Com ompsci psci 201 201 Arrays, s, ArrayList yList, , APIs Is - - PowerPoint PPT Presentation

Com

• mpsci

psci 201 201 Arrays, s, ArrayList yList, , APIs Is

1/22/2020 Compsci 201, Spring 2020 1

Susan Rodger January 22, 2020

Today is the last day of drop/add It is almost too late…

• You cannot

t switch your r section tion withou

• ut

t a perm rmission ission numbe mber. .

• DO

DO NOT drop rop your section tion until til you can get et a perm rm numbe mber r for the e ot

• ther

er section tion

• Go

Go to to https ps://w //www.cs.d s.duke.ed .edu/under /undergra rad/ d/registr egistrati tion

• n
• Scroll
• ll down

n belo low offic ice e hour

• urs

s til til you u see Permis missi sion

• n numbe

mber for Sprin ing 2020 course rses

• Then

en fill out that at form

• You must

t use e permi missi sion

• n numbe

mber to toda day! y!

• We will

ll not

• t be hand

nding ng them em out t afte ter r 5pm

1/22/2020 Compsci 201, Spring 2020 2

D is for …

• Debugging
• A key skill in making your programs correc

ect, your program will run without this skill

• Digital
• All about the 1s and 0s

1/22/2020 Compsci 201, Spring 2020 3

Plan for Today

• What

at work is here, what at work is coming ing

• P0, P1, APTs
• Unders

rstanding tanding arra rays s and ArrayList List: : Trad adeoff eoffs

• APTS always use arrays, convert to ArrayList?
• Reaso

soning ning empiric irically and analytic ticall ally

• Working well enough and working at scale

1/22/2020 Compsci 201, Spring 2020 4

Announcements

• Assig

ignment nment P0 due due las ast t week

• No late penalty since first assignment til Jan 24
• APT-1

1 now du due e Thur urs, s, January nuary 23 23

• Can still turn in Friday til 11:59pm
• Discussion

cussion 3 o

• n January

nuary 27

• Prediscussion, do before, out by Friday
• APT-2 out to

today, , du due January ry 28 (shor

• rt

t 1 due due)

• Assignme

ignment t P1 o

• ut

t Frida iday, , du due e Thursd rsday, , Jan 30

• Readin

ding on cal alen endar ar

• Slowing down …..
• try to read by date posted, ok if you need a few

more days

1/22/2020 Compsci 201, Spring 2020 5

Visualizations Help Understanding?

• Javatut

utor to to visua uali lize e code: : http tp:// ://pytho thont ntut utor

• r.com/j

com/java. a.html html

• Using the java.awt.Color class
• Both String and Color are immuta

utable ble

• Once created, cannot ever change

1/22/2020 Compsci 201, Spring 2020 6

Arrays and APIs

• Acces

cessing ing a list st of sand ndwic ich h ingred edients ients

https:/ ://w /www2. 2.cs. s.duk uke.e .edu du/cse /csed/ d/newapt/s t/sand ndwich ichbar.h .html tml

• Ingredients specified as string "lettuce tomato"
• String split method

String[] d = "lettuce tomato".split(" ")

1/22/2020 Compsci 201, Spring 2020 7

APTs: parsing/processing data

• String <-> String[]: parameter and return types

String s = "apple pear lemon orange"; String [] ar = s.split(" “); // ar is ["apple", "pear", "lemon", "orange"]

• Bridge

ge fro rom arra ray to to string? ring?

String[] a = {"one", "two", "three"} String b = String.join(":",a); // b is "one:two:three"

1/22/2020 Compsci 201, Spring 2020 9

Helper methods Creating your own API

• Sand

ndwic ichBar hBar APT writeup up

https://www2.cs.duke.edu/csed/newapt/sandwichbar.html

• Clue/hint that indexing loop appropriate
• Helper

er functi ction: array of strings contains all

• strings. Be creative in wanting an API for that

1/22/2020 Compsci 201, Spring 2020 11

WOTO SandwichBar

• How to

to te test st locally? ally?

• Write main method. Create new object …
• Faster to debug locally, even use debugger?

1/22/2020 Compsci 201, Spring 2020 12

Helper methods Creating your own API

• Sand

ndwic ichBar hBar APT writeup up

• Helper

er funct ctio ion: array of strings contains all

• strings. Be creative in wanting an API for that

1/22/2020 Compsci 201, Spring 2020 13

Scoring WOTOs going forward

• Each

h regular gular WOTO O fo form is worth h 2 pts

• If 2 WOTOs

Os in a lect ctur ure, , each ch worth th 2 pt pts

• Some

e WOTO O fo forms ms correc ectne tness s will count nt and each ch pro roblem lem is worth th pts s (likel ely 1 pt pt each) ch)

• We are dro

ropping ping all WOTOs Os done befor efore e to today

• SUBMIT

MIT ALL WOTO O FORM RMS – they count! nt!

• Work Together

her with h ot

• thers

ers

1/22/2020 Compsci 201, Spring 2020 16

WOTO (4 minutes)

http:/ ://bit.l bit.ly/ y/201 201sp sprin ring20 g20-01 0122 22-1

1/22/2020 Compsci 201, Spring 2020 17

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

• Paper on this: http://bit.ly/5qOjoH
• Software and steam engines

Source Code, Byte Code

• High level

el languages es compil mpiled d to to low level el lan anguages es

• In C/C++ low-level specific

ic to to platf tfor

• rm
• In Java byte code is low-level
• Exec

ecute e by machin ine: : real al or virtual al

• JVMs must

t be ported ed to to plat atform

• rm
• Andr

droid

• id doesn

sn't use e JVM

• Dalvik and now ARM
• Be grat

ateful!! l!!!! !!!

• High level source code

1/22/2020 Compsci 201, Spring 2020 19

From Hello.java to Hello.class

1/22/2020 Compsci 201, Spring 2020 20

public class Hello { public static void main(String[] args){ System.out.println("hello world"); } } Code: 0: aload_0 1: invokespecial #1 4: return Code: 0: getstatic #2 3: ldc #3 5: invokevirtual #4 8: return

Project P1 out soon

• See

e course rse websit ite fo for det etails ails, discus cussio ion n D3

• Not quite this, but …

https://www.youtube.com/watch?v=DoLe1c-eokI

• Read as

assignment signment befo efore e star arting ting to to code

• Think before fingers on keys
• Use Piazza

za and Helper er Hours

1/22/2020 Compsci 201, Spring 2020 21

Projects in 201: Review

• Start with

th GitLab Lab pro rojec ect, t, fork k reposit

• sitory
• ry
• Starter code and cloud-based storage for projects
• Clone repo to your machine: you need Git for this
• You use Duke

e Compsc sci GitLab ab websit site

• You manage your code/projects with GitLab
• Make

e chan anges es, , comple mplete e pro roject ject

• Push changes frequently using Git … hourly/daily

1/22/2020 Compsci 201, Spring 2020 22

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

1/22/2020 Compsci 201, Spring 2020 23

Assignments P0 and P1

• Simple

le Java a class ass with driver er pro rogra rams ms

• How is a program run? What is main?
• Testing methods and testing classes
• Examp

mple le of 201 work-flow

• GitLab for starter code and your code
• Using SSH and GitLab together
• Submitting via Gradescope, Analysis

1/22/2020 Compsci 201, Spring 2020 24

What is a static method?

• Objects

ects are inst stance ances s of a class ss

• Thus objects have instance variables
• Typically private, accessed in methods
• Static

tic meth ethod d belongs ngs to to class ss, not

• t obje

bject ct

• No instance variables
• Accesses static variables and methods
• More on this later, for now? …

1/22/2020 Compsci 201, Spring 2020 25

Static Methods

• The class

ss Math ath has as many ny (java.lan .lang)

• sqrt, cos, abs, …
• The class

ss Arrays s has as many ny (java. a.uti util)

• sort, fill, toString, asList, …
• Invoke as

as Math.s ath.sqrt(2 (25.0 5.0) ) or String.j ring.join

• in(…)
• Math is a class. Convention for name?
• There aren't different Math objects, none!
• No state needed, all code in methods

1/22/2020 Compsci 201, Spring 2020 26

WOTO: Correctness Counts

http://b ://bit.l it.ly/201spr y/201spring20 ing20-0122 0122-2

1/22/2020 Compsci 201, Spring 2020 27