Question

I have a set of numbers List<int> (for example) : 1, 3, 4, 5, 7, 12, 13, 14, 15, 20, 22, 24, 28, 29, 30

I want to have them grouped as sequential like :

Sequence1 = from 1 amount 1 (1)
Sequence2 = from 3 amount 3 (3, 4, 5)
Sequence3 = from 7 amount 1 (7)
Sequence4 = from 12 amount 4 (12, 13, 14, 15)
Sequence5 = from 20 amount 1 (20)
Sequence6 = from 22 amount 1 (22)
Sequence7 = from 24 amount 1 (24)
Sequence8 = from 28 amount 3 (28, 29, 30)

I know how to do it with a for and checking for each number. Is there an more elegant way or an algorithm, or some sql/lambda command that would help me ?

Answer 1

If the input is sorted, and you really want to avoid a foreach loop, you can use:

list.Select((value,index)=>new {value,index}).GroupBy(x=>x.value-x.index,x=>x.value).Select(g=>g.AsEnumerable())

One could also write a general helper method:

public static IEnumerable<IEnumerable<T>> SplitBetween<T>(this IEnumerable<T> sequence, Func<T,T,bool> predicate)
{
  T previous=default(T);
  List<T> list=new List<T>();
  int index=0;
  foreach(T current in sequence)
  {
    if((index>0)&&predicate(previous,current))
    {
      yield return list.ToArray();
      list.Clear();
    }
    list.Add(current);
    previous=current;
    index++;
  }
  if(list.Count>0)
    yield return list.ToArray();
}

And then use it with list.SplitBetween((previous,current) => previous+1 != current)