Compsci 201, Arrays, ArrayList, Simulation

Owen Astrachanola@cs.duke.edu

http://bit.ly/201spring19January 23, 2019

1/23/19 Compsci 201, Spring 2019: Arrays, ArrayList, Simulation 1

D is for …

• Debugging• A key skill in making your programs run

• Digital• All about the 1s and 0s

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ArrayList, Simulation2

Plan for the Week

• How to succeed in finishing Assignment P1, P0• Java and Git knowledge

• Understanding arrays and ArrayList: Tradeoffs• Toward diyad on Friday

• Applying these concepts to P1: Nbody simulation• Solving a problem that requires computational

simulation, not only mathematical analysis

1/23/19 Compsci 201, Spring 2019: Arrays, ArrayList, Simulation 3

GroundHog Day

• We had not quite finished SandwichBar APT

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Reading and thinking

• SandwichBar APT writeup• Clue/hint that indexing loop appropriate• Helper function: array of strings contains all

strings. Another: array of strings contains one

• How to access all strings: “cheese tomato bread”• Using String.split(" ") function

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If Only there was an API for that …

• As semester progresses, you'll learn …• In package java.util: ArrayList, HashSet, more• Collections. and Arrays. for help

• Preview – for now solve problems using what you know rather than increasing what you know!

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Questions about APTs and P0

• When are APTs due? What is late policy?

• What about optional APTs and reading

• Both String and Person are immutable

• Can make new objects, cannot change them

• A variable can change what it references, but what it references cannot change

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ArrayList, Simulation7

WOTO Review

http://bit.ly/201spring19-jan18-1

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Nancy Leveson: Software Safety• (@MIT) Mathematical and engineering

aspects, invented the discipline• Air traffic control• Microsoft word

“There will always be another software bug; never trust human life solely on software” huffington post?

• Therac 25: Radiation machine• http://en.wikipedia.org/wiki/Therac-

25• http://bit.ly/5qOjoH

• Software and steam engines

Assignments P0 and P1

• Simple Java class with driver program• How is a program run? What is main?• See documentation and create methods

• Example of 201 work-flow• GitLab for starter code and your code• Using SSH and GitLab together• Submitting via Gradescope, Analysis

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Project P1

• See course website for details• Not quite this, but …

• Read assignment before starting to code• Think before fingers on keys

• Use Piazza and Helper Hours

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What is a static method?

• Objects are instances of a class• Thus objects have instance variables• Typically private, accessed in methods

• Static method belongs to class, not object• No instance variables• Accesses static variables and methods• More on this later, for now? …

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Static Methods

• The class Math has many (java.lang)

• sqrt, cos, abs, …

• The class Arrays has many (java.util)

• sort, fill, toString, …

• Invoke as Math.sqrt(25.0)

• Math is a class. Convention for name?

• There aren't different Math objects, none!• No state needed, all code in methods

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ArrayList, Simulation13

WOTO: Static

http://bit.ly/201spring19-jan23-1

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Charles Isbell

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ArrayList, Simulation15

http://www.pbs.org/newshour/bb/online-graduate-programs-offer-degrees-significant-savings/

For me, the differences are simple to state: Computationalists grok that models, languages and machines are equivalent.

• Context matters: Threads

• Machine learning researcher

• Systems that interact intelligently with many other intelligence agents

• Exec. Assoc. Dean @ Georgia Tech

• Rethinking education: OnlineMasters in Computer Science

Projects in 201: Briefly

• Start with GitLab project, fork repository

• Starter code and cloud-based storage for projects

• Clone repo to your machine: you need Git for this

• You use Duke's Compsci GitLab website

• You manage your code/projects with GitLab

• Make changes, complete project

• Push changes frequently using Git … hourly/daily

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ArrayList, Simulation16

What is Git?

• Git is a free and open source distributed version control system designed to handle everything from small to very large projects with speed and efficiency. https://git-scm.com/

• In teams, huge win. Individually, huge win• Git is complicated when it doesn’t work, lots of

commands• Git relies on SSH to be secure

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Submission and Grading

• After completing assignment --- and before!

• Push code using Git

• Submit code on Gradescope. Wait a minute

• Connect to GitLab repository, grade

• (alternatives exist, we don't plan to use them)

• Submit before deadline!

• Autograder runs and tells you what happened!

• Don’t submit until you think code works!

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ArrayList, Simulation18

Working on P0 and P1

• Make sure you can start and finish before coding

• What does that mean? Issues with grading?

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ArrayList, Simulation19

Array and ArrayList

• Array can hold primitive or Object types• int[] and String[] work• Fixed size, cannot grow

• Use java.util class ArrayList for growth, more• https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/ArrayList.html

• Only hold object types, not primitives• Use .get(),.set() and not [] for indexing

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Tradeoffs: Algorithmic Approaches

• https://coursework.cs.duke.edu/201spring19/classwork-spring19/tree/master

• Read words from a file, store in ArrayList

• See ArrayListUniqiue.java, list grows as needed, to be investigated

• Create sorted list of unique words in list

• Compare three different algorithmic approaches

• Tradeoffs in understanding code

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ArrayList, Simulation21

Process every word, add to sorted

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ArrayList, Simulation22

• Code in methodA: process each word in list, add X to list of sorted, unique words

• If X already in sorted-list? Nothing to do

• If X greater than all words in list? Add at end

• Some word greater than X? shift to make room

Details of shifting to add X

• In code, s is the String being added to sorted-list, and the sorted-list is variable ret• Shift from end to index > X, then add X

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ArrayList, Simulation23

Code Analysis and Dissection

• What does break do? Use of boolean done?

• What if words in list already in order? …

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ArrayList, Simulation24

Tradeoff: Words Already Sorted

• There are duplicates in list, but it's sorted

• Look at X, each element of list in order, consider adding X to no-dupes sorted list

• X is >= last element of no-dupes

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ArrayList, Simulation25

What if we use API, other classes

• A set contains no duplicates, a TreeSet maintains unique elements in sorted order

• Create set, contains no duplicates

• Create ArrayList from set

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ArrayList, Simulation26