circular queue

1// Circular Queue implementation in C++
2
3#include <iostream>
4#define SIZE 5 /* Size of Circular Queue */
5
6using namespace std;
7
8class Queue {
9   private:
10  int items[SIZE], front, rear;
11
12   public:
13  Queue() {
14    front = -1;
15    rear = -1;
16  }
17  // Check if the queue is full
18  bool isFull() {
19    if (front == 0 && rear == SIZE - 1) {
20      return true;
21    }
22    if (front == rear + 1) {
23      return true;
24    }
25    return false;
26  }
27  // Check if the queue is empty
28  bool isEmpty() {
29    if (front == -1)
30      return true;
31    else
32      return false;
33  }
34  // Adding an element
35  void enQueue(int element) {
36    if (isFull()) {
37      cout << "Queue is full";
38    } else {
39      if (front == -1) front = 0;
40      rear = (rear + 1) % SIZE;
41      items[rear] = element;
42      cout << endl
43         << "Inserted " << element << endl;
44    }
45  }
46  // Removing an element
47  int deQueue() {
48    int element;
49    if (isEmpty()) {
50      cout << "Queue is empty" << endl;
51      return (-1);
52    } else {
53      element = items[front];
54      if (front == rear) {
55        front = -1;
56        rear = -1;
57      }
58      // Q has only one element,
59      // so we reset the queue after deleting it.
60      else {
61        front = (front + 1) % SIZE;
62      }
63      return (element);
64    }
65  }
66
67  void display() {
68    // Function to display status of Circular Queue
69    int i;
70    if (isEmpty()) {
71      cout << endl
72         << "Empty Queue" << endl;
73    } else {
74      cout << "Front -> " << front;
75      cout << endl
76         << "Items -> ";
77      for (i = front; i != rear; i = (i + 1) % SIZE)
78        cout << items[i];
79      cout << items[i];
80      cout << endl
81         << "Rear -> " << rear;
82    }
83  }
84};
85
86int main() {
87  Queue q;
88
89  // Fails because front = -1
90  q.deQueue();
91
92  q.enQueue(1);
93  q.enQueue(2);
94  q.enQueue(3);
95  q.enQueue(4);
96  q.enQueue(5);
97
98  // Fails to enqueue because front == 0 && rear == SIZE - 1
99  q.enQueue(6);
100
101  q.display();
102
103  int elem = q.deQueue();
104
105  if (elem != -1)
106    cout << endl
107       << "Deleted Element is " << elem;
108
109  q.display();
110
111  q.enQueue(7);
112
113  q.display();
114
115  // Fails to enqueue because front == rear + 1
116  q.enQueue(8);
117
118  return 0;
119}