Thread: Deque array base assistant

I need help with my deque class i mainly have a issue with my toString where i can naturally insert from the front and print normally
however when i add anything from the rear and print nothing prints out.

I will like the tostring to print from front to rear. Thank you

public class MyDeque {
	private int maxSize;
	private Item[] queArray;
	private int front;
	private int rear;
	private int nItems;
 
	public MyDeque(int s)  // constructor
	{
		maxSize = s;    // initialize maxsize
		queArray = new Item[maxSize]; // type item 
		front =0;
		rear = maxSize-1;
		nItems = 0; 
	}
	public void insertFront(Item item) // put item at rear of queue
	{
		if(front == 0)
			front = maxSize;
		queArray[--front] = item;
		nItems++;
	}
	public void insertRear(Item item) // put item at rear of queue
	{
		if(rear==maxSize-1)
			rear = -1;
		queArray[++rear] = item;
		nItems++;
	}
	public Item removeFront() //take item from front of queue
	{
		Item temp = queArray[front++];
 
		if(front == maxSize)
			front = 0;
		nItems--;
		return temp;
	}
	public Item removeRear() //take item from front of queue
	{
		Item temp = queArray[rear--];
 
		if(rear == -1)
			rear = maxSize -1 ;
		nItems--;
		return temp;
	}
	public Item peekFront() //peek at front of queue
	{
		return queArray[front];
	}
	public Item peekRear() //peek at last of queue
	{
		return queArray[rear];
	}
	public boolean isEmpty() //true if queue is empty
	{
		return(nItems == 0);
	}
	public boolean isFull() //true if queue is full
	{
		return (nItems == maxSize);
	}
 
	[U]public String toString(){
		String msg = "";
		for (int i = front; i <= rear; i++){
 
			msg += queArray[i].toString() + "\n";
		}
		return msg;
	}[/U]
 
}

the println will only print the front data insertion ...the rear is not printing. thanks for your help

At the first time:
When we insert front, your front = max_size -1
Then we insert rear, your rear = 0
Then look at your toString():
for(i = front; i <= rear)
-> front > rear -> print nothing.

I think for queue & dequeue it's better if you use a linkedlist to implement.
If you wanna fixed array, you need to check your front, tail carefully.

thanks zkidkid

so i will need a flow control within the tostring to check the front and rear ??

Originally Posted by gto11520

thanks zkidkid

so i will need a flow control within the tostring to check the front and rear ??

Yes if you "flow control" mean to make sure "front" always lower than "rear" ( front <= rear)

For better implement, I also suggest you take a look of LinkedList.

Originally Posted by zkidkid

Yes if you "flow control" mean to make sure "front" always lower than "rear" ( front <= rear)

For better implement, I also suggest you take a look of LinkedList.

Thanks. I will test this tonight

again i still have issue with my tostring in my dequre class and it isnt working properly. it can't print because its a one way traffic inserting front until full will print normal. or one way traffic inserting rear untill full will print normal. once i combine insert front and then insert rear then print nothing because my loop condition is not true 2<= 0 not true so doesnt print anything.

i tried putting if then condition and then i get nullpointer error or array out of bounds

Please put your code so that we could help.
at the first step, you could only implement queue(E) and dequeue() method to get understand clearly first and then implement others.
it would be easier & better to implement by using linkedlist instead fixed array.

1. i havent learned linked list quite yet. so i'm not gonna attempt to use linkedlist

2. below code throws me an error for null pointer

3. implmentating queue(E) has no error by inserting i had no issues. by removing no issues

4. the issue lies with the tostring my logic is off. lets say maxsize is 3 so therefore when i insertfront front, this was initialize to zero in beginning so the first condition tested true hence predecrement making front = 2

5. now lets say i insertrear, rear was initialize to maxsize-1 which a tree statement for the condition therefore rear is set to -1 . then preincrement rear which makes rear = 0

6. for (int i = front; i <= rear; i++){

msg += queArray[i].toString() + "\n";
}
* front = 2 rear = 0 ..... 2 <= 0 this condition is false therefore the loop never runs which is a problem for me

however lets say i just insertfront then front = 2 and rear = maxsize -1 which 2 because maxsize is 3
2 <= 2 is true therefore loop runs and only print front never rear.

public String toString(){
String msg = "";
if (front <= rear){
for (int i = front; i <= rear; i++){
System.out.print("testing inner loop");
msg += queArray[i].toString() + "\n";
}
}
else
for (int i = front; i >= rear; i--){
System.out.print("testing inner loop");
msg += queArray[i].toString() + "\n";
}

return msg;
}

NullPointer at this code: queArray[i].toString()
your queArray[i] == null.

Please put your whole code.
It's easier than you think.

public class MyDeque {
private int maxSize;
private Item[] queArray;
private int front;
private int rear;
private int nItems;

public MyDeque(int s) // constructor
{
maxSize = s; // initialize maxsize
queArray = new Item[maxSize]; // than requested
front =0;
rear = maxSize-1;
nItems = 0;
}
public void insertFront(Item item) // put item at rear of queue
{
if(front == 0)
front = maxSize;
queArray[--front] = item;
nItems++;
}
public void insertRear(Item item) // put item at rear of queue
{
if(rear==maxSize-1)
rear = -1;
queArray[++rear] = item;
nItems++;
}
public Item removeFront() //take item from front of queue
{
Item temp = queArray[front++];

if(front == maxSize)
front = 0;
nItems--;
return temp;
}
public Item removeRear() //take item from front of queue
{
Item temp = queArray[rear--];

if(rear == -1)
rear = maxSize -1 ;
nItems--;
return temp;
}
public Item peekFront() //peek at front of queue
{
return queArray[front];
}
public Item peekRear() //peek at last of queue
{
return queArray[rear];
}
public boolean isEmpty() //true if queue is empty
{
return(nItems == 0);
}
public boolean isFull() //true if queue is full
{
return (nItems == maxSize);
}

public String toString(){
String msg = "";
if (front <= rear){
for (int i = front; i <= rear; i++){
System.out.print("testing inner loop");
msg += queArray[i].toString() + "\n";
}
}
else
for (int i = front; i >= rear; i--){
System.out.print("testing inner loop");
msg += queArray[i].toString() + "\n";
}

return msg;
}
}

Please post your code correctly, like you did in post #1. You can edit your last post and add the code tags.

Originally Posted by Norm

System.out.print("testing inner loop");

Some comments on the print statements you've used. The same message is printed by both so you won't know which one printed.
The messages don't have show the values of the variables that controlled if they were to be printed: front and rear.
The print are inside the loop so they'l print several times. That could be useful so you'll see how many times the loop executed.

Thanks let me try this

Hi @gto11520,
Let image that you have a fixed array for 5 items. [][][][][][]
You put 1 front(value 1) & 1 rear (value2) with your code then you will have:

public void insertFront(Item item) // put item at rear of queue
{
if(front == 0)
front = maxSize;
queArray[--front] = item;
nItems++;
}

[][][][][1]
it's fine.
Then now I put 1 rear:

public void insertRear(Item item) // put item at rear of queue
{
if(rear==maxSize-1)
rear = -1;
queArray[++rear] = item;
nItems++;
}

So you will have:
[2][][][][1]


So how could this bellow code work ?

public String toString(){
String msg = "";
if (front <= rear){
for (int i = front; i <= rear; i++){
System.out.print("testing inner loop");
msg += queArray[i].toString() + "\n";
}

Let I talk you again, think about it more carefully to understand it & then implement later.

Let Imagine that you have an unlimited fixed array.
front=-1, rear = -1;
At first time, you put 1 item, then:
...[][front,rear][]...
front = 100 (for example)
rear = 100

Now you insert front:
...[front][rear][]....
front = 99, rear = 100

Now you still insert front:
...[front][item][rear][].....
front = 98
rear = 100

Now you insert rear:

...[front][item][item][rear]...
front = 98
rear = 101

got it ?

In case you just have a fixed array, if you couldn't put front -> return No Space Error, if you couldn't put rear -> return No Space Error.
After successful implement this above, you could think more to re-allocate your array or etc ...