Collections

Collections is similar to array which is used to store collection of values. Collections provide a more flexible way to work with groups of objects. Unlike arrays, the group of objects you work with can grow and shrink dynamically as the needs of the application change. For some collections, you can assign a key to any object that you put into the collection so that you can quickly retrieve the object by using the key. A collection is a class, so you must declare an instance of the class before you can add elements to that collection.

Some frequently used Collections are as Follows
1. List
2. ArrayList
3. Stack
4. Queue
5. Dictionary

Collection Syntax
List List <datatype> listName = new List<datatype>();
ArrayList ArrayList arraylistName = new ArrayList();
Stack Stack stackName = new Stack();
Queue Queue queueName = new Queue();
Dictionary Dictionary<datatype1, datatype1> dictionaryName = new Dictionary<datatype1, datatype1>();

NOTE:
We are also having some other Collections like Hashtable, SortedList, BitArray. If we clearly understand how above 5 works, we can easily understand these also.

CODE

EG 1 . Writing collections inside main method.

//All Collections except List present inside namespace System.Collections
using System.Collections;

namespace projectName{
    class Program{
        static void Main() {
            //-------- LIST --------
            //<int> is a generic which represents that it can hold only int values inside this List.
            List<int> intList = new List<int>();
            intList.Add(100);
            intList.Add(189);
            intList.Add(165);
            intList.Add(1985);
            intList.Remove(165); //We can remove value from list, this is not possible in ARRAY
            foreach (int eachElement in intList) {
                Console.WriteLine(eachElement);
            }
            Console.WriteLine();// For Empty Line

            //-------- ARRAY LIST --------
            //ArrayList is not having generic so we can store all kind of datatype values.
            ArrayList sampleArrayList = new ArrayList();
            sampleArrayList.Add("CodersCapsule");
            sampleArrayList.Add(23);
            sampleArrayList.Add(3.14);
            sampleArrayList.Add(true);
            sampleArrayList.Add('@');
            //object will convert to specific datatype depending on the value what we trying to store inside.
            foreach (object eachElement in sampleArrayList) {
                Console.WriteLine(eachElement);
            }
            Console.WriteLine();

            //-------- STACK --------
            //In stack what goes first will come last and what goes last will come first(Check Output)
            Stack stackList = new Stack();
            stackList.Push("CodersCapsule");
            stackList.Push(45.654);
            stackList.Push(false);
            foreach (object eachElement in stackList) {
                Console.WriteLine(eachElement);
            }
            Console.WriteLine();

            //-------- QUEUE --------
            //Queue is direct opposite of stack,here what goes first will come first, what goes last will come Last.
            Queue queueList = new Queue();
            queueList.Enqueue(654.364);
            queueList.Enqueue(456);
            queueList.Enqueue("CodersCapsule");
            foreach (object eachElement in queueList) {
                Console.WriteLine(eachElement);
                }
            Console.WriteLine();

            //-------- DICTIONARY --------
            //Dictionary is used to store Key, value pair. <datatype1(key),datatype2(value)>
            Dictionary<int, string> cityAndPincode = new Dictionary<int, string>();
            cityAndPincode.Add(6001,"Chennai");
            cityAndPincode.Add(5001,"Mumbai");
            cityAndPincode.Add(4001,"Bangalore");
            foreach (KeyValuePair<int, string> eachElement in cityAndPincode) {
                Console.WriteLine(eachElement.Key+" : "+eachElement.Value);
            }
        }
    }
}

OUTPUT
100
189
1985

CodersCapsule
23
3.14
True
@

False
45.654
CodersCapsule

654.364
456
CodersCapsule

6001 : Chennai
5001 : Mumbai
4001 : Bangalore

CODE EXPLANATION
In the above code, We used List, ArrayList, Stack, Queue and Dictionary collections.

1. We created a List called intList. List will have generic<datatype> which means here we can store only collection of specified datatype values. We added collection of int values using Add(). The main advantage of using Collections over Array is, here we can remove elements which is not possible when it comes to Array.

2. We created a ArrayList called sampleArrayList . ArrayList cannot have any generic hence we can store all kind of datatype values. We added values inside ArrayList using Add().

3. We created a Stack called stackList. stackList also cannot have generic hence we can store all kind of datatype values. In Stack whatever addded first will come last, and whatever added last will come first. We added values inside Stack using Push(). In order to get values we can also use Pop().

4. We created a Queue called queueList . queueList also cannot have generic hence we can store all kind of datatype values. In Queue whatever addded first will come first, and whatever added last will come last. We added values inside Queue using Enqueue(). In order to get values we can also use Dequeue().

5. We created a Dictionary called cityAndPincode. cityAndPincode having key value pair<datatype1,datatype2> where first datatype refers to key and second datatype refers to value. Using key we can pick out values. We added values inside Dictionary using Add().


What are the advantages of using Collections?
Collections provide a more flexible way to work with groups of elements. Unlike arrays, the group of elements you work with can grow and shrink dynamically as the needs of the application change. And in Collections we can remove elements stored inside which is not possible in Array.

Exercises
1. Create an integer list with 5 interger elements. Sort all values in ascending and descending order (Note: List having internal method called Max() and Min() to find maximum value and minimum value).

(Click Here, For more exercises)