How to “zip” or “rotate” a variable number of lists?

Question

If I have a list containing an arbitrary number of lists, like so:

var myList = new List>()
{
    new List() { \"a\",          


        

Answer 1

(from count in Range(myList[0].Count)
select new List<string>(
    from count2 in Range(myList.Count)
    select myList[count2][count])
    ).ToList();

It ain't pretty, but I think it'll work.

Answer 2

You can condense for loops using Range:

var result = Enumerable.Range(0, myList.Min(l => l.Count))
    .Select(i => myList.Select(l => l[i]).ToList()).ToList();

Answer 3

How about using SelectMany and GroupBy with some indexes?

// 1. Project inner lists to a single list (SelectMany)
// 2. Use "GroupBy" to aggregate the item's based on order in the lists
// 3. Strip away any ordering key in the final answer
var query = myList.SelectMany(
    xl => xl.Select((vv,ii) => new { Idx = ii, Value = vv }))
       .GroupBy(xx => xx.Idx)
       .OrderBy(gg => gg.Key)
       .Select(gg => gg.Select(xx => xx.Value));

From LinqPad:

Answer 4

Here is a recursive ZipMany implementation inspired from an answer of @Enigmativity to a different question.

public static IEnumerable<IEnumerable<T>> ZipMany<T>(params IEnumerable<T>[] sources)
{
    return ZipRecursive(sources);
    IEnumerable<IEnumerable<T>> ZipRecursive(IEnumerable<IEnumerable<T>> ss)
    {
        if (!ss.Skip(1).Any())
        {
            return ss.First().Select(i => Enumerable.Repeat(i, 1));
        }
        else
        {
            return ZipRecursive(ss.Skip(1).ToArray()).Zip(ss.First(), (x, y) => x.Prepend(y));
        }
    }
}

Advantage: avoids problems related to disposition of enumerators.
Disadvantage: Scales badly. The recursion overhead becomes noticeable at around 3000 enumerables, and grows exponentially afterwards.

Answer 5

Take a look at the linqlib project on codeplex, it has a rotate function that does exactly what you need.

Answer 6

You can roll your own ZipMany instance which manually iterates each of the enumerations. This will likely perform better on larger sequences than those using GroupBy after projecting each sequence:

public static IEnumerable<TResult> ZipMany<TSource, TResult>(
    IEnumerable<IEnumerable<TSource>> source,
    Func<IEnumerable<TSource>, TResult> selector)
{
   // ToList is necessary to avoid deferred execution
   var enumerators = source.Select(seq => seq.GetEnumerator()).ToList();
   try
   {
     while (true)
     {
       foreach (var e in enumerators)
       {
           bool b = e.MoveNext();
           if (!b) yield break;
       }
       // Again, ToList (or ToArray) is necessary to avoid deferred execution
       yield return selector(enumerators.Select(e => e.Current).ToList());
     }
   }
   finally
   {
       foreach (var e in enumerators) 
         e.Dispose();
   }
}