Circular Queue toString method ignoring some conditions

Question

when I call my toString() method it doesn't work if after the index wraps around (front > rear). I have included the code below. After, I enqueue(5) and enqueue(6), the toString seems to be completely ignored. At first I thought I wasn't overriding the default toString in Java but the first print statement clearly tells me I am. Anyways, check the code out:

public class driver {
public static void main(String[] args) {

    Queue queue = new Queue(4);
    System.out.println(queue);
    queue.enqueue(1);
    queue.enqueue(2);
    queue.enqueue(3);
    queue.enqueue(4);
    System.out.println(queue);
    queue.dequeue();
    System.out.println(queue);

    queue.dequeue();
    System.out.println(queue);

    queue.enqueue(5);
    queue.enqueue(6);

    System.out.println(queue);


}

public static class Queue {

    int front;
    int rear;
    int capacity;
    int[] queue;

    public Queue(int size) {
        this.capacity = size;
        this.front = this.rear = -1;
        this.queue = new int[capacity];
    }

    @Override
    public String toString() {
        String str = "";
        if (front > rear) {
            int i = front;
            while (i != rear) {
                str = str + queue[i % queue.length] + " ";
                i++;
            }
            //str= str+queue[rear];
        }
        if (front < rear) {
            for (int i = front; i <= rear; i++) {
                str = str + queue[i];
            }
        }
        if (front == rear) {
            str = "This Queue is Empty. Please Enqueue";
        }

        return str;
    }


    public boolean isFull() {

        return ((rear == this.queue.length - 1 && front == 0) || rear == front - 1);

    }

    public boolean isEmpty() {
        return (front == -1);
    }

    public void enqueue(int elem) {
        if (isFull()) {
            System.out.println("Full Queue - dequeue an element if you need to add an element in the queue");
        } else {
            if (isEmpty()) {
                this.queue[++rear] = elem;
                front = 0;
            } else {
                rear = (rear + 1) % this.queue.length;
                this.queue[rear] = elem;
            }
        }
    }

    public int dequeue() {
        if (isEmpty()) {
            System.out.println("empty queue. Enqueue some elments. ");
            return -1;
        } else {
            int store = this.queue[front];

            if (rear == front) {
                front = rear = -1;
            } else {

                front = front + 1 % this.queue.length;

            }
            return store;

        }

    }

}

*there is a return curly bracket here too lol still new to posting questions. P.S can someone help me because apparently I posted too much code in my question. Any workarounds?

It looks to me first glance that `i` should be decremented instead of incremented in the first case. (`i` starts out as `front`, and then you add one to it and see if it is equal to `rear`, but `front > rear` already... I am surprised you don't just enter an infinite loop there) — Slabgorb, Jul 15 '19 at 18:58
There is an infinite loop @Slabgorb. He just needs to loop it back around to 0 once capacity is reached — EDToaster, Jul 15 '19 at 18:59

EDToaster · Answer 1 · 2019-07-15T19:22:35.547

The issue is with i++ in your while loop of toString.

Consider the case where front = 3 and rear = 1 then you start the loop with i = front = 3. However, you keep incrementing until you reach i == rear which will never happen since rear < front

What you want is for i to loop back around to 0 once it reaches capacity. You can do this by removing i++ and replacing it with i = (i + 1) % capacity;

It seems like you have a different bug in your code, because when I run it I see

This Queue is Empty. Please Enqueue
1234
234
34
3 4 5

You'll need to figure this one out by yourself.

You can even condense it into a single for statement

for(int i = front; i != rear; i = (++i) % capacity)

ooh I love this trick `i = (i + 1) % capacity` that's good stuff - I would have done it with about three or four statements — Slabgorb, Jul 15 '19 at 19:02

Circular Queue toString method ignoring some conditions

1 Answers1