Circular queue with display operation

A circular queue is an array treated as a ring: after index N − 1 the next position is 0. Variables: array CQ[0..N-1], front = rear = -1 (empty).

Algorithms

ENQUEUE(x)
1. if (rear + 1) % N == front:  print "Queue overflow"; return
2. if front == -1:  front = rear = 0
   else            rear = (rear + 1) % N
3. CQ[rear] = x

DEQUEUE()
1. if front == -1:  print "Queue underflow"; return
2. x = CQ[front]
3. if front == rear:  front = rear = -1
   else               front = (front + 1) % N
4. return x

DISPLAY()                      /* does not change front or rear */
1. if front == -1:  print "Queue empty"; return
2. i = front
3. loop:
       print CQ[i]
       if i == rear:  break
       i = (i + 1) % N

C implementation

#include <stdio.h>
#define N 5
int cq[N], front = -1, rear = -1;

void enqueue(int x) {
    if ((rear + 1) % N == front) {               /* full */
        printf("Queue overflow\n"); return;
    }
    if (front == -1) front = rear = 0;           /* first element */
    else rear = (rear + 1) % N;
    cq[rear] = x;
}

int dequeue(void) {
    if (front == -1) {                           /* empty */
        printf("Queue underflow\n"); return -1;
    }
    int x = cq[front];
    if (front == rear) front = rear = -1;        /* last element */
    else front = (front + 1) % N;
    return x;
}

void display(void) {
    if (front == -1) { printf("Queue empty\n"); return; }
    int i = front;
    while (1) {
        printf("%d ", cq[i]);
        if (i == rear) break;
        i = (i + 1) % N;
    }
    printf("\n");
}

Example (N = 5)

display() at the end prints 30 40 50 60 70 (from front = 2 wrapping to rear = 1) and leaves the queue unchanged. Each operation takes O(1) time except display, which is O(n).