使用 LINQ 将数据列表聚合并分组到新列表中，但 Sum 为 0

Question

I have a list of data retrieved from SQL and stored in a class. I want to now aggregate the data using LINQ in C# rather than querying the database again on a different dataset.

Example data I have is above. Date, Period, Price, Vol and I am trying to create a histogram using this data. I tried to use Linq code below but seem to be getting a 0 sum.

1. Period needs to be a where clause based on a variable
2. Volume needs to be aggregated for the price ranges
3. Price needs to be a bucket and grouped on this column

I dont want a range. Just a number for each bucket.

Example output I want is (not real data just as example):

Bucket    SumVol
18000       50
18100       30
18200       20

Attempted the following LINQ query but my SUM seems to be be empty. I still need to add my where clause in, but for some reason the data is not aggregating.

var ranges = new[] { 10000, 11000, 12000, 13000, 14000, 15000, 16000, 17000, 18000, 19000, 20000 };
var priceGroups = eod.GroupBy(x => ranges.FirstOrDefault(r => r > x.price))
    .Select(g => new { Price = g.Key, Sum = g.Sum(s => s.vol)})
    .ToList();

var grouped = ranges.Select(r => new
{
    Price = r,
    Sum = priceGroups.Where(g => g.Price > r || g.Price == 0).Sum(g => g.Sum)
});

Answer 1

Use LINQ instead to query the DB is great, mainly because you are saving process avoiding a new call to your DB. And in case you don't have a high update BD (that change the data very quickly) you can use the retrived data to calculate all using LINQ

Answer 2

First things first... There seems to be nothing wrong with your priceGroups list. I've run that on my end and, as far as I can understand your purpose, it seems to be grabbing the expected values from your dataset.

var ranges = new[] { 10000, 11000, 12000, 13000, 14000, 15000, 16000, 17000, 18000, 19000, 20000 };
var priceGroups = eod.GroupBy(x => ranges.FirstOrDefault(r => r > x.price))
      .Select(g => new { Price = g.Key, Sum = g.Sum(s => s.vol) })
      .ToList();

Now, I assume your intent with the grouped list was to obtain yet another anonymous type list, much like you did with your priceGroups list, which is also an anonymous type list... List<'a> in C#.

var grouped = ranges.Select(r => new
{
    Price = r,
    Sum = priceGroups.Where(g => g.Price > r || g.Price == 0).Sum(g => g.Sum)
});

For starters, your are missing the ToList() method call at the end of it. However, that's not the main issue here, as you could still work with an IEnumerable<'a> just as well for most purposes.

As I see it, the core problem is at your anonymous property Sum attribution. Why are your filtering for g.Price > r || g.Price == 0 g.Price > r || g.Price == 0 ?

There is no element with Price equal to zero on your priceGroups list. Those are a subset of ranges , and there is no zero there. Then you are comparing every value in ranges against that subset in priceGroups , and consolidating the Sums of every element in priceGroups that have Price higher than the range being evaluated. In other words, the property Sum in your grouped list is a sum of sums.

Keep in mind that priceGroups is already an aggregated list. It seems to me you are trying to aggregate it again when you call the Sum() method after a Where() clause like you are doing. That doesn't make much sense.

What you want (I believe) for the Sum property in the grouped list is for it to be the same as the Sum property in the priceGroups list, if the range being evaluated matches the Price being evaluated. Furthermore, where there is no matches, you want your grouped list Sum to be zero, as that means the range being evaluated was not in the original dataset. You can achieve that with the following instead:

Sum = priceGroups.FirstOrDefault(g => g.Price == r)?.Sum ?? 0

You said your Sum was "empty" in your post, but that's not the behavior I saw on my end. Try the above and, if still not behaving as you would expect, share a small dataset for which you know the expected output with me and I can try to help you further.