Question

I have an int array, it’s a concatenated array from multiple similar arrays all starting at 1.

1, 2, 3, 4
1, 2
1, 2, 3
1, 2

int[] list = { 1, 2, 3, 4, 1, 2, 1, 2, 3, 1, 2 };

What I am trying to achieve is to get the “last set” of the result which is {1, 2}.

Attempt:

int[] list = { 1, 2, 3, 4, 1, 2, 1, 2, 3, 1, 2 };

List<int> lastSet = new List<int>();
var totalSets = list.Count(x => x == 1);

int encounter = 0;
foreach (var i in list)
{
    if (i == 1)
        encounter += 1;

    if (encounter == totalSets)
        lastSet.Add(i);
}

lastSet.ToList().ForEach(x => Console.WriteLine(x));

Is there a better way to achieve this using LINQ, perhaps SkipWhile, GroupBy, Aggregate?

Answer 1

If you can either make your list be an actual List<int> or if it doesn’t bother you to create a copy of the list via .ToList(), you can do this:

var list = new[]{ 1, 2, 3, 4, 1, 2, 1, 2, 3, 1, 2 }.ToList();
var lastSet = list.Skip(list.LastIndexOf(1)).ToList();

Otherwise, Aggregate can work, but it’s a little ugly:

var lastSet = list.Aggregate(new List<int>{1}, (seed, i) => {
    if(i == 1) {seed.Clear(); }
    seed.Add(i);
    return seed;
})

Update

As dtb points out, you can use Array.LastIndexOf rather than creating a List:

var list = new[]{ 1, 2, 3, 4, 1, 2, 1, 2, 3, 1, 2 };
var lastSet = list.Skip(Array.LastIndexOf(list, 1)).ToList();