Data Structures and Algorithms

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Data Structures and Algorithms

COMP 103 - 2021 T2

.

Peter Andreae

( “Pondy” )

Hui Ma

Thomas K

ühne

Marcus Frean (CC)

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Welcome!

• COMP 103 = 2nd core course for COMP, CGRA, SWEN, CYBR, ECEN, NWEN

• Core principles of Computer Science and essential programming skills

• Builds on COMP 102/112

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What’s the course about?

• Using and understanding Collections

• lists, sets, maps, stacks, queues, …

• Writing code with Trees and linked structures.

• How to write code using collections efficiently

• Designing and implementing algorithms

• Using recursion

Recurring theme: Efficiency

• How fast is it?

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Concepts

• Fundamental principles:

• collections, linked structures, algorithms, recursion, efficiency,

• Focusing on:

• Using collections, rather than implementing collections.

• Becoming good, effective, efficient programmers.

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Observations from COMP 102

• Not engaging in lectures is dangerous for your learning!

• Many were uncomfortable about asking questions in 102:

• in lectures, in labs, on the forum

because other students put them down

Putting people down for asking questions or for trying to learn is

• unprofessional

• obnoxious

• unacceptable

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Admin: People

• Coordinator:

Marcus Frean [Cotton 337] (from week 4) [email protected]

• Lecturers:

Peter Andreae ("Pondy") [Cotton 336] [email protected]

Hui Ma [Cotton 259] [email protected]

Thomas Kühne [Cotton 233] [email protected]

• Tutors:

Ghassem Narimani [Cotton 251] [email protected]

• Programmer:

Monique Damito [Cotton327] [email protected]

• Course URL:

http://ecs.victoria.ac.nz/Courses/COMP103_2021T2

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Prereqs

Prerequisite for COMP 103

• You need to have passed

COMP102 or COMP112

• If you did not meet this,

you will need to take 102/112 again

• Passing COMP 102/112 also required for ENGR 110 and CGRA151

• COMP 132 is a course on programming with data this trimester

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Prereqs

• What is assumed from COMP102/112?

• Programming in Java

• Methods, parameters and variables

• Conditionals, loops (if, if...else, for, while)

• UI for getting input, output, drawing shapes

• Files

• Classes, Objects, Fields, Methods

• Using GUI’s (Graphical User Interface) with buttons, mouse, etc

• ArrayLists and arrays

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Lectures

• Mon, Thu, Fri, 2pm in KK LT303

• Slides

• on course webpage

• Video recordings on blackboard

• Questions:

• Goals

• Provide a framework for your learning

• Provide key content and explanations

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Assessment

•

6 Assignments

[30%]

•

Mid-term test

[15%]

50 mins, week 7

•

Tutorials

[ 5%]

(group mark)

•

Final Test

[50%]

2 hours, in exam period (does NOT boost test 1!)

Mandatory Requirements

• submit reasonable attempts for

at least 5 out of 6 Assignments

(may also be used for borderline cases and aegrotats)

• Note: You have 48 "late hours" in total. Use them carefully!

• If you are late, have used up your late hours and don't have an approved extension. • DO NOT SUBMIT!

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Tutorials

No scheduled labs in COMP103; Weekly tutorials instead:

• 6 options: Mon: 3pm Tue: 12, 4pm, 5pm(zoom) Wed: 11am, 1pm

• 10 tutorials in total

• Starting next week Sign up now!

(zoom session available only if outside Wellington Region)

• What will happen?

• Work in groups on problems, with tutor support

• Answers get handed in (worth 5% in total, group mark)

• Aim: help you to better understand both lectures and assignments

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Assignments

• Critical for your learning!

• 6 Assignments  total of 30%

• 2 weeks each

• due: Tuesday 10am (Assig1&2)

Friday 10am (4&5)

Sunday 11:50pm (3 & 6)

• Video Solutions will be available.

• Late assignments cannot be marked. Remember the 48 late hours.

• Must be

individual

work. (not like COMP 102)

• We will talk about how to collaborate and help each other without plagiarism

• Won’t be as constraining as COMP 102 assignments

• You may need to do more of the design of the structure of the program

• Programming Style will be a component of the marks (up to 5% off for bad style)

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Getting Help.

[email protected]

• individual help where you can submit your code and questions to a tutor

• online help-desk

Physical & Zoom Help desks

• help in person for individuals or small groups

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Workshops

• Workshops (optional) covering all 1st year subjects with tutors to support.

• Excellent for working with discussion and interaction with other students.

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• JAVA FOUNDATIONS:

Introduction to Program Design & Data Structures

by Lewis, DePasquale and Chase

• 4th _{edition is best}

• Same textbook as comp 102/112 ☺

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PLAGIARISM UNACCEPTABLE

• We want you to LEARN, TALK to each other, learn TOGETHER, and

HELP each other, but

PLAGIARISM is UNACCEPTABLE!

• Got help from anybody other than lecturer or tutor?

STATE IT ON THE ASSIGNMENT!

• Copied bits of code from anywhere other than lecture slides or

textbook?

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Admin

• Class rep

• Workshops

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Course Structure

Topics:

• Programming with unstructured and linear collections

• Different Kinds of collections:

• Lists, Sets, Bags, Maps, Stacks, Queues, Priority Queues

• Algorithms using collections.

• Complexity

• Recursion

• Programming with Linked collections

• Building, traversing tree structured collections

• Building, traversing graph/network structured collections

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Recurring Themes

• Good Design:

• Which choices should you make?

• Choosing appropriate implementations for collections

• Making the right choice the first time

• Efficiency:

• How fast is it?

• How much memory does it take?

• By analysis, and by benchmarking

• Testing:

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Programming with Libraries

• Modern programs (especially GUI and network)

are too big to build from scratch.

⇒

Have to reuse code written by other people

• Libraries are collections of code designed for reuse.

• Java has a huge collection of standard libraries….

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Libraries for COMP 103

• ecs100

UI, etc

• java.io, java.nio

Classes for dealing with files

• java.util

Collection classes (you've used ArrayList, now …….)

Other utility classes

• We will use these libraries in almost every program

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Collections of information

• Lots of information that computers deal with comes in Collections

• Need to be able to store information in a way that reflects its structure

• Need to be able to manipulate the information in lots of different ways

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Collections in Java

What is a Collection in Java?

• An object that contains other objects

• What defines the collection type?

• What you can do to it

• What structure it imposes on its contents

• What properties it ensures

• Mostly we won’t care much it works inside:

• As long as it has the right behaviour, the inside doesn't matter?

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“Standard” Collections

Bag

Set

Map

List

_Queue

Stack

Tree

Graph

Collection

What's the difference?

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Collections: What are the differences?

• Different structures

• No structure – just a collection of values

• Linear structure of values – the order matters

• Set of key-value pairs

• Grid/table

• Hierarchical structures

• networked structures

• Different constraints

• duplicates allowed/not allowed

• get, put, remove anywhere

• get, put, remove only at the ends, or only at the top, or …

• get, put, remove by position, or by value, or by key, or …

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Abstract Data Types

Set, Bag, Queue, List, Stack, Map, etc are

Abstract Data Types

• an ADT is a type of data, described at an abstract level:

• Specifies the operations that can be done to an object of this type

• Specifies how it will behave.

• Doesn’t specify how it is implemented underneath – “black box”

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Eg: Set ADT

(simple version)

• Conceptual:

• Collection of items with no structure and no duplicates.

• Operations:

• add(value ),

• remove(value ),

• contains(value )→boolean

• Behaviour:

• A new set contains no values.

• A set will contain a value iff the value has been added to the set and it has not been removed since adding it.

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Java Collections Library (in java.util)

Defines interfaces ⇒

Abstract Data Types

and classes:

Collection Set SortedSet List Queue HashSet LinkedHash

Set TreeSet ArrayList LinkedList PriorityQueue

implements

interfaces ≈ ADTs

(not ordinary classes)

classes

Deque

Collections Arrays

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Java Collections library

Interfaces:

• Collection

= Bag (most general)

• List

= ordered collection

• Set

= unordered, no duplicates

• Queue

ordered collection, limited access (add at one end, remove from other)

• Map

= key-value pairs (or mapping)

• …

Specify the Types:

Classes

• List classes:

ArrayList, LinkedList, Vector

• Set classes:

HashSet, TreeSet, EnumSet, LinkedHashSet,…

• Map classes:

EnumMap, HashMap, TreeMap,

LinkedHashMap, WeakHashMap, …

• …

Implement the interfaces

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Java Interfaces and ADT’s

• A Java Interface corresponds to an Abstract Data Type

• Specifies what methods can be called on objects of this type

(specifies name, parameters and types, and type of return value)

• Behaviour of methods is only given in comments (but cannot be enforced)



No constructors - can’t make an instance: new Set() new List()



No fields - doesn’t say how to store the data

public interface Set <E> {

public boolean add(E item); /*…description…*/

public boolean remove(E item); /*…description…*/

public boolean contains(E item); /*…description…*/

…

// (plus lots more methods in the Java Set interface)

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Using Java Collection library

• Your program can

• Declare a variable, parameter, or field of the interface type

private List <String> images; // defined using the ADT – interface

Set <Student> students;

• Create a collection object

images= new ArrayList <String> (); // constructed using a class

students= new HashSet <Student> ();

• Call methods on that variable, parameter, or field

images.add(UIFileChooser.open("Choose an image file"); images.set(i, images.remove(j));

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Why?

• Why use the Interface type to declare the field/variable then use a class to make

the object?

• Can’t make an object of the interface type:

List <Double> myNumbers = new List <Double>();

• Why not just use the class?

ArrayList <Double> myNumbers = new ArrayList <Double>();

• More flexible design to use the Interface type:

List <Double> myNumbers = new ArrayList <Double>();

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Stack is an exception

• There is no Interface for Stack, just a class:

Stack <Action> undoStack = new Stack <Action> (); // constructed using a class

• Stacks have four stack operations:

• empty() -> boolean is the stack empty or not

• push(item) push an item onto the top of the stack

• pop() -> item removes and returns the item at the top of the stack (error if the stack is empty)

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Comments on code style for 103

• I will drop “this.” except when needed.

• instead of this.loadFromFile(fname) just loadFromFile(fname)

• instead of this.shapes.addShape(shape) just shapes.addShape(shape)

• I may leave out { } when surrounding just one statement

• instead of if (i < name.length) { name[i] = null;

}

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Undo

How does it work?

• Have to keep a record of all the actions as they are done

• Have to keep enough information to be able to undo them later

• Undo button steps backwards through the record of actions, undoing the next one.

What kind of collection do we need for the record of actions?

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Brick Builder

• What are the actions to remember?

• What info do we need to remember for each action?

• To incorporate undo:

• make a stack of undo record

• at each action in the program, add a new record to the stack

• add an undo button and method which pops the top record from the stack and undoes the action

• create a new class to store all the information for each undo record

• fields

• constructors