Data Structure

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Chapter 12: Data Structures : 

Chapter 12: Data Structures Presentation slides for Java Software Solutions Foundations of Program Design Third Edition by John Lewis and William Loftus Java Software Solutions is published by Addison-Wesley Presentation slides are copyright 2002 by John Lewis and William Loftus. All rights reserved. Instructors using the textbook may use and modify these slides for pedagogical purposes.

Data Structures: 

2 Data Structures Now we explore some convenient techniques for organizing and managing information Chapter 12 focuses on: collections Abstract Data Types (ADTs) dynamic structures and linked lists queues and stacks non-linear data structures predefined collection classes

Collections: 

Collections A collection is an object that serves as a repository for other objects A collection usually provides services such as adding, removing, and otherwise managing the elements it contains Sometimes the elements in a collection are ordered, sometimes they are not Sometimes collections are homogeneous , sometimes the are heterogeneous

Abstract Data Types: 

4 Abstract Data Types Collections can be implemented in many different ways An abstract data type (ADT) is an organized collection of information and a set of operations used to manage that information The set of operations defines the interface to the ADT As long as the ADT fulfills the promises of the interface, it doesn't really matter how the ADT is implemented Objects are a perfect programming mechanism to create ADTs because their internal details are encapsulated

Abstraction: 

5 Abstraction Our data structures should be abstractions That is, they should hide unneeded details We want to separate the interface of the structure from its underlying implementation This helps manage complexity and makes it possible to change the implementation without changing the interface What do we mean by “ makes it possible to change the implementation without changing the interface“? Why is changing the implementation without changing the interface desirable?

What about a stack?: 

What about a stack? Is a stack an Abstract Data Type (ADT) with a collection of data and operations that are allow on the data? How many operations can we legally perform to manipulate a stack? Do we care about how these operations are implemented? We only care about the what, not about the how! A set of operations defines the interface to the ADT. What are they for a stack? push & pop

Static vs. Dynamic Structures: 

7 Static vs. Dynamic Structures A static data structure has a fixed size This meaning is different from the meaning of the static modifier (variable shared among all instances of a class) Arrays are static; once you define the number of elements it can hold, the number doesn’t change A dynamic data structure grows and shrinks at execution time as required by its contents A dynamic data structure is implemented using links

Object References: 

8 Object References Recall that an object reference is a variable that stores the address of an object A reference also can be called a pointer References often are depicted graphically: student John Smith 40725 3.58

References as Links: 

9 References as Links Object references can be used to create links between objects Suppose a Student class contains a reference to another Student object John Smith 40725 3.57 Jane Jones 58821 3.72 Note Jane’s null pointer.

References as Links: 

10 References as Links References ( pointers ) can be used to create a variety of linked structures, such as a linked list : info next info next info null info next Figure 12.1 A linked list studentList

Intermediate Nodes: 

11 Intermediate Nodes The objects being stored should not be concerned with the details of the data structure in which they may be stored For example, the Student class should not have to store a link to the next Student object in the list Instead, we can use a separate node class with two parts: 1) a reference to an independent object and 2) a link to the next node in the list The internal representation becomes a linked list of nodes

Magazine Collection: 

Magazine Collection Let’s explore an example of a collection of Magazine objects The collection is managed by the MagazineList class, which has an private inner class called MagazineNode Because the inner class MagazineNode is private to MagazineList , the MagazineList methods can directly access MagazineNode data without violating encapsulation 1 st - see MagazineRack.java (page 641) 2 nd - see Magazine.java (page 644) 3 rd - see MagazineList.java (page 642)

Fig. 12.2 – inserting new node into list: 

Fig. 12.2 – inserting new node into list A method called insert could be defined to place a node anywhere in the list, for example to keep it sorted Inserting a node into the middle of a list list info next info next info next info null info next insert this new node

Fig. 12.3 - deleting a node from a list: 

Fig. 12.3 - deleting a node from a list A method called delete could be defined to remove a node from the list Deleting a node from a list list info next info next info next info null info next What must we be careful of when deleting a node? (Hint: what could we lose?)

Fig 12.4 - doubly linked list: 

15 Fig 12.4 - doubly linked list It may be convenient to implement as list as a doubly linked list , with next and previous references A doubly linked list list info next info next info next info null info next prev prev prev prev null What is contents of the first nodes “prev” pointer ? What does a doubly linked list allow ?

Fig. 12.5 - list with front & rear references: 

16 Fig. 12.5 - list with front & rear references It may be convenient to use a separate header node , with a count and references to both the front and rear of the list info next info next info null info next front rear count: 4 list What will this structure allow to occur quicker than without “rear” pointer? header node list with front and rear references

Other Dynamic List Implementations: 

Other Dynamic List Implementations A linked list can be circularly linked -- last node in the list points to the first node in the list If the linked list is doubly linked , the first node in the list also points to the last node in the list Discuss example implementing priority control of resource allocation – i.e., CPU time sharing via circular queue

Dynamic Implementations: 

Dynamic Implementations The representation should facilitate the intended operations and should make them easy to implement.

Classic Data Structures: 

Classic Data Structures Classic linear data structures include queues and stacks Classic nonlinear data structures include trees , binary trees , graphs , and digraphs

Fig. 12.6 – a queue data structure: 

20 Fig. 12.6 – a queue data structure A queue is similar to a list but adds items only to the rear of the list and removes them only from the front It is called a FIFO data structure: First-In, First-Out Analogy: a line of people at a movie ticket window enqueue dequeue first item in, first item out last item in, last item out

Queues: 

21 Queues We can define the operations for a queue enqueue - add an item to the rear of the queue dequeue (or serve) - remove an item from the front of the queue empty - returns true if the queue is empty As with our linked list example, by storing generic Object references, any object can be stored in the queue Queues often are helpful in simulations or any situation in which items get “backed up” while awaiting processing (Jobs waiting their turn to be processed.)

Queues: 

Queues A queue can be represented by a singly-linked list. Operationrs: enqueue – add an item to rear dequeue – remove an item from front empty – returns true if queue is empty Is it more efficient if the references point from front to the rear? queue info4 null info3 next rear info2 next info1 next front queue info4 next info3 next info2 next info1 null rear front Two representations of same queue

Queues: 

Queues A queue can be represented by an array What may happen as queue grows via enqueue with no immediately occurring dequeues? 0 1 2 3 4 5

Stacks: 

24 Stacks A stack ADT is also linear, like a list or a queue Items are added and removed from only one end of a stack It is therefore LIFO : Last-In, First-Out Analogies: a stack of plates in a cupboard a stack of bills to be paid a stack of hay bales in a barn

Fig. 12.7 – stack data structure: 

25 Fig. 12.7 – stack data structure Stacks often are drawn vertically: pop push  f irst item in, last item out  last item in, first item out

Stacks: 

26 Stacks Some stack operations: push - add an item to the top of the stack pop - remove an item from the top of the stack peek (or top) - retrieves the top item without removing it empty - returns true if the stack is empty A stack can be represented by a singly-linked list; it doesn’t matter whether the references point from the top toward the bottom or vice versa A stack can be represented by an array, but the new item should be placed in the next available place in the array rather than at the end of the array (What can happen with an array implementation?)

Stacks: 

Stacks The java.util package contains a Stack class Like ArrayList operations, the Stack operations operate on Object references See (not during class) Decode.java (page 649) which reverses the strings in a message. The words in the message are separated by a single space. The Stack class is used to push the characters of the word onto the stack and then pops the characters out in reverse order.

Decode.java: 

Decode.java S m a r t push t r a m S pop S m a r t reverse a string 1 2 3 4 5 2 3 4 5 1 Decode.java reverses the strings in a message

Fig 12.8 - Trees : 

Fig 12.8 - Trees A tree is a non-linear data structure that consists of a root node and potentially many levels of additional nodes that form a hierarchy Nodes that have no children are called leaf nodes Non-root and non-leaf nodes are called internal nodes A tree data structure leaf nodes root node imagine an upside down tree internal nodes Tree can represent inheritance relationship between classes.

Organization chart represented via a tree data structure: 

Organization chart represented via a tree data structure leaf nodes root node president VP VP VP VP mgr mgr mgr mgr

Binary Trees: 

Binary Trees A binary tree is defined recursively. Either it is empty (the base case) or it consists of a root and two subtrees , each of which is a binary tree Binary trees and trees typically are represented using references as dynamic links, though it is possible to use fixed representations like arrays leaf nodes root node

Fig. 12.9 - graph: 

Fig. 12.9 - graph A graph is a non-linear structure (also called a network ) Unlike a tree or binary tree, a graph does not have a root – no primary entry point. Any node can be connected to any other node by an edge Can have any number of edges and nodes Analogy: the highway system connecting cities on a map a graph data structure

Fig. 12.10 - digraphs: 

Fig. 12.10 - digraphs Each edge of directed graph or digraph has a specific direction denoted by arrows. Edges with direction sometimes are called arcs Analogy #1: airline flights between airports (see below) Analogy #2: Solution to a problem (on board – miles on arcs) a directed graph airline routes represented via digraph What else could be provided? A R B C D E F L J P Y W X N S

Graphs and Digraphs: 

Graphs and Digraphs Both graphs and digraphs can be represented using dynamic links or using arrays. As always, the representation should facilitate the intended operations and make them convenient to implement

Collection Classes: 

Collection Classes The Java standard library contains several classes that represent collections, often referred to as the Java Collections API. (API  Application Programmer Interface ) Java Collections API supports the organization and management of data . Their underlying implementation is implied in the class names such as ArrayList and LinkedList Several interfaces are used to define operations on the collections, such as List , Set , SortedSet , Map , and SortedMap Set – a collection of items with no duplicates. Map – group of items that can be referenced by a key value.

Summary: 

Summary Chapter 12 has focused on: collections Abstract Data Types (ADTs) dynamic structures and linked lists queues and stacks non-linear data structures predefined collection classes