Using C# , LINQ how to pick items between markers again and again?

Question

Say I have an array of strings:

string[] strArray = {"aa", "bb", "xx", "cc", "xx", "dd", "ee", "ff", "xx","xx","gg","xx"};

How do I use LINQ to extract the strings between the "xx" markers as groups?

Say by writing them to the console as:

cc
dd,ee,ff
gg

Answer 1

A pure-functional solution (mutation-free):

string[] strArray = { "aa", "bb", "xx", "cc", "xx", "dd", 
                      "ee", "ff", "xx", "xx", "gg", "xx" };

var result = 
 strArray.Aggregate((IEnumerable<IEnumerable<string>>)new IEnumerable<string>[0],
   (a, s) => s == "xx" ? a.Concat(new[] { new string[0] })
      : a.Any() ? a.Except(new[] { a.Last() })
                   .Concat(new[] { a.Last().Concat(new[] { s }) }) : a)
         .Where(l => l.Any());

// Test
foreach (var i in result)
  Console.WriteLine(String.Join(",", i.ToArray()));

If you want to filter out the results past the last marker:

string[] strArray = { "aa", "bb", "xx", "cc", "xx", "dd", 
                      "ee", "ff", "xx", "xx", "gg", "xx"};

var result = 
  strArray.Aggregate(
    new { C = (IEnumerable<string>)null, 
          L = (IEnumerable<IEnumerable<string>>)new IEnumerable<string>[0] },
    (a, s) => s == "xx" ? a.C == null
        ? new { C = new string[0].AsEnumerable(), a.L }
        : new { C = new string[0].AsEnumerable(), L = a.L.Concat(new[] { a.C }) } 
        : a.C == null ? a : new { C = a.C.Concat(new[] { s }), a.L }).L
          .Where(l => l.Any());

// Test
foreach (var i in result)
  Console.WriteLine(String.Join(",", i.ToArray()));