GroupBy - Error Operator / cannot be applied to operands of type IEnumerable<float> and float

Question

Hi, 

I have the following records

A=0, B=1, C=4

A=0, B=2, C=1

A=1, B=2, C=2

A=2, B=0, C=3

I want to be able to GroupBy A and divide each C by the SUM of the GroupBy A ot produce the below

A=0, C=4, Cfraction = 0.8      // 0.8 = 4 / (4+1)

A=0, C=4, Cfraction = 0.2     //  0.2 = 1 / (4+1

A=1, C=4, Cfraction = 1.0

A=2, C=4, Cfraction = 1.0

var finalresult = records
                                     .GroupBy(a => a.A)
                                     .Select(row => new 
                                     {
                                         A = row.Key,
                                         Cfraction = (row.Select(c => (float)c.C) / row.Sum(csum => (float)csum.C)) //DOES NOT COMPILE Operator / cannot be applied to operands of type IEnumerable<float> and float
                                     }).ToList();

---

Answer 1

I am not that solid with LINQ, but I am pretty sure you can't do it like that in SQL either. You have to do it in two steps there. I asume it is the same for LINQ.

Here is how I would do it in SQL, should be translateable to LINQ fairly easily:

1. Group and Sum:

SELECT A, C, SUM(C) = 'sumOfC' FROM TABLE source GROUP BY (A)

You have to keep original C around for the 2nd step to work on the first ones result.

2. Get teh fraction from that intermediate result:

SELECT A, C/sumOfC = 'fraction' FROM (/*The querry from step 1*/) GROUP BY (A)

Answer 2

row is an IGrouping that contains several records and you cannot divide a collection of objects with a float. That's what the compiler tells you.

And the expected result you have posted is not grouped by A since you have two records where A = 0.

You could first store the sum of C for each A in a Dictionary and then lookup the sum in this dictionary for each row in records. This will produce the expected results:

List<Record> records = new List<Record>();
records.Add(new Record() { A = 0, B = 1, C = 4 });
records.Add(new Record() { A = 0, B = 2, C = 1 });
records.Add(new Record() { A = 1, B = 2, C = 2 });
records.Add(new Record() { A = 2, B = 0, C = 3 });

var group = records.GroupBy(a => a.A).ToDictionary(row=>row.Key, row=>row.Sum(csum => (float)csum.C));

var finalresult = records.Select(row => new { A = row.A, Cfraction = (float)row.C / group[row.A] }).ToList();

 

Hope that helps.

Please remember to close your threads by marking helpful posts as answer and then start a new thread if you have a new question. Please don't ask several questions in the same thread.