Question

Assume that you have two IEnumerbale objects. How we can merge them (in some condition e.g merge in merge sort …) and create a unique IEnumerable? I tried this with Zip, but in Zip the two list sizes should be equal (maybe you didn’t get exception but maybe we have some data lost.)

In addition, I try it by using Enumerable.Range(…).Select(…) but i didn’t get an acceptable result.

Furthermore, my question is totally different from using Union or this one, in fact as I said like merge in merge sort I like to preserve lists order (in fact just want to fill some gaps in first list).

It’s easy to handle it with for loop, but i can’t see any full linq way.

Edit:

Sample input:

lst1 = {5,10,12}
lst2 = {7,9,16,20,25}

result: {5,7,9,10,12,16,20,25}

this can be done with a for loop and two pointer in O(n + m) but I’m looking for linq solution in O(n+m)

for loop solution:

        var lst1 = new List<int> { 5, 10, 12 };
        var lst2 = new List<int> { 7, 9, 16, 20, 25 };

        var result = new List<int>();

        int j = 0;
        for (int i = 0; i < lst1.Count; i++)
        {
            while (j < lst2.Count && lst2[j] < lst1[i])
            {
                result.Add(lst2[j]);
                j++;
            }
            result.Add(lst1[i]);
        }

        while (j < lst2.Count)
        {
            result.Add(lst2[j]);
            j++;
        }
        Console.WriteLine(string.Join(",", result.ToArray()));

Answer 1

There is no such method in LINQ. And I don’t think it’s possible to combine the existing methods to do exactly what you want (if it was, it would be overly complicated).

But implementing such method yourself isn’t that hard:

static IEnumerable<T> Merge<T>(this IEnumerable<T> first,
                               IEnumerable<T> second,
                               Func<T, T, bool> predicate)
{
    // validation ommited

    using (var firstEnumerator = first.GetEnumerator())
    using (var secondEnumerator = second.GetEnumerator())
    {
        bool firstCond = firstEnumerator.MoveNext();
        bool secondCond = secondEnumerator.MoveNext();

        while (firstCond && secondCond)
        {
            if (predicate(firstEnumerator.Current,  secondEnumerator.Current))
            {
                yield return firstEnumerator.Current;
                firstCond = firstEnumerator.MoveNext();
            }
            else
            {
                yield return secondEnumerator.Current;
                secondCond = secondEnumerator.MoveNext();
            }
        }

        while (firstCond)
        {
            yield return firstEnumerator.Current;
            firstCond = firstEnumerator.MoveNext();
        }

        while (secondCond)
        {
            yield return secondEnumerator.Current;
            secondCond = secondEnumerator.MoveNext();
        }
    }
}

And you could use it like this:

lst1.Merge(lst2, (i, j) => i < j)