Select the next N elements of an IEnumerable<T>

Question

Say you've got some IEnumerable called S of length N. I would like to select all continuous subsequences of length n <= N from S.

If S were, say, a string, this'd be pretty easy. There are (S.Length - n + 1) subsequences of length n. For example, "abcdefg" is length (7), so that means it has (5) substrings of length (3): "abc", "bcd", "cde", "def", "efg".

But S could be any IEnumerable, so this route isn't open. How do I use extension methods to solve this?

Answer 1

Actually, you can use LINQ to solve this problem, e.g.

var subList = list.Skip(x).Take(y);

where list is an IEnumerable

Answer 2

F# has a library function called Seq.windowed for this.

http://research.microsoft.com/en-us/um/cambridge/projects/fsharp/manual/FSharp.Core/Microsoft.FSharp.Collections.Seq.html

// windowed : int -> seq<'a> -> seq<array<'a>>
let windowed n (s: seq<_>) =    
    if n <= 0 then Helpers.invalid_arg2 "n" "the window size must be positive"
    { let arr = Array.zero_create n 
      let r = ref (n-1)
      let i = ref 0 
      use e = s.GetEnumerator() 
      while e.MoveNext() do 
          do arr.[!i] <- e.Current
          do i := (!i + 1) % n 
          if !r = 0 then 
              yield Array.init n (fun j -> arr.[(!i+j) % n])
          else 
              do r := (!r - 1) }

Answer 3

You can use the Select extension that provides the index to create an object containing the index and value, then divide the index with the length to place them into groups:

var x = values.Select((n, i) => new { Index = i, Value = n }).GroupBy(a => a.Index / 3);

Answer 4

IEnumerable<IEnumerable<T>> ContiguousSubseqences<T>(this IEnumerable<T> seq, Func<T,bool> constraint)
{
    int i = 0;
    foreach (T t in seq)
    {
        if (constraint(t))
            yield return seq.Skip(i).TakeWhile(constraint);
        i++;
    }
}