Combine entries from two lists by position using LINQ

.net, c#, linq

Solution

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>`

Problem

Original source

Related problems

• How do I merge (or zip) two IEnumerables together?