Is there a LINQ query that will give me the sum of several maximum values?

Question

I've got a table of transaction data for several locations and I need to find the sum of each location's maximum from a subset of locations. So imagine the following table:

location  year  transactions
  123     2009      57
  124     2009      23
  125     2009      45
  123     2010      64
  124     2010      12
  125     2010      66

So if I'm looking just for the data for locations 123 and 124 the code should pick out the value 64 for location 123 in 2010 and 23 for location 124 in 2009.

I've got the following code which works in that it finds the maximum value for each location and then adds it to the running total.

int total = 0;

foreach (var location in locationIds)
{
    int? temp = transactions.Where(t => t.Location == location)
                            .Max(t => t.Transactions);
    if (temp.HasValue)
    {
        total += temp.Value;
    }
}

Is there a more elegant way of coding this?

Answer 1

I'm using the naming you used in the table in your example above. Judging from your code, you may need to change some of the names accordingly...but I can't be positive:

var locationsToInclude = new List<int> { 123, 124 };

var sum = transaction
    .Where(t => locationsToInclude.Contains(t.location))
    .GroupBy(t => t.location)
    .Sum(g => g.Max(t => t.transactions));

Answer 2

This works over all locations. I only want to get the values for a subset.

Then how about this small modification to Justin's answer?

var sum = transaction
    .GroupBy(x => x.Location)
    .Where(g => locationIds.Contains(g.Key))
    .Sum(g => g.Max(x => x.Transactions));