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# Queue using circular 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.

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

  • Let SIZE be the size of the array i.e. number of elements.

  • To implement queue using circular array :

    1. In the enqueue() method we will make rear = (rear+1)%SIZE instead of rear++.
    2. In the dequeue() method we will make front = (front+1)%SIZE, instead of front++.
    3. And to check if the queue is full we will use the condition : if (rear+1)%SIZE == front, it would mean that the queue is full.

# 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 or not
bool isempty()
{
 if(front == -1 && rear == -1)
 return true;
 else
 return false;
}

//function to enter elements in queue
void enqueue ( int value )
{
 //queue is full
 if ((rear + 1)%SIZE == front)
    cout<<"Queue is full \n";
 else
 {
  //first element inserted
  if( front == -1)
     front = 0;
 //insert element at rear
 rear = (rear+1)%SIZE;
   A[rear] = value;
 }
}

//function to delete/remove element from queue
void dequeue ( )
{
 if( isempty() )
  cout<<"Queue is empty\n";
 else
 //only one element
 if( front == rear )
  front = rear = -1;
 else
  front = (front + 1)%SIZE;
}

//function to show the element at front
void showfront( )
{
 if( isempty())
 cout<<"Queue is empty\n";
 else
 cout<<"element at front is:"<<A[front];
}

//function to display queue
void displayQueue()
{
 if(isempty())
  cout<<"Queue is empty\n";
 else
 {
  int i;
  if( front <= rear )
  {
   for( i=front ; i<= rear ; i++)
   cout<<A[i]<<" ";
  }
  else
  {
   i=front;
   while( i < SIZE)
   {
   cout<<A[i]<<" ";
   i++;
   }
   i=0;
   while( i <= rear)
   {
   cout<<A[i]<<" ";
   i++;
   }
  }
 }
}

//Main Function
int main()
{
 int choice, flag=1, value;
 while( flag == 1)
 {
  cout<<"\n1.enqueue 2.dequeue 3.showfront 4.displayQueue 5.exit\n";
  cin>>choice;
  switch (choice)
  {
  case 1: cout<<"Enter Value:\n";
          cin>>value;
          enqueue(value);
          break;
  case 2: dequeue();
          break;
  case 3: showfront();
          break;
  case 4: displayQueue();
          break;
  case 5: flag = 0;
          break;
  }
 }

 return 0;
}

Time Complexity - Each Operation is O(1)