Compsc sci 201 201 Wo Work, Nbody dy, , ArrayL yLists ts - - PowerPoint PPT Presentation

Compsc sci 201 201 Wo Work, Nbody dy, , ArrayL yLists ts

1/29/2020 Compsci 201, Spring 2020 1

Susan Rodger January 29, 2020

F is for …

• Folder
• aka Directory – where things are stored in Git
• Function
• Abstraction – a method in Java

1/29/2020 Compsci 201, Spring 2020 2

PFTW

• Get

etting ng t thing ngs done i ne in 201

• How to succeed and enjoy the effort
• Mund

ndan ane J e Java-is isms ms

• From char to autoboxing: primitives
• What is this?
• Gener

neric classes es: H How Ar ArrayList st wo works

• Design, create, test, measure

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Getting Things Done in 201

• What do these d

dat ata m mean f n for y you, u, f for m me, f for t the communit unity o

• f 286 s

stud udent ents i in Compsc sci 201? 201?

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From Last Time … Go over WOTO: Correctness Counts

http://b ://bit.l .ly/20 /201s 1spring2 g20-012 124-2

1/24/2020 Compsci 201, Spring 2020 8

Object class, equals method

• In

In Java vaDoc

• Signa

natur ure o e of equals als met etho hod

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@Override .equals

• Create a new P

Point m met etho hod

• Use annotation @Override, help with errors

boolean equals(Object o) { …

• Must u

use t e this is s signa natur ure, t to implem lement ent:

• Cast parameter appropriately
• Compare instance fields

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Point inherits Object.equals

• This d

s doesn sn’t w t work f k for Point o t objects ts!

• Def

efault ult simply uses ==, no idea about points

• a.equals(b) if a and b ref

eference nce t the same o e object ect

• Two different (0,0) points not the same

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Point equals fixed!

• Must u

use s e same s e signa natur ure

• Must c

st cast st O Object to t to Point

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Contract for Equality

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• Reflex

lexiv ive e x.equals(y) then y.equals(x)

• Trans

ansit itivit ivity: : x.eq(y), y.eq(z) then n x.eq(z)

• Check x.equals(x) as

as a a special c ecial cas ase w e with = h ==

• Check this.getClass() == o.getClass()
• Don’t want to have an apple == orange
• Cas

ast O Object ct p param ameter er a and u use i e instance ance va variab iables les

• See Point as example

Amanda Randles, Duke 2005

• ACM Grace M

Murray H Hoppe pper Aw Award ( d (<= 35 yo yo)

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For developing HARVEY, a massively parallel circulatory simulation code capable of modeling the full human arterial system at subcellular resolution and fostering discoveries that will serve as a basis for improving the diagnosis, prevention, and treatment of human diseases. //XXX and Amanda Peters //Compsci 100: Huffman Coding //November 19, 2002 I felt like working in a pair was a really successful way to complete the program. It helped the most when it came to working out basic logic and finding errors. I found it really helpful because he often would see the basic logic to the code and I could help more with the implementation. I feel like it was a successful group and we both contributed a lot.

Reading Points

• We'll t

ll typic ically lly u use a e a Scanne nner t to read value ues

• Use .hasNext(),.hasNextDouble(), …
• If/while there's more to read? Call .next()
• Met

etho hod .next() retur urns ns a a String ing

• Method .nextDouble() returns a double …
• See

e PointRea eader er.j .java cla lass, us useful in in NB NBod

• dy

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Scanner Sources for Reading

• Construct

uct a a Scanne anner f from System em.in .in

• Reads from keyboard/console
• .hasNextX()true until end-of-file OR no X
• Control-D on OS X, Control-Z on Windows
• Cons

nstruct a a Scanner f from a a File ile

• Reads from file, ex

exception c could uld hap happen

• .hasNextX()true until all of file read OR no X
• Each call of .nextX() returns the next X, internally

the Scanner "remembers" where it last read

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Scanner hasNext and next

• Think

nk a about ut s scanner nner a as a a long r reel/s l/sour urce ce of data

• If .hasNext() returns true, there is

something to read by Scanner cursor/reader

• Calling .next() returns and

and ad advances cursor

• Scanner object maintains cursor internally
• Source: f

: file, S , String ing, , terminal inal, … , …

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N-Body Simulation

• Class Celes

lestialB ialBody repres esent ents C Celes lestia ial B l Body

• Planet, Sun, Asteroid
• Models an object in 2D space, not 3D
• Position, Velocity, Mass, Image for display
• Class NB

NBod

• dy drives

es t the s simula ulation

• Compute gravitational forces: physics
• Time-step simulation
• compute all forces, update ,display

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Class CelestialBody

• Illus

lustrates s stand andard J Java i idioms

• Constructors, Methods, Instance Variables
• State i

is private: s : six i instanc nce v e variab iable les

• myXPos, … using my

my convention - this object

• Initialized by constructors
• Methods ar

are p e pub ubli lic

• Include access

ssors aka getters for state

• No setters, cannot change myXPos other than

via the update method, a mutator

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The Object Concept

• Every i

instan ance ce v variab able a and ever ery non-sta tatic tic m meth thod d acces cessed ed/cal called ed a after Obj bject. t.Dot

• b.getX(), b.calcForcExertedBy(other)
• From w

withi hin a n a class, e e.g., Celestia tialB lBody

• myXPos, getX(),this.myXPos,
• All are equivalent as is this.getX()
• Some

e prefer er a always using t

• this. – clear

earer er?

1/29/2020 Compsci 201, Spring 2020 23

NBody numbers

• Floating

ing p point nt i issue ues, p , proble lems, q , quand ndaries ies

• When is (a + b) + c != a + (b + c)
• When is a/b * c != a*c / b
• Watch for this in Gradescope tests!!

1/29/2020 Compsci 201, Spring 2020 25

Debugging Arithmetic

• Order

er o

• f operatio

ions ns w with f h floating ing p point v values ues c can n result ult i in overflo low, u , under erflo low, m , more

• Small number + Big number …

1/29/2020 Compsci 201, Spring 2020 26

Debugging double Arithmetic

• Integer v

values ues a are n not the s same as e as D Double v le values ues

• 1/0 is … whereas 1.0/0 is …

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Completing NBody

• Please r

e read t the e TL;DR d docum cument nt

• Test at each step, push constantly using Git
• Afte

ter usin ing suppli pplied T d Test… c classes, p proceed d to to simu mula lati tion

• Must be able to read data file to simulate
• Understand the basics, read carefully
• Analysis

is: comple mplete b before s submit mittin ting to to Gradesc scop

• pe

for final s submissi ssion

• n

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Now look at DNAMaxNucleotide

• Retur

urn n the s strand and f from s strand ands a array w with m h most

• ccur

urrence nces o

• f nucleo

cleotid ide e nuc

• nuc. R

. Retur urn l n longes est such s ch strand nd

• Exa

Exampl ple

1/29/2020 Compsci 201, Spring 2020 29

Algorithm - DNAMaxNucleotide

• Does

es t thi his co code m e mak ake t the he alg algorithm c cle lear?

• Why must count be a helper method?
• Why can't max = 0 before loop?

1/29/2020 Compsci 201, Spring 2020 30

Two Versions of Helper Method

• Iter

erating ing o

• ver each c

ch char aract acter er o

• f a string

ng

• Note that nuc is a one-character string
• How does s.substring(a,b) work?

1/29/2020 Compsci 201, Spring 2020 31

Critique of another implementation

• Where d

e does t this is s solut lutio ion c n come from?

• Strings are immutable, s.replace(…)
• Replace every "a" with "" (nothing)
• Is this better? Different? More clever? More of a

hack? …

1/29/2020 Compsci 201, Spring 2020 32

WOTO

http:// //bi bit.ly/2 /201spr pring20-01 0129 29-1

1/29/2020 Compsci 201, Spring 2020 33

Donald Knuth

• aka “The Donald”
• Turing award (and others)
• Author of “The Art of

Computer Programming”

• Arguably most important book

written in Computer Science

• First publication: Mad Magazine

1/29/2020 Compsci 201, Spring 2020 34

If you optimize everything you will always be unhappy. Everyday life is like programming, I guess. If you love something you can put beauty into it.

https://www.youtube.com/watch?v=cK7yyjXfbc4

From Array to ArrayList

• Have int[], String[], CelestialBody[]
• Array of any type, but d

doesn sn't g 't grow

• Can't use .contains with array, can't print
• The

The java.ut utils ils.Arrays cla lass ha has s some hel help

1/29/2020 Compsci 201, Spring 2020 35

java.util.ArrayList

• Growab

able a le array w with m h many u useful m ul met etho hods

• https://docs.oracle

le.com/en en/java/javase/ e/11 11/docs/ap api/ i/java. a.base/ e/java/ a/util/ il/Lis ist.html

• Can only contain Object types (no primitives)
• Convert f

fro rom a m arra ray?

• Arrays.asList
• It's a List!
• String yes, int no

1/29/2020 Compsci 201, Spring 2020 36

From Array to ArrayList

• Can m

n make c e conv nver ersio ion w n with O Object ect, e e.g., ., S String ing

• Use Arrays.asList as a bridge, be careful

1/29/2020 Compsci 201, Spring 2020 37

Primitive Array? do it yourself

• No bridge

ge f fro rom m Arrays.asL sList st since p nce primit itiv ive

• Loop and use autoboxing/unboxing
• Conversion of int to Integer and vice versa

1/29/2020 Compsci 201, Spring 2020 38

Objects, Primitives, Arrays/Lists

• ar

array c can ho an hold ld any any type: int[], String[]

• ArrayList only

ly O Object ct t types es, n not p primit itiv ives

• Autoboxing allows for add/get int ::: Integer
• ArrayList<Object> a,

a, a.toArray(…) array ay

• Syntax is not intuitive, see examples in code
• Arrays.asList(Object[]) to

to ArrayList

• Actually returns List, not ArrayList, …

1/29/2020 Compsci 201, Spring 2020 39

• - real world APT?
• https://l

://lee eetcod

• de.com/p

/prob

• blem

ems/u /unique-mo morse-cod

• de-word

rds/

• "a" > ".-", "b" > "-..."
• "z" > "--.."
• Note "gin" > "--...-." and "zen" > "--...-."
• Giv

iven an ar an array o

• f strings, ho

how many any uniq unique enc encodin ings ar are t e ther here?

• Also given String[] of 26 Morse codes, where

code[0] = ".-" for "a"

1/29/2020 Compsci 201, Spring 2020 40

High Level Ideas

• Fir

irst s step: w wha hat alg algorithm/method w will y ill you u us use?

• Verify that it's correct. High level isn't easy
• Is it n

nece cessary t to look a at/p /proce cess e ever ery string ing?

• What value i

ue is retur urne ned, h , how to det eter ermine v ine value ue?

• Stop, t

, think nk, d don't c code, … …

1/29/2020 Compsci 201, Spring 2020 41

Solution

• What'

t's a s a hig high le level solut lutio ion n using ng k known t n tools ls?

• What is the method makeMorse() ?
• Talk to your interviewer … it's a dialog

1/29/2020 Compsci 201, Spring 2020 42

From Nothing to Done

• Basic id

idea eas: ho how do we e acc access enc encodings in in an an ar array wher here c code[ e[1] is is for ' 'b', " "-…" …"

• Arithmetic with char values, 'b' – 'a' == 1
• What about (int) 'b' == 97?
• https://youtu.be/xLpfbcXTeo8?t=49
• Loop over c

r chara racters rs i in a Stri ring? g?

• Index k with s.charAt(k)
• Or for(char ch : s.toCharArray()){

1/29/2020 Compsci 201, Spring 2020 43

WOTO with Live/Leet Code

• Id

Ideas f for

• r s

sol

• lving Le

LeetCode problem lem

• Given array of Strings, return number of unique

Morse code encodings

• How is a set useful here? Doable without?

1/29/2020 Compsci 201, Spring 2020 44

From DNAMax to Morse Code

• loop
• op on
• n 1

18-23, wh why do does ch ch-'a' a' serve as e as i index ex?

• Primitive char is an int except when printed

1/29/2020 Compsci 201, Spring 2020 45

ArrayList<…>

• Gener

eric a ic aka p a param ameter erized ed t type

• Any Object subtype can be in ArrayList<..>
• Integer, D

, Double, C Char, B , Boolean an are wrapper er classe sses f s for p primiti tives

• Mostly these work. But immutable. Cannot

increment an Integer, can create new one

1/29/2020 Compsci 201, Spring 2020 46

WOTO

http:// //bi bit.ly/2 /201spr pring20-01 0129 29-2

1/29/2020 Compsci 201, Spring 2020 50