转换列表 <int> 到字典 <int,int> 使用LINQ

Question

Say I have the following list:

  List<int> list = new List<int>() { 5, 5, 6, 6, 6, 7, 7, 7, 7 };

How could I convert the list to a Dictionary where the value is the count of each distinct number in the list by using LINQ? For example, the above list should be converted to a dictionary with the elements:

key -> value

5 -> 2

6 -> 3

7 -> 4

Answer 1

var result = list.GroupBy(i => i).ToDictionary(g => g.Key, g => g.Count());

Answer 2

Efficent solution (only 1 iteration over collection):

var output = new Dictionary<int, int>();
foreach (var val in list)
{
    if (!output.ContainsKey(val)) 
    {
        output.Add(val, 1);
    }
    else
    {
        output[val] = output[val] + 1;
    }
}

Answer 3

var group = list.GroupBy(T => T).ToDictionary(T => T.Key, T => T.Count())

Answer 4

try this:

var dic =    list.GroupBy(c => c)
                 .Select(c => new {c.Key, Count = c.Count()})
                 .ToDictonary(c => c.Key, q => q.Count)