Question

I have:-

IEnumerable<IEnumerable<T>> items;

and I’d like to create:-

IEnumerable<IEnumerable<T>> results;

where the first item in “results” is an IEnumerable of the first item of each of the IEnumerables of “items”, the second item in “results” is an IEnumerable of the second item of each of “items”, etc.

The IEnumerables aren’t necessarily the same lengths. If some of the IEnumerables in items don’t have an element at a particular index, then I’d expect the matching IEnumerable in results to have fewer items in it.

For example:-

items = { "1", "2", "3", "4" } , { "a", "b", "c" };
results = { "1", "a" } , { "2", "b" }, { "3", "c" }, { "4" };

Edit: Another example (requested in comments):-

items = { "1", "2", "3", "4" } , { "a", "b", "c" }, { "p", "q", "r", "s", "t" };
results = { "1", "a", "p" } , { "2", "b", "q" }, { "3", "c", "r" }, { "4", "s" }, { "t" };

I don’t know in advance how many sequences there are, nor how many elements are in each sequence. I might have 1,000 sequences with 1,000,000 elements in each, and I might only need the first ~10, so I’d like to use the (lazy) enumeration of the source sequences if I can. In particular I don’t want to create a new data structure if I can help it.

Is there a built-in method (similar to IEnumerable.Zip) that can do this?

Is there another way?

Answer 1

Now lightly tested and with working disposal.

public static class Extensions
{
  public static IEnumerable<IEnumerable<T>> JaggedPivot<T>(
    this IEnumerable<IEnumerable<T>> source)
  {
    List<IEnumerator<T>> originalEnumerators = source
      .Select(x => x.GetEnumerator())
      .ToList();

    try
    {
      List<IEnumerator<T>> enumerators = originalEnumerators
        .Where(x => x.MoveNext()).ToList();

      while (enumerators.Any())
      {
        List<T> result = enumerators.Select(x => x.Current).ToList();
        yield return result;
        enumerators = enumerators.Where(x => x.MoveNext()).ToList();
      }
    }
    finally
    {
      originalEnumerators.ForEach(x => x.Dispose());
    }
  } 
}

public class TestExtensions
{
  public void Test1()
  {
    IEnumerable<IEnumerable<int>> myInts = new List<IEnumerable<int>>()
    {
      Enumerable.Range(1, 20).ToList(),
      Enumerable.Range(21, 5).ToList(),
      Enumerable.Range(26, 15).ToList()
    };

    foreach(IEnumerable<int> x in myInts.JaggedPivot().Take(10))
    {
      foreach(int i in x)
      {
        Console.Write("{0} ", i);
      }
      Console.WriteLine();
    }
  }
}