It can work with any IEnumerable<T> and not just List<T>

Answer 2

I'd create an extension method to do it.

public static List<T> MergeAll<T>(this List<T> first, List<T> second)
{
    int maxCount = (first.Count > second. Count) ? first.Count : second.Count;
    var ret = new List<T>();
    for (int i = 0; i < maxCount; i++)
    {
        if (first.Count < maxCount)
            ret.Add(first[i]);
        if (second.Count < maxCount)
            ret.Add(second[i]);
    }

    return ret;
}

This would iterate through both lists once. If one list is bigger than the other it will continue to add until it's done.