Marc Gravell said:
Aside, but I did manage to cobble something together that aggregates
efficiently while streaming, allowing usage as below... uses
Dictinary<TKey, ...> approach, and will have a little overhead
compared to doing it manually, but it is a little more expressive (but
works for LINQ-to-objects only; it won't parse down to the database!)
If anybody wants the source let me know (extensions are to
IEnumerable<T>).
Just to say I've just implemented this myself, or rather a variation,
due to something else I'm writing at the moment when I want to group
and count. Here's what the unit tests look like, so you can see the
kind of thing it does. Like Marc, let me know if you'd like it...
(It'll be part of MiscUtil when I get round to it.)
Note that my code only does group counting - no summing etc, at the
moment. I'm not sure whether either CountGroup or Marc's GroupCount
names really capture the functionality nicely. I did call mine
"CountBy" for a while... not sure whether that's better or not...
[Test]
public void CountGroupIdentity()
{
var query = new[] { "One", "Two", "Two", "Three" }.CountGroup();
query = from element in query
orderby element.Count, element.Key
select element;
AssertAreEqual(new[] { 1, 1, 2 },
query.Select(elt => elt.Count));
AssertAreEqual(new[] { "One", "Three", "Two" },
query.Select(elt => elt.Key));
}
[Test]
public void CountGroupWithMapping()
{
var query = new[] { "a", "b", "c", "aa", "bb",
"111", "222", "333", "444" }
.CountGroup(x => x.Length);
query = from element in query
orderby element.Count
select element;
AssertAreEqual(new[] { 2, 3, 4 },
query.Select(elt => elt.Count));
AssertAreEqual(new[] { 2, 1, 3 },
query.Select(elt => elt.Key));
}
[Test]
public void CountGroupWithMappingAndComparison()
{
var query = new[] { "a", "A", "a", "B", "b" }.CountGroup
(x => x, StringComparer.OrdinalIgnoreCase);
query = from element in query
orderby element.Count
select element;
AssertAreEqual(new[] { 2, 3 },
query.Select(elt => elt.Count));
AssertAreEqual(new[] { "B", "a" },
query.Select(elt => elt.Key));
}
.