Frequencies

Returns a Dictionary with the unique values of a collection as keys and their frequencies as the values.

• Use IEnumerable.GroupBy() to create groups for each distinct value in the collection.
• Use IEnumerable.ToDictionary() to convert the result of the previous operation to a Dictionary.

using System.Collections.Generic;
using System.Linq;

public static partial class _30s 
{
  public static Dictionary<T,int> Frequencies<T>(IEnumerable<T> values)
  {
    return values
      .GroupBy(v => v)
      .ToDictionary(v => v.Key, v => v.Count());
  }
}

char[] c = {'a', 'b', 'a', 'c', 'a', 'a', 'b'}; 

_30s.Frequencies(c); // { [a, 4], [b, 2], [c, 1] }