Combine lists of key value pairs where keys match

Question

I have a list of List<KeyvaluePair<DateTime, int>>, which I want to merge into one (union), and where there are pairs with the same key in more than one list, have their values summed.

Example:

input list 1:
date1, 1
date2, 2

input list 2:
date2, 3
date3, 4

input list 3:
date3, 5
date4, 6

desired output:
date1, 1
date2, 5
date3, 9
date4, 6

From what I've been reading about LINQ trickery this sort of thing is possible to do really neatly, but so far I've not been able to get my head around it. If there's a nice way to do this with LINQ I'd love to hear it, if not I'll be grateful for any other tidy looking answer too - my C++ brain can only think of long-winded procedural ways of doing this :)

Note that I'm using a List for a reason so please don't suggest using a dictionary or another datatype. Thanks.

Answer 1

var sums = list1.Concat(list2).Concat(list3)
            .GroupBy(pair => pair.Key, pair => pair.Value)
            .ToDictionary(g => g.Key, g => g.Sum());

sums here is a Dictionary<DateTime, int>, and you can easily access data using sums[date]. To keep your current data structure, you may replace ToDictionary with:

.Select(g => new KeyValuePair<DateTime, int>(g.Key, g.Sum())).ToList();

A bit more general LINQ way is to use an ILookup - this is similar to a dictionary of lists, so you get the individual numbers, if you need them (again, you can make a quick transformation to get to the list you want):

ILookup<DateTime,int> sums = list1.Concat(list2).Concat(list3)
    .ToLookup(pair=>pair.Key,pair=>pair.Value);
int number = sums[date].Sum();

Answer 2

This is a great site for examples on LINQ: http://msdn.microsoft.com/en-us/vbasic/bb688085.aspx

I guess this one http://msdn.microsoft.com/en-us/vbasic/bb737926.aspx#grpbysum solves your problem

Answer 3

ILookup<DateTime, int> lookup = source
  .SelectMany(list => list)
  .ToLookup(kvp => kvp.Key, kvp => kvp.Value);

Or

List<KeyValuePair<DateTime, int>> result = source
  .SelectMany(list => list)
  .GroupBy(kvp => kvp.Key, kvp => kvp.Value)
  .Select(g => new KeyValuePair<DateTime, int>(g.Key, g.Sum()))
  .ToList();