Add incremented property to IEnumerable by group using LINQ - linq

Question

To simplify the scenario, let's say we have a list of People with FirstName and LastName properties.
Our data looks like this:
Bob Smith
Jane Smith
Todd Grover
Larry Lewis
Jill Lewis
Frank Lewis
The first step would be to add an Integer property that gets incremented for each item:
Bob Smith 1
Jane Smith 2
Todd Grover 3
Larry Lewis 4
Jill Lewis 5
Frank Lewis 6
Ideally, I'd like to reset the counter for every new group to achieve this:
Bob Smith 1
Jane Smith 2
Todd Grover 1
Larry Lewis 1
Jill Lewis 2
Frank Lewis 3
Maybe LINQ isn't appropriate. It just seems like LINQ should be able to do this elegantly.

Answer 1

If you just want a counter that increments with each item:
var query = people
.Select((x, i) => new { x.FirstName, x.LastName, Index = i + 1 });
Or if you want to reset the counter for each group, grouped by LastName:
var query = people
.GroupBy(x => x.LastName)
.Select
(
x => x.Select((y, i) => new { y.FirstName, y.LastName, Index = i + 1 })
)
.SelectMany(x => x);
And to quickly display the query results:
foreach (var item in query)
{
Console.WriteLine(item.FirstName + "\t" + item.LastName + "\t" + item.Index);
}

Answer 2

You can use the second overload of the Select method, which incorporates an index parameter passed to the lambda expression, for example:
var people = new [] {
new { FirstName = "Bob", LastName = "Smith"},
new { FirstName = "Jane", LastName = "Smith"},
new { FirstName = "Todd", LastName = "Grover"},
new { FirstName = "Larry", LastName = "Lewis"},
new { FirstName = "Jill", LastName = "Lewis"},
new { FirstName = "Frank", LastName = "Lewis"},
}.ToList();
people.Select((p, index) => new {
FirstName = p.FirstName,
LastName = p.LastName,
Index = index
}
);
Result:

Answer 3

Assuming there is an integer property already on the record type, and the collection is already sorted, then you can abuse Aggregate (i.e. left-fold), something like this
collection.Aggregate( (old, next) => { if (namesDiffer(old, next)) next.Index = 1 else next.Index = old.Index +1; return next;}, aRecordWithEmptyName);
EDIT -- fixed return value; fingers had been on autopilot.