Determining the first available value in a list of integers

Question

I got a simple List of ints.

List myInts = new List();

myInts.Add(0);
myInts.Add(1);
myInts.Add(4);
myInts.Add(6);
myInts.Add(24);
         


        

Answer 1

You basically want the first element from the sequence 0..int.MaxValue that is not contained in myInts:

int? firstAvailable = Enumerable.Range(0, int.MaxValue)
                                .Except(myInts)
                                .FirstOrDefault();

Edit in response to comment:

There is no performance penalty here to iterate up to int.MaxValue. What Linq is going to to internally is create a hashtable for myInts and then begin iterating over the sequence created by Enumerable.Range() - once the first item not contained in the hashtable is found that integer is yielded by the Except() method and returned by FirstOrDefault() - after which the iteration stops. This means the overall effort is O(n) for creating the hashtable and then worst case O(n) for iterating over the sequence where n is the number of integers in myInts.

For more on Except() see i.e. Jon Skeet's EduLinq series: Reimplementing LINQ to Objects: Part 17 - Except