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.

You are using ( front == 0 && rear == -1 ) to detect BOTH full AND empty states AND whether to print a . or something else. I suggest you revisit that expression in all three places and rethink what it means, especially bearing in mind front and rear change as you add/remove entries. — OldCurmudgeon
– OldCurmudgeon, Commented Mar 14, 2013 at 14:06

Mihir Gogate · Accepted Answer · 2013-04-24 05:46:58Z

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]);   
              }
        }

    }

}    


}

Saurabh Mithe · Accepted Answer · 2016-06-05 12:55:38Z

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++;
                    }
                }
            }
        }

Vincent van der Weele · Accepted Answer · 2013-03-14 14:05:59Z

0

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;

}

answered Mar 14, 2013 at 14:05

Vincent van der Weele

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7 Comments

Envix Over a year ago

i know what you mean, but i am not not supposed to add other methods because its a requirement for this assignment, i can only use enqueue, dequeue and print methods.

OldCurmudgeon Over a year ago

Another alternative is to use front == rear as full and (rear + 1) % bufferSize == front as empty. This way you can fit the maximum number of elements in the buffer.

Envix Over a year ago

if you look at the comments in code i am not supposed to change the constructor. the problem is that the problem ends after enqueue(0) so maybe something is wrong with my print method?

Vincent van der Weele Over a year ago

There are not enough states front and rear can have. If you want them both in the range [0-5), which is the case after the first enqueue, there are 5 options for rear per value of front. However, there are 6 states: 0, 1, 2, 3, 4, and 5 elements in the queue. Conclusion: you need an extra variable.

Envix Over a year ago

but i can't add an extra variable, it was my mistake it should be 0-4 which is 5. but the problem is the program ends itself after enqueue(0) its not even printing the "<" in the print method. So i am thinking theres somthing wrong with the exception if in enqueu method?

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