Connecting with Computer Science, 2e Chapter 8 Data Structures

Objectives In this chapter you will: –Learn what a data structure is and how it’s used –Learn about single-dimensional and multidimensional arrays and how they work –Learn what a pointer is and how it’s used in data structures –Learn that a linked list allows you to work with dynamic information Connecting with Computer Science, 2e 2

Objectives –Understand that a stack is a linked list and how it’s used –Learn that a queue is another form of a linked list and how it’s used –Learn that a binary tree is a data structure that stores information in a hierarchical order –See an overview of several sorting routines Connecting with Computer Science, 2e 3

Why You Need to Know About…Data Structures Data structures: –Organize the data in a computer Efficiently access and process data –All programs use some form of data structure –There are many occasions for using data structures Connecting with Computer Science, 2e 4

Data Structures Defined as a way of organizing data Types of data structures in memory –Arrays, lists, stacks, queues, trees File structures organize storage media data Computer memory is organized into cells –Memory cell has a memory address and content –Memory addresses are organized consecutively –Data structures hide memory implementation details Connecting with Computer Science, 2e 5

Arrays Set of contiguous memory cells –Used for storing the same type of data Simplest memory data structure Consists of a set of contiguous memory cells –Memory cells store the same type of data Usefulness: –Storing similar kinds of information in memory Sorted or left as entered –One array name for a number of similar items Connecting with Computer Science, 2e 6

7 Figure 8-2, Arrays make program logic easier to understand and use Arrays

How an Array Works Java example –int[ ] aGrades = new int[5]; “int[ ]” indicates array will hold integers “aGrades” identifies the array name “new” keyword specifies new array being created “int[5]” reserves five memory locations “=” sign assigns aGrades as “manager” of the array “;” (semicolon) indicates end of statement reached Hungarian notation standard is used to name “aGrades” Connecting with Computer Science, 2e 8

9 Figure 8-3, Five contiguous memory cells managed by aGrades in a single-dimensional array How an Array Works

Element: memory cell in an array Dimension: levels created to hold array elements –aGrades: single-dimensional array (row of mailboxes) Offset specifies distance between memory locations –Array’s first position: referenced as position 0 –Next array position: referenced as position 1 Found by using starting memory location plus one offset –Third array position: referenced as position 2 Found by using starting memory location plus two offsets Connecting with Computer Science, 2e 10

Connecting with Computer Science, 2e 11 Figure 8-5, Arrays start at position 0 and use an offset to know where the next element is located in memory How an Array Works

Index (subscript) –Indicates memory cell to access in the array Looks at element’s position Placed between square brackets ( [ ] ) after array name Integer placed in “[ ]” for access –Example: aGrades[0] = 50; Position 0: first position –Array with positions 0 to 4 Five memory cells or addresses Upper bound: highest array position Lower bound: lowest array position Connecting with Computer Science, 2e 12

Connecting with Computer Science, 2e 13 Figure 8-6, The array with all elements stored How an Array Works

Multidimensional Arrays Multidimensional arrays –Consists of two or more single-dimensional arrays –Multiple rows stacked on top of each other Apartment building mailboxes and tic-tac-toe boards Creating the tic-tac-toe board –char[ ][ ] aTicTacToe = new char[3][3]; Assignment: aTicTacToe[1][1] = ’X’; –Place X in second row of the second column Arrays beyond three dimensions are difficult to manage Connecting with Computer Science, 2e 14

Connecting with Computer Science, 2e 15 Figure 8-7, A multidimensional array is like apartment mailboxes stacked on top of each other Figure 8-8, Tic-tac-toe board Multidimensional Arrays

Connecting with Computer Science, 2e 16 Figure 8-10, Second and third rows of the tic-tac-toe board Figure 8-9, First row of the tic-tac-toe board Multidimensional Arrays

Connecting with Computer Science, 2e 17 Figure 8-11, Storing a value in an array location Multidimensional Arrays

Connecting with Computer Science, 2e 18 Figure 8-12, Three-dimensional array Multidimensional Arrays

Uses of Arrays ( Lists ? ) Advantages –Allow sequential access of memory cells –Retrieve and store data with element array name and data type –Easy to create –Useful for people who write computer programs Disadvantages –Require a lot of overhead for insertions –Memory cell data is only accessed sequentially Connecting with Computer Science, 2e 19

Lists Hold dynamic lists of data –Lists vary in size –Examples: class enrollment, cars being repaired, e- mail in-boxes Appropriate whenever amount of data is unknown or can change Three basic list forms: –Linked lists –Queues –Stacks Connecting with Computer Science, 2e 20

Linked Lists Use noncontiguous memory locations to store data –Each element points to the next element in line Does not have to be contiguous with previous element –Used when exact number of items is unknown –Store data noncontiguously –Maintain data and address of next linked cell –Examples: names of students visiting a professor, points scored in a video game, list of spammers Basic constructs for more advanced data structures –Queues and stacks: pointer based Connecting with Computer Science, 2e 21

Linked Lists Pointers: memory variable containing the address of a memory cell as its data Illustration: linked list game –Students sit in a circle with piece of paper –Paper has box in the upper left corner and center –Upper left box indicates a student number –Center box divided into two parts –Students indicate favorite color in left part of center –Professor has a piece of paper with a number only Connecting with Computer Science, 2e 22

Connecting with Computer Science, 2e 23 Figure 8-14, Structure of a linked list Linked Lists

Piece of paper represents a two-part node –Data (the first part, the color) –Pointer (the student ID number) Professor’s piece: head pointer with no data Last student: pointer’s value is NULL Inserting new elements –No resizing needed –Create new “piece of paper” with dual node structure –Realign pointers to accommodate new node (paper) Connecting with Computer Science, 2e 24

Connecting with Computer Science, 2e 25 Figure 8-15, Inserting an element into a linked list Linked Lists

Similar procedure for deleting items –Modify pointer of element preceding target item –Students deleted from list without moving elements Use dynamic memory allocation –More efficient than arrays –Memory cells need not be contiguous Connecting with Computer Science, 2e 26

Connecting with Computer Science, 2e 27 Figure 8-16, Deleting an element from a linked list Linked Lists

Stacks List in which the next item to be removed is the item most recently stored –“Push” items on to the list to store new items –“Pop” items off the list to retrieve current items Examples –Restaurant spring-loaded plate holder or text editor Peeking –Looking at the stack’s top item without removing it LIFO data structure –Last or most recent item pushed (put) onto the stack Becomes first item popped (removed) from the stack Connecting with Computer Science, 2e 28

Connecting with Computer Science, 2e 29 Figure 8-17, The stack concept Stacks

Uses of a Stack Processes lines of program source code Source code is logically organized into procedures –Keep track of procedure calls with a stack –Address of procedure popped off stack Connecting with Computer Science, 2e 30

Back to Pointers Stack pointer –Keeps track of where to remove or add an item in a data structure Check stack before applying pop or push operations Stacks –Memory locations organized into logical structures Facilitates reading from them and writing to them Connecting with Computer Science, 2e 31

Connecting with Computer Science, 2e 32 Figure 8-18, Stack pointer is decremented when the item is popped off Back to Pointers

Queues Another type of linked list –Implements first in, first out (FIFO) storage system –Insertions made at the end –Deletions made at the beginning –Similar to that of a waiting line Connecting with Computer Science, 2e 33

Uses of a Queue Printer example –First item printed Document waiting longest –Current item deleted from queue Next item printed –New documents Placed at the end of the queue Insertions of new data occur at the rear of the queue Removal of data occurs at the front of the queue Connecting with Computer Science, 2e 34

Pointers Again Head pointer tracks beginning of queue Tail pointer tracks end of queue Queue containing no items –Both the head and tail pointer point to same location Dequeue operation –Remove item (oldest entry) from the queue Head pointer changed to point to the next item in list Enqueue operation –Item placed at list end –Tail pointer updated Connecting with Computer Science, 2e 35

Connecting with Computer Science, 2e 36 Figure 8-19, A queue uses a FIFO structure Pointers Again

Connecting with Computer Science, 2e 37 Figure 8-20, Removing an item from the queue Figure 8-21, Inserting an item into the queue Pointers Again

Trees Hierarchical data structure similar to organizational or genealogy charts –Node or vertex: position in the tree Connecting with Computer Science, 2e 38 Figure 8-22, Tree data structure

Trees Binary tree –Each node has at most two child nodes –Node can have zero, one, or two child nodes –Left child: child node to the left of the parent node –Right child: child node to the right of the parent node –Root: node that begins the tree –Leaf node: node that has no child nodes –Depth (level): distance from root node –Height: longest path length in the tree Connecting with Computer Science, 2e 39

Connecting with Computer Science, 2e 40 Figure 8-24, The level and height of a binary tree Figure 8-23, Tree nodes Trees

Uses of Binary Trees Binary search tree: a type of binary tree –Data value of left child node is less than the value of parent node –Data value of right child node is greater than the value of parent node Useful for searching through stored data –Storing information in a hierarchical representation Connecting with Computer Science, 2e 41

Connecting with Computer Science, 2e 42 Figure 8-25, A file system structure can be stored as a binary search tree Uses of Binary Trees

Searching a Binary Tree Three components in a binary search tree node: –Left child pointer, right child pointer, data Root pointer contains root node’s address –Provides initial access to the tree If left or right child pointers contain a null value –Node is not a parent to other nodes down that specific path If both left and right pointers contain null values –Node is not a parent down either path Binary tree must be defined properly to be searchable Connecting with Computer Science, 2e 43

Connecting with Computer Science, 2e 44 Figure 8-26, A node in a binary search tree Searching a Binary Tree

Search routine –Start at the root position –Determine if path moves to left child or right –Move in direction of data (left or right) –If left pointer NULL No node to traverse down the left side –If left pointer does have a value Path continues down that side –If value looking for is found Stop at that node Connecting with Computer Science, 2e 45

Connecting with Computer Science, 2e 46 Figure 8-27, Searching a binary tree for the value 8 Searching a Binary Tree

Connecting with Computer Science, 2e 47 Figure 8-28, Searching a binary tree for the value 1 Searching a Binary Tree

Sorting Algorithms Sorting: leverages data structures to organize data –Example of data being sorted: Words in a dictionary Files in a directory Index of a book Course offerings at a university Many algorithms for sorting –Each has advantages and disadvantages Focus: selection and bubble sorts Connecting with Computer Science, 2e 48

Selection Sort Selection sort: mimics manual sorting –Starts at first value in the list –Processes each element looking for smallest value –After smallest value found, it is placed in first position Moves first position value to location originally containing smallest value –Sort moves on looking for next smallest value –Continues to “swap places” Simple to use and implement Inefficient for large lists Connecting with Computer Science, 2e 49

Connecting with Computer Science, 2e 50 Figure 8-29, A selection sort Selection Sort

Bubble Sort Bubble: older and slower sort method –Start with the last element in the list –Compare its value to that of the item just above –If smaller, change positions and continue up list Continue comparison until smaller item found –If not smaller, next item compared to item above –Check until smallest value “bubbles” to the top –Process repeated for list less first item Simple to implement Inefficient for large lists Connecting with Computer Science, 2e 51

Connecting with Computer Science, 2e 52 Figure 8-30, A bubble sort Bubble Sort

Connecting with Computer Science, 2e 53 Figure 8-31, The bubble sort continues Bubble Sort

Other Types of Sorts Quicksort: incorporates “divide and conquer” logic –Two small lists are easier to sort than one large list –Uses recursion to break down problem –All sorted sub-lists are combined into single sorted set –Fast but difficult to comprehend Connecting with Computer Science, 2e 54

Other Type of Sorts Merge sort: similar to the quicksort –Continuously halves data sets using recursion –Sorted halves are merged back into one list –Time efficient, but not as space efficient as quicksort Insertion sort: simulates manual sorting of cards –Requires two lists –Not complex, but inefficient for list size fewer than 1000 Shell sort: uses insertion sort against expanding data set Connecting with Computer Science, 2e 55

One Last Thought Algorithms –Used everywhere in the computer industry –Knowing how to work with data structures and sorting algorithms is necessary to begin writing computer programs –Many algorithms are written and available for use Knowing the tools available and which sort routine will perform best for a situation saves time Connecting with Computer Science, 2e 56

Summary Data structures organize data Basic data structures –Arrays, lists, queues, stacks, trees Arrays –Store data contiguously –May have one or more dimensions Linked lists –Store data in dynamic containers –Use pointers for noncontiguous storage Pointer: contains memory cell address as its data Connecting with Computer Science, 2e 57

Summary Stack –Linked list structured as LIFO container Queue –Linked list structured as FIFO container Tree –Hierarchical structure consisting of nodes –Binary tree: nodes have at most two children –Binary search tree: efficient for searching for information Connecting with Computer Science, 2e 58

Summary Sorting algorithms –Organize data within structure Examples: selection sort, bubble sort, quicksort, merge sort, insertion sort, shell sort Sorting routines –Analyzed by code, space, and time complexities Connecting with Computer Science, 2e 59

The End Connecting with Computing Science, 2e 60