Split a List into groups of consecutive numbers [closed]

Answer 1

Here is an extension method taken from http://bugsquash.blogspot.com/2010/01/grouping-consecutive-integers-in-c.html

public static IEnumerable> GroupConsecutive(this IEnumerable list) {
    var group = new List();
    foreach (var i in list) {
        if (group.Count == 0 || i - group[group.Count - 1] <= 1)
            group.Add(i);
        else {
            yield return group;
            group = new List {i};
        }
    }
    yield return group;
}

You can use it like this:

var numbers = new[] { 1, 2, 3, 4, 6, 7, 9 };
var groups = numbers.GroupConsecutive();

Once C# 7 is released, this can made even more efficient with the use of Span to avoid creating new lists.

---

This updated version does it without allocating any lists.

public static class EnumerableExtensions
{
    public static IEnumerable> GroupConsecutive(this IEnumerable list)
    {
        if (list.Any())
        {
            var count = 1;
            var startNumber = list.First();
            int last = startNumber;

            foreach (var i in list.Skip(1))
            {
                if (i < last)
                {
                    throw new ArgumentException($"List is not sorted.", nameof(list));
                }
                if (i - last == 1)
                    count += 1;
                else
                {
                    yield return Enumerable.Range(startNumber, count);
                    startNumber = i;
                    count = 1;
                }
                last = i;
            }
            yield return Enumerable.Range(startNumber, count);
        }
    }
}