Mimic `rollup` function on generic list

Question

I have a generic list of type Element, for example.

public class Element
{
    public string Country { get; set; }
    public string City { get; set; }
    public int Population { get; set; }
}

With the following data.

var elements = new List<Element>
   {
       new Element { Country = "Country A", City = "Barrie", Population = 12 },
       new Element { Country = "Country A", City = "Barrie2", Population = 12 },
       new Element { Country = "Country A", City = "Barrie2", Population = 12 },
       new Element { Country = "Country A", City = "Barrie", Population = 12 },
       new Element { Country = "Country D", City = "Essex", Population = 12 },
       new Element { Country = "Country A", City = "Barrie", Population = 12 },
       new Element { Country = "Country A", City = "Barrie", Population = 12 },
       new Element { Country = "Country D", City = "Essex", Population = 12 },
       new Element { Country = "Country A", City = "Barrie", Population = 12 },
       new Element { Country = "Country A", City = "Barrie", Population = 12 }
   };

Essentially, I'd like a running total of the population grouped by country and city.

Something like.

Country A | Barrie  | `running total for Barrie`
Country A | Barrie2 | `running total for Barrie2`
          |         | `total for Country A`
Country D | Essex   | `running total for Essex`
          |         | `total for Country D`
          |         | `total for everything`

I couldn't find an extension (I say extension because I plan on using a rollup several times) anywhere so I figure I'd give it a shot. So I started with this simple query.

var groupedElements = elements
    .GroupBy(x => new { x.Country, x.City })
    .Select(x => new { Country = x.Key, City = x.Select(xx => xx.City), Population = x.Sum(xx => xx.Population) })
    .ToList();

This query works as expected so I think I'm on the right track. Next I think I have to figure out which property from groupedElements is aggregate because that's what we'll be doing a rollup on. How do I accomplish that? Or perhaps I could have a parameter that makes me specify what column I wish to do the aggregate function on.

Answer 1

I don't think this is as easy to do as you might think. Firstly, the items of the result sequence you want are not 'naturally' of the same type. You want a running total within each country, grouped totals for each country, and then an overall total.

Even if you could write a terse query to create this information, the caller would have to differentiate between each kind of total from the result. I wonder if it is even makes much sense to view the result that you want as a single IEnumerable<XXX> in the first place; it might make much more sense to create a nice OO solution like this and proceed from there:

public interface IGeographicEnity
{
   string Name { get; }
   int Population { get; }
}

public class City : IGeographicEntity {...}

public class Country : IGeographicEntity
{
   public IList<City> Cities { get {...} }       
   public int Population { get { return Cities.Sum(c => c.Population); } }
   ...
}

public class World : IGeographicEntity
{
    public IList<Country> Countries { get {...} }       
    public int Population { get { return Countries.Sum(c => c.Population); }
    ...
}

If you still want to stick with your original idea, here's the best I can come up with:

public class PopulationTotal
{
    // You can use subclassing instead of an enum to represent this
    public enum Kind
    { RunningTotalWithinCountry, TotalForCountry, TotalOverall }

    public string GroupName { get; private set; }
    public int Value { get; private set; }
    public Kind Kind { get; private set; }

    public static IEnumerable<PopulationTotal> GetTotals(IEnumerable<Element> elements)
    {
        int overallTotal = 0;

        foreach (var elementsByCountry in elements.GroupBy(e => e.Country))
        {
            int runningTotalForCountry = 0;

            foreach (var element in elementsByCountry)
            {
                runningTotalForCountry += element.Population;
                yield return new PopulationTotal
                                    {
                                        GroupName = element.City,
                                        Kind = Kind.RunningTotalWithinCountry,
                                        Value = runningTotalForCountry
                                    };
            }

            overallTotal += runningTotalForCountry;

            yield return new PopulationTotal
                                {
                                    GroupName = elementsByCountry.Key,
                                    Kind = Kind.TotalForCountry,
                                    Value = runningTotalForCountry
                                };
        }

        yield return new PopulationTotal
                            {
                                GroupName = null,
                                Kind = Kind.TotalOverall,
                                Value = overallTotal
                            };
    }
}

Usage:

var totals = PopulationTotal.GetTotals(elements);