Question

I have several numbered lists stored in a List<string>:

List<string> all = new List<string>()
{
  "1. Apple",
  "2. Banana",
  "3. Coconut",
  "1. Ant",
  "2. Beaver",
  "3. Cat"
  ...
}

I want to split this list into a list of lists where each list contains 1-3.

List<List<string>> split = new List<List<string>>()
{
  new List<string>() { "1. Apple", "2. Banana", "3. Coconut"},
  new List<string>() { "1. Ant", "2. Beaver", "3. Cat"}
}

There will ALWAYS be “1.” so I can use that as my delimiter. Is there a slick way to do this with LINQ without needing two nested for loops?

Update: I’d like this to be generalized for any length, not always 3.

Answer 1

Just another option to get desired result (assign group index to each item in sequence, then group by that index):

int groupIndex = 0;
List<List<string>> split = all.Select(s => {
                                   if (s.StartsWith("1."))
                                      groupIndex++;
                                   return new { groupIndex, s }; })
                              .GroupBy(x => x.groupIndex)
                              .Select(g => g.Select(x => x.s).ToList())
                              .ToList();

---

Another option – accumulate results (this will require one traverse over the list)

List<List<string>> split =
    all.Aggregate(new List<List<string>>(), (acc, s) =>
                    { 
                        if (s.StartsWith("1."))
                            acc.Add(new List<string>());
                        acc[acc.Count - 1].Add(s);
                        return acc; 
                    });