'Smart' grouping with LINQ

Question

I have a list of strings and I want to convert it to some kind of grouped list, whereby the values would be grouped by their location in the list (not normal grouping, but i

Answer 1

The extension method from this answer does pretty much what you ask (Microsoft also provide an implementation to group contiguous items in a sequence):

public static IEnumerable> 
    GroupConsecutive(this IEnumerable set, Func predicate)
{
    var i = 0;
    var k = 0;
    var ranges = from e in set
                 let idx = ++i
                 let next = set.ElementAtOrDefault(idx)
                 let key = (predicate(e, next)) ? k : k++
                 group e by key into g
                 select g;
    return ranges;
}

You could use it as follows:

void Main()
{
    LinkedList myList = new LinkedList();
    myList.AddLast("aaa");
    myList.AddLast("aaa");
    myList.AddLast("bbb");
    myList.AddLast("bbb");
    myList.AddLast("aaa");
    myList.AddLast("aaa");
    myList.AddLast("aaa");
    IGrouping ggg;

    var groups=myList.GroupConsecutive((a,b)=>a==b);

    ILookup lookup=groups.ToLookup(g=>g.First(),g=>g.Count());

    foreach(var x in lookup["aaa"])
    {
        Console.WriteLine(x); //outputs 2 then 3
    }
    foreach(var x in lookup["bbb"])
    {
        Console.WriteLine(x); //outputs 2
    }

}

Notice that the final container is an ILookup which behaves a little like a Dictionary, but allows one to store multiple values against a single key.