03 - Collections.md

Collections

• 1. Objectives
• 2. ArrayList
• 3. List<T>
• 4. Queues, Stacks, and Sets
• 5. Custom Collections
• 6. Bibliography

1. Objectives

• understand the advantages of using List<T> instead of ArrayList
• understand how to use the List<T> class
• have an overview of the available collection types
• know how to implement a custom collection with the help of IEnumerator<T> and IEnumerable<T>

2. ArrayList

Similar data can often be handled more efficiently when stored and manipulated as a collection MSDocs. You can find a guide for choosing the right type of collection at: https://docs.microsoft.com/en-us/dotnet/standard/collections/ . The document also discusses the algorithmic complexity of various operations.

Assignment

Sample code available – Check the “StandardCollections” Sample

1. Create a new project with the name “StandardCollections”

2. Add the following method in the “Program” class and call it from the Main method

    private static void ArrayListExample()
   {
   	var words = new ArrayList();
   	words.Add("melon");
   	words.Add("avocado");
   	string first = (string)words[0];
   
   	//int first = (int)words[0];
   }

3. List<T>

3. Add the following method in the “Program” class and call it from the Main method

   private static void ListExample()
   {
   	// New string-typed list
   	var words = new List<string>();
   	words.Add("melon");
   	words.Add("avocado");
   	words.AddRange(new[] { "banana", "plum" });
   	
   	// Insert at start
   	words.Insert(0, "lemon"); 
   	
   	// Insert at start
   	words.InsertRange(0, new[] { "peach", "nashi" }); 
   	words.Remove("melon");
   	
   	// Remove the 4th element
   	words.RemoveAt(3); 
   	
   	// Remove first 2 elements
   	words.RemoveRange(0, 2);
   	
   	// Remove all strings starting in 'n':
   	words.RemoveAll(x => x.StartsWith("n"));
   	
   	for (var i=0; i<words.Count; i++)
   	{
   		Console.WriteLine(words[i]);
   	}
   
   	foreach (var word in words)
   	{
   		Console.WriteLine(word);
   	}
   }

4. Add the following “Person” class

   internal class Person
   {
   	#region Properties
   	public string Name { get; set; }
   	public int Age { get; set; }
   	#endregion
   
   	public Person(string name, int age)
   	{
   		Name = name;
   		Age = age;
   	}
   }

5. Add the following method in the Program class and call it from the Main method

   private static void ListPersonExample()
   {
   	var personList = new List<Person>();
   
   	var rnd = new Random();
   	for (var i = 0; i < 10; i++)
   	{
   		personList.Add(new Person("Persoana " + i, rnd.Next(100)));
   	}
   
   	//Which interface is needed for Array.Sort(personList)
   
   	foreach (var p in personList) //equivalent to foreach (Person p in personList)
   		Console.WriteLine(p);
   }

4. Queues, Stacks, and Sets

• Queue , Stack<T>, LinkedList<T>

• SortedList<TKey, TValue> and many others: link

5. Custom Collections

Assignment

Sample code available – Check the “CustomCollections” Sample

1. Add the following PersonCollection class

   internal class PersonCollection :IEnumerable<Person>
   {
   	private Person[] _personArray;
   
   	//Indexer [int]
   	public Person this[int index]
   	{
   		get { return _personArray[index]; }
   		set { _personArray[index] = value; }
   	}
   
   	//Indexer [string]
   	public Person this[string name]
   	{
   		get { return _personArray.FirstOrDefault(x=>x.Name == name); }
   	}
   
   	public int Length
   	{
   		get { return _personArray.Length; }
   	}
   
   	public PersonCollection()
   	{
   		_personArray = new []
   		{
   			new Person("name1", 1),
   			new Person("name2", 2),
   			new Person("name3", 3)
   		};
   	}
   	
   	public IEnumerator<Person> GetEnumerator()
   	{
   		return new PersonEnumerator(this);
   	}
   
   	IEnumerator IEnumerable.GetEnumerator()
   	{
   		return GetEnumerator();
   	}
   }

2. Add the following PersonEnumerator class

   internal class PersonEnumerator : IEnumerator<Person>
   {
   	private int _nIndex;
   	private PersonCollection _personCollection;
   
   	public PersonEnumerator(PersonCollection personCollection)
   	{
   		_personCollection = personCollection;
   		_nIndex = -1;
   	}
   
   	public bool MoveNext()
   	{
   		_nIndex++;
   		return _nIndex < _personCollection.Length;
   	}
   
   	public void Reset()
   	{
   		_nIndex = -1;
   	}
   
   	public Person Current
   	{
   		get { return _personCollection[_nIndex]; }
   	}
   
   	object IEnumerator.Current
   	{
   		get { return Current; }
   	}
   
   	public void Dispose()
   	{
   		
   	}
   }

3. Add the following method in the Program class and call it from the Main method

   private static void PersonCollectionExample()
   {
   	var personList = new PersonCollection();
   
   	foreach (var p in personList)
   		Console.WriteLine(p);
   }

6. Bibliography

• https://docs.microsoft.com/en-us/dotnet/standard/collections/
• https://docs.microsoft.com/en-us/dotnet/api/system.collections
• https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic