Common elements

A List contains many elements in a C# program, but some may be duplicated. We can compute which ones are the most common, and which ones are the least common.

Using Dictionary, we can loop through our string List and keep track of how many times each element occurs. This is fast—the Dictionary has constant time lookup for each element.

In this program, we create a string List from a string array. Notice how there are some values, like "bird," that occur more than once in the string array.

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

class Program
{
    static void Main()
    {
        // Compute frequencies for this data.
        string[] values = { "bird", "cat", "bird", "dog", "bird", "man", "frog", "cat" };
        // Get a list.
        List<string> valuesList = new List<string>(values);
        // Call our methods.
        var freqs = GetFrequencies(valuesList);
        DisplaySortedFrequencies(freqs);
    }

    static Dictionary<string, int> GetFrequencies(List<string> values)
    {
        var result = new Dictionary<string, int>();
        foreach (string value in values)
        {
            if (result.TryGetValue(value, out int count))
            {
                // Increase existing value.
                result[value] = count + 1;
            }
            else
            {
                // New value, set to 1.
                result.Add(value, 1);
            }
        }
        // Return the dictionary.
        return result;
    }

    static void DisplaySortedFrequencies(Dictionary<string, int> frequencies)
    {
        // Order pairs in dictionary from high to low frequency.
        var sorted = from pair in frequencies
                     orderby pair.Value descending
                     select pair;

        // Display all results in order.
        foreach (var pair in sorted)
        {
            Console.WriteLine($"{pair.Key} = {pair.Value}");
        }
    }
}
bird = 3
cat = 2
dog = 1
man = 1
frog = 1

Notes, DisplaySortedFrequencies

Consider now the DisplaySortedFrequencies method. We use a LINQ expression to sort the Dictionary we created previously.

Results, summary

We find that our results indicate the string "bird" occurs 3 times. This approach works not just for string keys, but any sort of key (including ints).