Question

I have a List where I would like to add an internal counter when several items have the same name.

var myList = new List<string>();

myList.Add("a");
myList.Add("b");
myList.Add("b");
myList.Add("c");

And I want the result to be
a01
b01
b02
c01
after some fancy LINQ stuff.

Any great ideas out there?

Answer 1

Not saying that’s nice, but it’s a (mostly) Linq solution:

var indexed =   from index in myList.Aggregate(
                    new 
                    { 
                        Counters = new Dictionary<string, int>(), 
                        Items = new List<string>() 
                    }, 
                    (acc, cur) => 
                    { 
                        if (!acc.Counters.ContainsKey(cur))
                            acc.Counters.Add(cur, 0);
                        acc.Counters[cur] = acc.Counters[cur] + 1;
                        acc.Items.Add(cur + acc.Counters[cur]);
                        return acc;
                    }).Items
                select index;

The accumulation part is pretty ugly, but it does the job and all inside a Linq computation.

EDIT

If the initial list is already sorted, this expression is cleaner (but might be inefficient, you’d have to see how many items you have in your list):

var indexed =   from index in myList.Aggregate(
                    new 
                    { 
                        Counter = 0, 
                        Key     = (string)null,
                        Items   = Enumerable.Empty<string>() 
                    }, 
                    (acc, cur) => 
                    {
                        var counter = acc.Key != cur ? 1 : acc.Counter+1;
                        return new 
                        { 
                            Counter = counter,
                            Key     = cur,
                            Items   = acc.Items.Concat(
                                        Enumerable.Repeat(cur + counter, 1))
                        };
                    }).Items
                select index;