# Queue using array

• Queue is a linear data structure which follows FIFO i.e. First-In-First-Out method.

• The two ends of a queue are called Front and Rear.

• Initially when the queue is empty both front and rear are equal to -1.

• Insertion takes place at the Rear and the elements are accessed or removed from the Front.

• Applications of queues in computer world :

  1. Scheduling : CPU Scheduling , Job Scheduling.
  2. Buffers : I/O Buffers.

• Operations

  1. enqueue() : Insertion of new element in queue.
  2. dequeue() : Removal of element at front from queue.
  3. showfront() : To show the element at front.
  4. isempty() : To check if queue is empty.

• Suppose we have this queue of size 5.

• And we have to insert the element 8 in the queue, so we will increment rear by one and insert 8 at rear.

• While inserting (enqueue) we have to consider two special cases:

  1. Queue is empty - then initially front and rear will be -1 so we will increment rear and insert the element at rear and we will make front = 0 i.e. both front and rear will point to the first element.
  2. Queue is full - then we cannot insert any more elements in the queue.

• Similarly if we have to remove the first element from the queue, then we will simply increment front by one.

• While removing (dequeue) elements from the queue we also have two special cases:

  1. Queue is empty - in that case front and rear both will be equal to -1 and as there are no elements in the queue we cannot perform dequeue operation.
  2. Queue has only one element - in that case front will be equal to rear and we will make both equal to -1.

# Source Code - C++

#include <iostream>
using namespace std;

#define SIZE 5
int A[SIZE];
int front = -1;
int rear = -1;
//function to check if queue is empty
bool isempty()
{
 if(front == -1 && rear == -1)
 return true;
 else
 return false;
}
//function to insert element in queue
void enqueue ( int value )
{
 if (rear == SIZE-1)
  cout<<"Queue is full \n";
 else
 {
  if( front == -1)
   front = 0;
  rear++;
  A[rear] = value;
 }
}
//function to remove element from queue
void dequeue ( )
{
 if( isempty() )
  cout<<"Queue is empty\n";
 else
 if( front == rear )
  front = rear = -1;
 else
  front++;
}
//function to display element at front
void showfront( )
{
 if( isempty())
  cout<<"Queue is empty\n";
 else
  cout<<"element at front is:"<<A[front]<<"\n";
}
//function to display elements of the queue
void displayQueue()
{
 if(isempty())
  cout<<"Queue is empty\n";
 else
 {
  for( int i=front ; i<= rear ; i++)
   cout<<A[i]<<" ";
  cout<<"\n";
 }
}

int main()
{
 //inserting elements in queue
 cout<<"Inserting elements in queue\n";
 enqueue(2);
 displayQueue();
 enqueue(3);
 displayQueue();
 enqueue(5);
 displayQueue();
 enqueue(7);
 displayQueue();
 enqueue(8);

 displayQueue();
 // queue is full
 enqueue(9);
 //show element at front
 showfront();

 cout<<"Removing elements from queue\n";
 //removing elements from the queue
 dequeue();
 displayQueue();
 dequeue();
 displayQueue();
 dequeue();
 displayQueue();
 dequeue();
 displayQueue();
 dequeue();

 return 0;
}

Time Complexity - Each Operation is O(1)