Circular Queue Data Structure in C++

A queue is an abstract data structure that contains a collection of elements. Queue implements the FIFO mechanism i.e the element that is inserted first is also deleted first.

Queue cane be one linear data structure. But it may create some problem if we implement queue using array. Sometimes by using some consecutive insert and delete operation, the front and rear position will change. In that moment, it will look like the queue has no space to insert elements into it. Even if there are some free spaces, that will not be used due to some logical problems. To overcome this problem, we will use the circular queue data structure.

A circular queue is a type of queue in which the last position is connected to the first position to make a circle.

Example

#include <iostream>
using namespace std;
int cqueue[5];
int front = -1, rear = -1, n=5;
void insertCQ(int val) {
   if ((front == 0 && rear == n-1) || (front == rear+1)) {
      cout<<"Queue Overflow \n";
      return;
   }
   if (front == -1) {
      front = 0;
      rear = 0;
   }
   else {
      if (rear == n - 1)
         rear = 0;
      else
         rear = rear + 1;
   }
   cqueue[rear] = val ;
}
void deleteCQ() {
   if (front == -1) {
      cout<<"Queue Underflow\n";
      return ;
   }
   cout<<"Element deleted from queue is : "<<cqueue[front<<endl;
   if (front == rear) {
      front = -1;
      rear = -1;
   }
   else {
      if (front == n - 1)
         front = 0;
      else
         front = front + 1;
      }
   }
   void displayCQ() {
      int f = front, r = rear;
      if (front == -1) {
         cout<<"Queue is empty"<<endl;
         return;
      }
      cout<<"Queue elements are :\n";
      if (f <= r) {
         while (f <= r){
            cout<<cqueue[f]<<" ";
            f++;
         }
      }
       else {
         while (f <= n - 1) {
            cout<<cqueue[f]<<" ";
            f++;
         }
         f = 0;
         while (f <= r) {
            cout<<cqueue[f]<<" ";
            f++;
         }
      }
      cout<<endl;
}
int main() {
   int ch, val;
   cout<<"1)Insert\n";
   cout<<"2)Delete\n";
   cout<<"3)Display\n";
   cout<<"4)Exit\n";
   do {
      cout<<"Enter choice : "<<endl;
      cin>>ch;
      switch(ch) {
         case 1:
            cout<<"Input for insertion: "<<endl;
            cin>>val;
            insertCQ(val);
            break;
         case 2:
            deleteCQ();
            break;
         case 3:
            displayCQ();
            break;
         case 4:
            cout<<"Exit\n";
            break;
            default: cout<<"Incorrect!\n";
      }
   }
   while(ch != 4);
      return 0;
}

Output

1)Insert
2)Delete
3)Display
4)Exit
Enter choice :
1
Input for insertion:
10
Enter choice :
1
Input for insertion:
20
Enter choice :
1
Input for insertion:
30
Enter choice :
1
Input for insertion:
40
Enter choice :
1
Input for insertion:
50
Enter choice :
3
Queue elements are :
10 20 30 40 50
Enter choice :
2
Element deleted from queue is : 10
Enter choice :
2
Element deleted from queue is : 20
Enter choice :
3
Queue elements are :
30 40 50
Enter choice :
4
Exit