Check if a list of integers increments by one

Question

Is there a way, with LINQ, to check if a list of integers are "sequential" - ie 1,2,3,4,5 or 14,15,16,17,18?

Answer 1

You could do this via Enumerable.Zip:

bool sequential = values.Zip(values.Skip(1), (a,b) => (a+1) == b).All(x => x);

This works by taking each pair of values, and checking to see if the second is 1 more than the first, and returning booleans. If all pairs fit the criteria, the values are sequential.

---

Given that this is a list of integers, you can do this slightly more efficiently using:

bool sequential = values.Skip(1).Select((v,i) => v == (values[i]+1)).All(v => v);

This will only work on sequences which can be accessed by index. Note that we use values[i], not values[i-1], as the Skip call effectively shifts the indices.

Answer 2

bool isSequential = Enumerable.Range(values.Min(), values.Count())
                              .SequenceEqual(values);

Answer 3

One more option is to use Aggregate to iterate sequence only once.

Note that unlike All suggested by Reed Copsey Aggregate can't stop in the middle when condition fails...

var s = new int[] {3,4,5,6}.ToList();

var isSequential = s.Aggregate
  (
      new {PrevValue = 0, isFirst = true, Success = true} ,
      (acc, current) => 
          new {
                  PrevValue = current,
                  isFirst = false, 
                  Success = acc.Success && (acc.isFirst || (acc.PrevValue == current - 1))
               }
  )
  .Success;

Fancier version would be to have iterator that carries previous value along or special code that would split iterator on "First and the rest" allowing to implement Reed's solution with single iteration for any enumerable.