使用Linq计算C＃中的平均评分

Question

I have an array ratings where ratings[i] = j means there are j ratings of i, where i is between 1 and 10. How can I use linq to calculate the average rating ?

for (int i = 1; i <= 10;i++)
            {
                sum += i * ratings[i];
                nr += ratings[i];
            }
ratingAvg = sum / nr;

If i use ratings.Sum() / ratings.Count() it doesn't do what I want. I would need to do something like ratings.Sum(w => index_of_w_in_the_array * w.value).

Answer 1

There are several ways to do this, for example using Select to involve the index of each rating in the calculation:

var average = ratings.Select((r, n) => r * n).Sum() / ratings.Sum();

If the ratings themselves are integers you 'll need to cast one side of the above to double so that you don't get integer division on the result.

Answer 2

Just keep a running sum and a running count of items. For each item in the list add to the total count, and add to the running sum based on the count of that value multiplied by the value itself:

int sum = 0;
int count = 0;
for(int i = 0; i < array.Length; i++)
{
    sum += array[i] * i;
    count += array[i];
}
double average = sum / (double)count;

Here is a LINQ solution, that looks nice, but is going to be quite a bit less efficient:

var average = array.SelectMany((n, i) => Enumerable.Repeat(n, i))
    .Average();

Answer 3

ratings.Sum() / ratings.Count() should work. If it doesn't, you need to be a lot more detailed than "doesn't do what I want."

And, if all else fails, try:

ratings.Average();