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

You can do this by using the Select extension taking a Func:

var rotatedList = myList.Select(inner => inner.Select((s, i) => new {s, i}))
                        .SelectMany(a => a)
                        .GroupBy(a => a.i, a => a.s)
                        .Select(a => a.ToList()).ToList();

This will give you another List>.

Breakdown

.Select(inner => inner.Select((s, i) => new {s, i}))

For each inner list, we project the list's content to a new anonymous object with two properties: s, the string value, and i the index of that value in the original list.

.SelectMany(a => a)

We flatten the result to a single list

.GroupBy(a => a.i, a => a.s)

We group by the i property of our anonymous object (recall this is the index) and select the s property as our values (the string only).

.Select(a => a.ToList()).ToList();

For each groups, we changed the enumerable to a list and another list for all the groups.