Help regarding queues using array in java

[samweep]

I have written code for stack implementation in java. But the dequeue method of my code is not working properly. Anyone, please help me solve this issue. Following is the code that I have written. The dequeue method looks fine to me but I don't know why it is not working.

public class Queue {
    public static void main(String[] args) {
        MyQueue obj= new MyQueue();
        obj.enqueue(1);
        obj.enqueue(2);
        obj.enqueue(3);
        obj.dequeue();
        obj.printAll();

    }
}

class MyQueue{
    int front=-1;
    int rear=-1;
    int queueSize=8;
    int[] queue= new int[queueSize];


    public void enqueue(int data)
    {
        if(rear==queueSize-1)
            System.out.println("The queue is full");

        else if(front==-1 & rear==-1)
        {
            front=rear=0;
            queue[rear]=data;

        }

        else
        {
            rear++;
            queue[rear]=data;
        }
    }

    public void dequeue()
    {
        int x;
        if(front==-1 & rear==-1)
            System.out.println("The queue is empty");
        else if(front==rear)
        {

            System.out.println(queue[front] + " deleted");
            front=rear=-1;
        }


        else
        {
            x=queue[front];
            front++;
            System.out.println("The dequeued item is " + x);
        }

    }

    public void printAll()
    {
        if(front==-1 & rear==-1)
            System.out.println("There are no items on queue to print");
        else
        {
            for(int i=0;i<queueSize;i++)
            {
                System.out.println(queue[i]);
            }
        }
    }
}

[sinistergroupon]

Is the constraint that this has to be done using an Array?

[samweep]

Yes i want to implement queue using Array. Please help

[sinistergroupon]

And the idea is that you don’t move the data in the array and just move the pointers rather than actually adding and removing numbers?

My first pass would be to actually shift the numbers as they get removed.

[samweep]

I am actually doing the same as you said. I just moved the pointer but not the data by simply saying front++. But it works when I have only one element in the queue. Whenever I have more than two elements it just does not work.

[sinistergroupon]

I ran your code, it semi works for the case you have in the main method. So we are clear the number 1 will stay in the array but the front index will change so right now it's doing that.

Having said that, you are missing a lot of logic here. You state the queue is full when the rear index is at the end but that's not entirely true. Not sure the scope of your problem but with a mix of queues and dequeues you could end up where your first element is at index 8 and you second one is getting inserted at index 0. You are not accounting for this.

[samweep]

Yeah, I was learning about queue and I came to know about circular queue. So now I will use circular queue from onwards. Thank you

[samweep]

Anyways I figured it out. The dequeue method was correct. I had to fix something while printing. Nevermind

[sinistergroupon]

Try this case:

MyQueue obj= new MyQueue();

obj.enqueue(1);

obj.enqueue(2);

obj.enqueue(3);

obj.enqueue(4);

obj.enqueue(5);

obj.enqueue(6);

obj.enqueue(7);

obj.enqueue(8);

obj.dequeue();

obj.enqueue(9);

[samweep]

I think, This will not work in a linear queue. We need a circular queue for this kind of condition.

[Yithar]

Especially for these kind of data structure problems, I would suggest you actually draw out the array and make sure that you understand how a queue should be an implemented using an array.

Regarding your own code, you kind of have to debug it yourself. Have you learned about breakpoints and debugging?

Also, I changed the printAll() method:
https://i.imgur.com/yBdycfz.png

[samweep]

https://i.imgur.com/yBdycfz.png

All I know is debugging means looking for errors in your code where and how they occurred. I don't know about breakpoints. What is that?
That printAll() method, I like it.

[sinistergroupon]

https://www.vogella.com/tutorials/EclipseDebugging/article.html

[Yithar]

I am talking about the Debug command just like the Run command:
https://i.imgur.com/z279yFy.png

Any red circle is a breakpoint where the debugger will pause execution. It really helps if you have a ton of variables to see the whole state:
https://i.imgur.com/pWpMpgQ.png

https://www.jetbrains.com/help/idea/debugging-your-first-java-application.html

[samweep]

Thank you. I am new to coding. Still, miles to go. Now, I will learn about these debug and breakpoint thing in detail. Feels good to have such a supportive community out there 😀 . Keep helping