How to find consecutive same values items as a Linq group

Question

var schedules = new List{
    new Item { Id=1, Name = \"S\" },
    new Item { Id=2, Name = \"P\" },
    new Item { Id=3, Name = \"X\" },
    new Item { Id=4,         


        

Answer 1

You can do it by maintaining the count of items that you found so far. This helps you find consecutive items, because the value of count(name) - index is invariant for them:

IDictionary count = new Dictionary();
var groups = schedules
    .Select((s, i) => new {
        Item = s
    ,   Index = i
    })
    .GroupBy(p => {
        var name = p.Item.Name;
        int current;
        if (!count.TryGetValue(name, out current)) {
            current = 0;
            count.Add(name, current);
        }
        count[name] = current + 1;
        return new { Name = name, Order = current - p.Index };
    })
    .Select(g => g.ToList())
    .Where(g => g.Count > 1)
    .ToList();

This produces the desired output for your example:

{ Item = Id=3 Name=X, Index = 2 }
{ Item = Id=4 Name=X, Index = 3 }
-----
{ Item = Id=5 Name=P, Index = 4 }
{ Item = Id=6 Name=P, Index = 5 }
{ Item = Id=7 Name=P, Index = 6 }

Demo.

Note: If Order = current - p.Index expression looks a little like "magic", consider removing the final Select and Where clauses, and enumerating group keys.