ordered containers cmput 115 - lecture 21 department of computing science university of alberta...

Ordered ContainersOrdered Containers

Cmput 115 - Lecture 21

Department of Computing Science

University of Alberta©Duane Szafron 2000

Some code in this lecture is based on code from the book:Java Structures by Duane A. Bailey or the companion structure package

Revised 2/25/00

©Duane Szafron 2000

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About This LectureAbout This Lecture

In this lecture we will learn about Ordered containers.

An ordered container is a container where the order of the elements depends not on the order they are added, but rather on comparisons of the elements that are added.


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OutlineOutline

Ordered Containers OrderedStructure Interface OrderedStructure Example OrderedVector class OrderedList class


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Ordered ContainersOrdered Containers An ordered container is a container whose elements are

ordered by comparing them with each other.

This requires a binary operation to be defined that applies to any pair of elements that can be added to the container.

In Java, we use the compareTo(Object) method from the Comparable Interface.

As each element is added to the container it immediately goes to the proper location in the container based on comparing it with all other elements that are in the container.


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OrderedStructure HierarchyOrderedStructure Hierarchy

The structure package adds the OrderedStructure interface below the Collection interface.

Store

Collection

List OrderedStructure


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Structure Interface - StoreStructure Interface - Store

public interface Store {public int size();//post: returns the number of elements contained in // the store.

public boolean isEmpty();// post: returns the true iff store is empty.

public void clear();// post: clears the store so that it contains no // elements.

}

code based on Bailey pg. 18


7code based on Bailey pg. 19

Structure Interface - CollectionStructure Interface - Collectionpublic interface Collection extends Store {

public boolean contains(Object anObject);// pre: anObject is non-null// post: returns true iff the collection contains the object

public void add(Object anObject);// pre: anObject is non-null// post: the object is added to the collection. // Replacement policy is not specified

public Object remove(Object anObject);// pre: anObject is non-null// post: removes object “equal” to anObject and returns it,// otherwise returns nil

public Iterator elements();// post: return an iterator for traversing the collection }


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Structure Interface - OrderedStructureStructure Interface - OrderedStructure

public interface OrderedStructure extends Collection {}


The unusual thing about the OrderedStructure interface is that it does not add any new methods to those provided by Collection.

However, any class that implements this interface must ensure that when elements are added, they go to the correct location.

In essence, it changes the post condition of the add method in Collection:

// post: the object is added to the collection. The // replacement policy is not specified


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OrderedStructure ExampleOrderedStructure Example

public static void main (String[ ] args) {

OrderedStructure container;RandomInt generator;int index;Iterator iterator;

container = new OrderedVector();generator = new RandomInt(1);for (index = 0; index < 100; index++) {

container.add(new Integer(generator.next(100)))};iterator = container.elements();while(iterator.hasMoreElements())

System.out.print(iterator.nextElement() + ‘ ‘);} code based on Bailey pg. 158

1 1 1 2 2 3 3 3 44 5 7 7 7 8 9 9 1013 13 14 14 15 17...


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OrderedVector - OrderedVector - State and ConstructorState and Constructor

class OrderedVector implements OrderedStructure {

protected Vector data;

public OrderedVector(){// post: intitalizes the OrderedVector to have 0 elements

this.data = new Vector();}



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OrderedVector - OrderedVector - Store InterfaceStore Interface

/* Interface Store Methods */public int size() {//post: returns the number of elements contained in the store.

return this.data.size();}

public boolean isEmpty() {// post: returns the true iff store is empty.

return this.size() == 0;}

public void clear();// post: clears the store so that it contains no elements.

this.data.clear(); }code based on Bailey pg. 178


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OrderedVector - OrderedVector - contains(Object)contains(Object)

/* Interface Collection Methods */

public boolean contains(Object anObject) {// pre: anObject is non-null// post: returns true iff the collection contains the object

int index;

index = this.indexOf ((Comparable) anObject);return (index < this.size()) &&

(this.data.elementAt(index).equals(anObject));}



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OrderedVector - OrderedVector - add(Object)add(Object)

public void add(Object anObject);// pre: anObject is non-null// post: the object is added to the collection at the// appropriate position based on comparing it to the// other elements.

int index;

index = this.indexOf ((Comparable) anObject);this.data.insertElementAt(anObject, index);

}



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OrderedVector - OrderedVector - remove(Object)remove(Object)

public Object remove(Object anObject);// pre: anObject is non-null// post: removes object “equal” to anObject and returns it,// otherwise returns nil

int index;Object result;

index = this.indexOf((Comparable) anObject));if (index < this.size()) && (this.data.elementAt(index).equals(anObject)) { result = this.data.elementAt(index); //finds this.data.removeElementAt(index); //removes return result;}return null; } code based on Bailey pg. 177


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OrderedVector - OrderedVector - elements()elements()

public Iterator elements();// post: return an iterator for traversing the collection

return this.data.elements();}



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The Search ProblemThe Search Problem

To complete this class, we need to solve the search problem for a sorted container.

Given a container, find the index of a particular element, called the key.

If it is not there, find the index where it should be.

10 25 30 50 55 60 70 75 800 1 2 3 4 5 6 7 8 9

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?


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Binary Search AlgorithmBinary Search Algorithm

L HM middle = (low + high) / 2

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L HM

low = middle + 1

middle = (low + high) / 2

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high = middle - 1

HML

10 25 30 50 55 60 70 75 800 1 2 3 4 5 6 7 8 9


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Element not found 1Element not found 1

L HM middle = (low + high) / 2

10 25 30 50 55 60 70 75 800 1 2 3 4 5 6 7 8 9

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high = middle - 1

HML

10 25 30 50 55 60 70 75 800 1 2 3 4 5 6 7 8 9

L HM

low = middle + 1


10 25 30 50 55 60 70 75 800 1 2 3 4 5 6 7 8 9


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Element not found 2Element not found 2

H

low < high

middle = (low + high) / 2LM

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low = middle + 1

L HM

10 25 30 50 55 60 70 75 800 1 2 3 4 5 6 7 8 9


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Element past end 1Element past end 1

L HMmiddle = (low + high) / 2

10 25 30 50 55 60 70 75 800 1 2 3 4 5 6 7 8 9

90


HML

10 25 30 50 55 60 70 75 80

0 1 2 3 4 5 6 7 8 9

low = middle + 1

L HM

low = middle + 1


10 25 30 50 55 60 70 75 800 1 2 3 4 5 6 7 8 9


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Element past end 2Element past end 2

H

low < high

middle = (low + high) / 2 LM

10 25 30 50 55 60 70 75 800 1 2 3 4 5 6 7 8 9

90

low = middle + 1

L HM

10 25 30 50 55 60 70 75 800 1 2 3 4 5 6 7 8 9


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OrderedVector - OrderedVector - indexOf(Object) 1indexOf(Object) 1

/* Protected Methods */ protected int indexOf(Comparable anObject) {

// pre: anObject is non-null// post: returns index of object in the collection or where// it should be placed if it is not in the collection

Comparable midObject;int low;int high;int middle;int comparison;

low = 0;high = this.data.size();middle = (low + high) / 2;



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OrderedVector - OrderedVector - indexOf(Object) 2indexOf(Object) 2

while (low < high) { midObject =

(Comparable) this.data.elementAt(middle); comparison = midObject.compareTo(anObject); if (comparison) < 0)

low = middle + 1; else if (comparison > 0)

high = middle - 1; else

return middle; middle = (low + high) / 2;}return low;

}code based on Bailey pg. 174


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Time Complexity of OrderedVectorTime Complexity of OrderedVector

The indexOf(Object) method does O(log(n)) comparisons to find the index.

This means that it takes O(log(n)) comparisons for the add(Object), remove(Object) and contains(Object) methods.

However, it also requires O(n) assignments to move elements in methods add(Object) and remove(Object).

In Java, for most objects, the compareTo(Comparable) method is slower than assignment so the time complexity of add(Object) and remove(Object) is O(log(n)).


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OrderedListOrderedList

We can also implement the OrderedStructure Interface using a linked list in a class called OrderedList.

However, we do not simply bind an instance variable to a linked list object like a SinglyLinkedList since we require access to the middle of the list to put added elements in the correct location.

Therefore we use SinglyLinkedListElements and link them together manually.


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OrderedList - OrderedList - difference from difference from OrderedVectorOrderedVector

The important difference between OrderedList and OrderedVector is that the internal implementation of OrderedVector has access to the indexes of the underlying Vector elements.

– This allows us to find the index of a particular element so that it can be found, added, or removed.

– It also allows us to do a binary search since we can divide the search list in half using the indexes.


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OrderedList - Sequential SearchOrderedList - Sequential Search

In OrderedList, we create an analog of the indexOf(Object) method called previousOf(Object) which returns the node before the node containing the object, or the node before the node where the object should be inserted.

Unfortunately, we must do a sequential search instead of a binary search.

However, we can stop early if we encounter an element that is larger than the one we are looking for.


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OrderedList - OrderedList - State and ConstructorState and Constructor

class OrderedList implements OrderedStructure {

protected SinglyLinkedListElement head;protected int count;

public OrderedList(){// post: intitalizes the OrderedList to have 0 elements

this.clear();}



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OrderedList - OrderedList - Store InterfaceStore Interface

/* Interface Store Methods */public int size() {//post: returns the number of elements in the store.

return this.count;}

public boolean isEmpty() {// post: returns the true iff store is empty.

return this.size() == 0;}

public void clear();// post: clears the store so that it contains no elements.

this.head = null;this.count = 0; }



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OrderedList - OrderedList - contains(Object)contains(Object)

/* Interface Collection Methods */ public boolean contains(Object anObject) {

// pre: anObject is non-null// post: returns true iff the collection contains the object

SinglyLinkedListElement previous;SinglyLinkedListElement current;previous = this.previousOf((Comparable)

anObject);if (previous == null) // no previous element, first node

current = this.head;else current = previous.next();if (current == null) return false;else return current.value().equals(anObject); }



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OrderedList - OrderedList - add(Object)add(Object)

public void add(Object anObject);// pre: anObject is non-null// post: the object is added at the appropriate position // based on comparing it to the other elements.

SinglyLinkedListElement previous;SinglyLinkedListElement current;Comparable comparable;previous = this.previousOf((Comparable)

anObject);if (previous == null) // no previous element, first node

this.head =new SinglyLinkedListElement(anObject, this.head);

else previous.setNext(new SinglyLinkedListElement(anObject, previous.next()));

this.count++;}



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OrderedList - OrderedList - remove(Object) 1remove(Object) 1

public Object remove(Object anObject);// pre: anObject is non-null// post: removes object “equal” to anObject and returns it,// otherwise returns null

SinglyLinkedListElement previous;SinglyLinkedListElement current;Comparable comparable;previous = this.previousOf((Comparable) anObject);if (previous == null) // no previous element, first node

current = this.head;else current = previous.next();if ((current == null) || !current.value().equals(anObject)) return null;



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OrderedList - OrderedList - remove(Object) 2remove(Object) 2

if (previous == null) // no previous element, first node

this.head = current.next();else previous.setNext(current.next());this.count--;

}



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OrderedList - OrderedList - elements()elements()

public Iterator elements();// post: return an iterator for traversing the collection

return new SinglyLinkedListIterator(this.head);}



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OrderedList - OrderedList - previousOf(Object) 1previousOf(Object) 1

/* Protected Methods */ protected SinglyLinkedListElement previousOf(Object

anObject) {// pre: anObject is non-null// post: returns the node before the node that contains the// given object, if the object is in the collection or the// node before where it should be placed if it is not in the collection

SinglyLinkedListElement cursor;SinglyLinkedListElement previous;Comparable key;

cursor = this.head;previous = null;key = (Comparable) anObject;



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OrderedList - OrderedList - previousOf(Object) 2previousOf(Object) 2

while ((cursor != null) && (((Comparable) cursor.value()).compareTo(key) < 0)) {

previous = cursor; cursor = cursor.next();}return previous;

}



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Some Principles from the TextbookSome Principles from the Textbook

16. Declare parameters of overriding methods with the most general types possible. (e.g., equals)

17. Avoid multiple casts of the same object by assigning the value to a temporary variable.

18. Consider your code from different points of view.

principles from Bailey ch. 9

ordered containers cmput 115 - lecture 21 department of computing science university of alberta...

Documents

collection interface

comparable interface

public void clear post

public int size post

public boolean isempty

ordered containers

orderedstructure hierarchy

nonnull post