Circular Queue java not printing right

Question

hey guys i have a problem when trying to print out the circular queue array heres my code:

public class CircularQueue {

    private int [] queue;
    private int front, rear;

    // do not change the constructor
    CircularQueue() {
        queue = new int [5];
        front = 0;
        rear = -1;
    }

    // FILL IN:
    // throws DSException if out of space
    public void enqueue ( int item  ) throws DSException {
            if ( front == 0 && rear == -1 ){
                    throw new DSException();
            }
            queue[rear+1] = item;
            rear = (rear+1)%queue.length;
    }

    // FILL IN:
    // throws DSException if no element in the queue
    // return the dequeued value
    public int dequeue () throws DSException {
            if ( front == 0 && rear == -1 ){
                    throw new DSException();
            }

            int temp = queue[front];
            queue[front] = 0;
            front = (front+1)%queue.length;
            return temp;

    }


    // FILL IN:
    // return the value at beginning of the queue
    // throws DSException if no element in the queue
    public int first () throws DSException {
            return front;
    }

    // FILL IN:
    // print the circular queue in the following format
    // - print "+" before the value at the front
    // - print "-" after the value at the rear
    // - print "." for the places without valid element.

    public void print () {
        System.out.print("      <");
        for ( int i = 0; i < queue.length; i++ ){
                if ( front == 0 && rear == -1 ){
                        System.out.print("."+"\t");
                } else if ( i == front ) {
                        System.out.print("+"+ queue[i]);
                } else if ( i == rear ) {
                        System.out.print( queue[i]+ "-");
                } else if ( i == front && i == rear) {
                        System.out.print("+"+ queue[i] +"-");
                } else {
                        System.out.print( queue[i] );
                }
        }
        System.out.print(">\n");
    }    

}

and here's the result EMPTY: <. . . . . > ENQUEUE (0):

i am supposed to enqueue 0-4 and dequeue some element but it stops after enqueue 0.

Answer 1

A CircularQueue can be in 3 states whose invariants are given below :
Empty : front == -1 && rear == -1
Full : (rear+1)%queue.length == front
Neither empty nor full : Does not satisfy the conditions mentioned above


public class CircularQueue {

 private int [] queue; private int front, rear; // do not change the constructor CircularQueue() { queue = new int [5]; front = -1; rear = -1; } // FILL IN: // throws DSException if out of space public void enqueue ( int item ) throws DSException,Exception { if ( front == -1 && rear == -1 ){ front = 0; rear = 0; queue[rear] = item; } else if((rear+1)%queue.length == front) { throw new Exception("Full"); } else { rear = (rear+1)%queue.length; queue[rear] = item; } } // FILL IN: // throws DSException if no element in the queue // return the dequeued value public int dequeue () throws DSException { if ( front == -1 && rear == -1 ){ throw new DSException(); } else { int ret = queue[front]; if(rear==front) { rear = -1; front = -1; } else { front = (front+1)%queue.length; } return ret; } } // FILL IN: // return the value at beginning of the queue // throws DSException if no element in the queue public int first () throws DSException { if(front==-1 && rear ==-1) { throw new DSException(); } return queue[front]; } // FILL IN: // print the circular queue in the following format // - print "+" before the value at the front // - print "-" after the value at the rear // - print "." for the places without valid element. public void print () { if(front==-1 && rear == -1) { for(int i=0;i<queue.length;i++) { System.out.print("."); } } else { if(front<=rear) { for(int i=0;i<=front-1;i++) { System.out.print("."); } System.out.print("+"); for(int i=front;i<=rear;i++) { System.out.print(queue[i]); } System.out.print("-"); for(int i=rear+1;i<=queue.length-1;i++) { System.out.print("."); } } else { for(int i=0;i<=rear;i++) { System.out.print(queue[i]); } System.out.print("-"); for(int i=rear+1;i<=front-1;i++) { System.out.print("."); } System.out.print("+"); for(int i=front;i<=queue.length-1;i++) { System.out.print(queue[i]); } } } } } 

private int [] queue; private int front, rear; // do not change the constructor CircularQueue() { queue = new int [5]; front = -1; rear = -1; } // FILL IN: // throws DSException if out of space public void enqueue ( int item ) throws DSException,Exception { if ( front == -1 && rear == -1 ){ front = 0; rear = 0; queue[rear] = item; } else if((rear+1)%queue.length == front) { throw new Exception("Full"); } else { rear = (rear+1)%queue.length; queue[rear] = item; } } // FILL IN: // throws DSException if no element in the queue // return the dequeued value public int dequeue () throws DSException { if ( front == -1 && rear == -1 ){ throw new DSException(); } else { int ret = queue[front]; if(rear==front) { rear = -1; front = -1; } else { front = (front+1)%queue.length; } return ret; } } // FILL IN: // return the value at beginning of the queue // throws DSException if no element in the queue public int first () throws DSException { if(front==-1 && rear ==-1) { throw new DSException(); } return queue[front]; } // FILL IN: // print the circular queue in the following format // - print "+" before the value at the front // - print "-" after the value at the rear // - print "." for the places without valid element. public void print () { if(front==-1 && rear == -1) { for(int i=0;i<queue.length;i++) { System.out.print("."); } } else { if(front<=rear) { for(int i=0;i<=front-1;i++) { System.out.print("."); } System.out.print("+"); for(int i=front;i<=rear;i++) { System.out.print(queue[i]); } System.out.print("-"); for(int i=rear+1;i<=queue.length-1;i++) { System.out.print("."); } } else { for(int i=0;i<=rear;i++) { System.out.print(queue[i]); } System.out.print("-"); for(int i=rear+1;i<=front-1;i++) { System.out.print("."); } System.out.print("+"); for(int i=front;i<=queue.length-1;i++) { System.out.print(queue[i]); } } } } }

Answer 2

This is my approach. But I am assuming here that the empty blocks in the array are initialized with zero, and that any valid entry would be non-zero. Also, it will print zeros in case the queue is partially filled.

public void print() {
            if (isEmpty()) {
                System.out.println("Queue is empty");
            } else {
                System.out.println("In order from latest to oldest");
                int i = front;
                while (i < array.length) {
                    System.out.print(array[i] + " ");
                    i++;
                }
                i = i % array.length;
                if(array[i] != 0) {
                    while(i < front) {
                        System.out.print(array[i] + " ");
                        i++;
                    }
                }
            }
        }

Answer 3

Problem with circular arrays and front/rear indices is that 'full' and 'empty' are indistinguishable. You will have to add a boolean 'empty', which is initially true, and is used in the tests.

private int [] queue;
private int front, rear;
private boolean empty;

// do not change the constructor
CircularQueue() {
    queue = new int [5];
    front = 0;
    rear = -1;
    empty = true;
}

// FILL IN:
// throws DSException if out of space
public void enqueue ( int item  ) throws DSException {
        if (!empty && (front - rear + queue.length) % queue.length == 1){
                throw new DSException();
        }
        queue[rear+1] = item;
        rear = (rear+1)%queue.length;
        empty = false;
}

// FILL IN:
// throws DSException if no element in the queue
// return the dequeued value
public int dequeue () throws DSException {
        if (empty){
                throw new DSException();
        }            

        int temp = queue[front];
        queue[front] = 0;
        front = (front+1)%queue.length;
        empty = (front - rear + queue.length) % queue.length == 1;
        return temp;

}