A Mutable Log

A blog by Devendra Tewari

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Lists and sorting in .NET

.NET Framework provides a generic SortedList class that keeps objects sorted by Key while they are added. It requires Key to be unique, probably for performance reasons. The example below demonstrates the alternative List class for sorting and searching, and the limitation of SortedList while trying to do the same thing. You'll probably get better performance out of SortedList. List has a BinarySearch method that can improve search performance significantly.

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

namespace ListExample
{
    class Person
    {
        public string name;
        public byte age;

        public Person(string name, byte age)
        {
            this.name = name;
            this.age = age;
        }
    }

    class CompareByAge : IComparer<Person>
    {
        public int Compare(Person x, Person y)
        {
            return x.age - y.age;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Person luisa = new Person("Luisa", 7);
            Person victoria = new Person("Victoria", 10);
            Person alice = new Person("Alice", 10);
            Person lara = new Person("Lara", 1);
            Person rafaela = new Person("Rafaela", 9);
            Person noname = new Person("Noname", 6);
            Person p;

            Console.WriteLine("Sorting with List:");
            List<Person> list = new List<Person>();
            list.Add(luisa);
            list.Add(victoria);
            list.Add(alice);
            list.Add(lara);
            list.Add(rafaela);

            // Sort using System.Collections.Generic.IComparer<T>
            list.Sort(new CompareByAge());

            // Sort using the System.Comparison<T> delegate
            //list.Sort(delegate(Person x, Person y) {
            //    return x.age - y.age;
            //});

            foreach (Person person in list)
            {
                Console.WriteLine(person.name);
            }

            Console.WriteLine();

            Console.WriteLine("Find with List:");
            //p = list[list.FindIndex(0, person => person.age >= noname.age)];
            //p = list.Find(person => person.age >= noname.age);
            int i = list.BinarySearch(noname, new CompareByAge());
            i = i < 0 ? ~i : i;
            if (i == list.Count)
            {
                Console.WriteLine("No one that old here");
            }
            else
            {
                p = list[i];
                Console.WriteLine(p.name);
            }

            Console.WriteLine();

            Console.WriteLine("Sorting with SortedList:");
            SortedList<byte, Person> sortedList = new SortedList<byte, Person>();
            sortedList.Add(luisa.age, luisa);
            sortedList.Add(victoria.age, victoria);
            //sortedList.Add(alice.age, alice); // throws System.ArgumentException
            sortedList.Add(lara.age, lara);
            sortedList.Add(rafaela.age, rafaela);

            foreach (Person person in sortedList.Values)
            {
                Console.WriteLine(person.name);
            }

            Console.WriteLine();

            Console.WriteLine("Find with SortedList:");
            try
            {
                p = sortedList.First(keyValue => keyValue.Key >= noname.age).Value;
                Console.WriteLine(p.name);
            }
            catch(InvalidOperationException)
            {
                Console.WriteLine("No one that old here");
            }

            Console.ReadLine();
        }
    }
}

Here’s the output produced by the example

Sorting with List:
Lara
Luisa
Rafaela
Victoria
Alice

Find with List:
Luisa

Sorting with SortedList:
Lara
Luisa
Rafaela
Victoria

Find with SortedList:
Luisa