Question

How can you determine the current items position whilst looping through the collection?

I’m working through decision data, grouped by each client, but I have some business logic which depends on the “position” in the set, i.e. 1st, 2nd, 3rd, etc. in conjunction with other properties of the record, e.g. if it’s the 3rd decision about a client and their rating in the instance is A then …

var multiples = from d in context.Decision_Data
                group d by d.Client_No
                    into c
                    where c.Count() > 1
                    select c;

foreach (var grouping in multiples)
{
    foreach (var item in grouping)
    {
        // business logic here for processing each decision for a Client_No
        // BUT depends on item position ... 1st, 2nd, etc.
    }

UPDATE: I appreciate I could put a counter in and manually increment, but it feels wrong and I’d of thought there was something in .NET to handle this ??

Answer 1

Something like this:

foreach (var grouping in multiples)
{
    foreach (var x in grouping.Select(index,item) => new {index, item})
    {
        // x.index is the position of the item in this group
        // x.item is the item itself
    }
}

Side note: you can make the implementation of your LINQ query a bit more efficient. Count() > 1 will completely enumerate each group fully, which you are likely to do in the foreach anyway. Instead you can use Skip(1).Any(), which will stop iterating the group as soon as it finds two items. Obviously this will only make a real difference for (very) large input lists.

var multiples = from d in context.Decision_Data
                group d by d.Client_No
                    into c
                    where c.Skip(1).Any() 
                    select c;