How do I turn my custom linked list into an array?

Question

I have to implement a method:

E[] toArray(E[] a) // Pass an array, convert to singly linked list, then return the array.

from java.util Interface List<E>

As I mentioned, I have to pass an array, convert it to a singly linked list, sort it, then return the array.

In the Node class I have this to work with:

public Node(E v, Node<E> next) {
    // pre: v is a value, next is a reference to remainder of list
    // post: an element is constructed as the new head of list
    data = v;
    nextElement = next;
}

public Node(E v) {
    // post: constructs a new tail of a list with value v
    this(v,null);
}

public Node<E> next() {
    // post: returns reference to next value in list
    return nextElement;
}

public void setNext(Node<E> next)  {
    // post: sets reference to new next value
    nextElement = next;
}

public E value() {
    // post: returns value associated with this element
    return data;
}

public void setValue(E value) {
    // post: sets value associated with this element
    data = value;
}

Am I barking up the wrong tree or can someone help me with this here? Sorry if this is the wrong place for such questions.

Where did the requirement to sort the list come from? What have you tried so far with respect to implementing E[] toArray(E[] a)? — Mark Elliot
– Mark Elliot, Commented Oct 21, 2010 at 3:17
Why are you turning it into a linked list and back to an array again? Is this just for an exercise? — shoebox639
– shoebox639, Commented Oct 21, 2010 at 3:22

XecP277 · Accepted Answer · 2010-10-21 05:33:29Z

The following code will create the single linked list and copy that back to new copy of the array. For the sort you need to make sure you make the \"E"\ type implementes comparable. One way is to change the generic declarator of \"E"\, to <E extends Comparable<? super E>>.


    E[] toArray(E[] a)
    {
        E[] result ;
        Class<?> type ;
        Node<E> head, temp, current ;

        /*
         * Makes a copy of the original array
         */
        type = a.getClass().getComponentType() ;
        result = (E[])java.lang.reflect.Array.newInstance(type, a.length);
        for(int idx = 0; idx < a.length; idx++)
            result[idx] = a[idx] ;

        /*
         * Sort the array (selection copy)
         */
        for(int idx = 0; idx < result.length; idx++)
        {
            int best = idx ;
            for(int jdx = idx + 1; jdx < result.length; jdx++)
            {
                if (result[best].compareTo(result[jdx]) > 0)
                {
                    best = jdx ;
                }
            }
            // swap
            if (best != idx)
            {
                E temporal = result[idx] ;
                result[idx] = result[best] ;
                result[best] = temporal ;
            }
        }

        /*
         * Compose the single linked list (SORTED)
         */
        head = new Node<E>(null, null) ;

        // Create the single linked list
        current = head ;
        for(E element : result)
        {
            temp = new Node<E>(element, null) ;
            current.setNext(temp) ;
            current = current.next();
        }

        /*
         * Copy the single linked list to the array 
         * (Not needed, added just for educational purpose,
             * the result array is already SORTED)
         */

        current = head.next ;
        // copies the values to the result array
        for(int index = 0; current != null ; current = current.next)
            result[index++] = current.value();

        return result ;
    }

Eric Towers · Accepted Answer · 2010-10-21 03:15:07Z

0

Obvious wrong answer:

    E[] toArray(E[] a)
    { return a; }  // Prove I didn't make the linked list.

I assume there are some side effect to constructing and sorting the linked list that you completely skipped? Or maybe the return is supposed to be the sorted list, not the array, possibly the sorted list coerced back into an array?

answered Oct 21, 2010 at 3:15

Eric Towers

4,2551 gold badge18 silver badges17 bronze badges

5 Comments

meesh Over a year ago

straight from the horse's mouth:

meesh Over a year ago

Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. If the list fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this list.

meesh Over a year ago

If the list fits in the specified array with room to spare (i.e., the array has more elements than the list), the element in the array immediately following the end of the list is set to null. (This is useful in determining the length of the list only if the caller knows that the list does not contain any null elements.)

Eric Towers Over a year ago

The only way it would be smaller is if you're supposed to eliminate duplicates or otherwise drop array members as they pass through this process. So, are you supposed to eliminate array elements as they pass through? The only way it could get larger is if you're supposed to invent new elements that weren't in the array initially, which also hasn't been mentioned up to now.

meesh Over a year ago

That's a good point. I'll do my best to figure out an answer.

shoebox639 · Accepted Answer · 2010-10-21 04:04:48Z

0

I hope this does what you want:

/**
 * You need to pass in the class of the object you are creating
 * because you can't instantiate a generic class.
 */
public static <E> E[] toArray(E[] a, Class<E> clazz) {
    Node<E> root = null;
    Node<E> prev = null;
    Node<E> curr = null;

    for (E e : a) {
        curr = new Node<E>(e);
        if (prev != null) {
            prev.setNext(curr);
        }
        else {
            root = curr;
        }
        prev = curr;
    }

    curr =  root;

    // Cast is unsafe. I don't know a better way to do this.
    E[] newArray = (E[]) Array.newInstance(clazz, a.length);
    int i = 0; 
    while (curr != null) {
        newArray[i] = curr.value();
        curr = curr.next();
        i++;
    }

    return newArray;
}

As I say in the code, I don't know how to correctly instantiate a generic class. Can someone correct me?

answered Oct 21, 2010 at 4:04

shoebox639

2,31215 silver badges14 bronze badges

10 Comments

meesh Over a year ago

I would love to be able to correct you but you're way ahead of me, so all I can say do is try to follow the logic. Thanks for the input.

shoebox639 Over a year ago

By the way is this AP comp sci homework? I remember doing stuff like this in high school. I'll take silence as tacit agreement ;)

meesh Over a year ago

Sorry to bother. Didn't know if I was in the right place or not.

shoebox639 Over a year ago

Heh, just saying, if I were a teacher I'd troll here days before an assignment is due.

meesh Over a year ago

Well, to be honest, they don't seem to care. They encourage us to get help online and from each other and such.

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