Copy Elements From Stack to Queue

Question

I'm supposed to copy elements from a stack to a queue.

I haven't been able to think of a way to keep the stack the way it is and still copy its elements to a queue.

I ended up with this method which removes the elements completely from the stack and adds them to the queue:

public void CopyFromStack(){
E t;
int c = w.size();
while(c != 0){
t = w.pop();
enqueue(t);
c--;  }}

Trying to push elements back is not an option because it'll do it backwards.

Edit: This is the class that contains my method which is my queue class:

public class Queue<E> {
protected int size;
protected Node<E> head;
protected Node<E> tail;
NodeStack<E> w = new NodeStack<E>();
NodeStack<E> w2 = new NodeStack<E>();

public Queue(){
    size = 0;
    head = tail = null;}

public boolean isEmpty(){
    return size==0;}

public void enqueue(E elem) { 
Node<E> node = new Node<E>(); 
node.setElement(elem); 
node.setNext(null); 
if (size == 0) head = node; 
else tail.setNext(node);
tail = node; 
size++;  }

public E dequeue() {
if (size == 0) System.out.print("Queue is empty."); 
E tmp = head.getElement(); 
head = head.getNext(); 
size--;
if (size == 0) tail = null; 
return tmp; } 

public String toString(){
    String s = "";
    E t;
    int c = size;
    while(c != 0){
        t = dequeue();
        s += t + " ";
        enqueue(t);
        c--; }
    return s;}

public int FindItem(E elem){
    int index=0;
    int c = size;
    E t;
while(c != 0){
t = dequeue();
if (t == elem)
return index;
else index++;
c--;}
System.out.print("Not found!");
return -1;}

public void CopyToStack(){
System.out.print("Elements copied to the stack are: ");
E t;
int c = size;
while(c != 0){
t = dequeue();        
w.push(t);
enqueue(t);
c--;
System.out.print(w.pop()+" "); }}

public void CopyFromStack(){
E t;
int c = w.size();
while(c != 0){
t = w.pop();
enqueue(t);
c--;  }}

Please show the rest of your stack code so we can help you. — Jonas Czech
– Jonas Czech, Commented Apr 25, 2015 at 17:42
The first line in your question contradicts the rest of the lines. You say want to copy elements from a queue to a stack and then you say you haven't been able to keep the stack the way it is and still copy its elements to a queue. Kindly edit your question to remove this ambiguity. Do you want to copy from a stack to queue or queue to stack? — Chetan Kinger
– Chetan Kinger, Commented Apr 25, 2015 at 17:47
What Stack class are you talking about? Is it yours? If so, why doesn't it provide a way to iterate over its content without modifying it? — JB Nizet
– JB Nizet, Commented Apr 25, 2015 at 17:51
Cant you just get the iterator for the stack and iterate through them. This will not alter the stack? — AppX
– AppX, Commented Apr 25, 2015 at 18:02
I edited my mistake in the question. I hope it makes more sense now. — CuriousAD
– CuriousAD, Commented Apr 25, 2015 at 18:52

paulsm4 · Accepted Answer · 2015-04-25 22:12:54Z

0

Q: I haven't been able to think of a way to keep the stack the way it is A:

A: That's because reading from a "classic" stack is destructive. "Reading" an element == removing that element from the stack.

TWO SOLUTIONS:

1) Modify your stack implementation so that you can "peek" each element

... or ...

2) Create a new stack containing all the elements from the first one.

Q: I ended up with this method ... Trying to push elements back is not an option because it'll do it backwards.

"A: This is a variation on "Option 2): above.

SOLUTION: Just create a new stack object, and push each element at the same time as you enqueue the element.

PS:

The standard JRE implementation of Stack includes peek() and search() methods. But I don't think they would help you here. If you wanted "Option 1)", you'd have to implement your own, custom stack.

================== UPDATE ==================

Note, too:

You should always indent your methods, and indent your "if" and "loop" blocks within your methods.
You should use "camel-case" (lower-case first letter) for your method names.

Here are the "official" Java coding conventions. They were useful in 1995; they're useful today:

http://www.oracle.com/technetwork/java/index-135089.html

There's actually a third option: Java's "Stack" happens to implement "iterator". Here's an example:

EXAMPLE CODE:

package com.testcopy;

import java.util.ArrayDeque;
import java.util.Iterator;
import java.util.Queue;
import java.util.Stack;

public class TestCopy {

    public static void main (String[] args) {

        TestCopy app = new TestCopy ();
        app.run ();
    }

    public void run () {
        // Create and populate stack
        Stack<String> myStack = new Stack<String> ();
        mkData(myStack);

        // Copy to queue
        Queue<String> myQueue = new ArrayDeque<String> ();
        copyFromStack (myStack, myQueue);

        // Print 
        int i=0;
        for (String s : myQueue) {
            System.out.println ("myQueue[" + i++ + "]: " + s);
        }
    }

    @SuppressWarnings("unchecked")
    public void mkData (Stack stack) {
        stack.push("A");
        stack.push("B");
        stack.push("C");
        // Stack should now contain C, B, A
    }

    public void copyFromStack (Stack stack, Queue queue) {
        @SuppressWarnings("rawtypes")
        Iterator it = stack.iterator ();
        while (it.hasNext()) {
            queue.add(it.next());
        }
    }
}

EXAMPLE OUTPUT:

myQueue[0]: A
myQueue[1]: B
myQueue[2]: C

edited Apr 25, 2015 at 22:12

answered Apr 25, 2015 at 17:50

paulsm4

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5 Comments

Chetan Kinger Over a year ago

What if the original stack is huge? What if you have limited memory to work with. Questions like these implicitly ask for an optimized solution.

paulsm4 Over a year ago

And what if the original stack isn't huge or you do have sufficient memory? The point is: both "Option 1" and "Option 2" are applicable. Which to choose? "It depends". And remember: "Option 2 " involves just three extra lines: 1) Create the new stack, 2) push the elements, and 3) replace the old stack with the new stack once everything's been copied.

Chetan Kinger Over a year ago

But what if Option 1 is not possible because the Stack class is third party? The point I am trying to make is that creating a copy of the stack is kind of common sense and I don't believe the OP would post a question if that was a probable solution.

CuriousAD Over a year ago

Creating a new stack was first thought, but I was hoping I could find a way to avoid that.

CuriousAD Over a year ago

Your solution was most helpful. Thank you.

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