English Conundrum s In English, add “s” to end of word to make plural s But for 1 special...

English Conundrum

In English, add “s” to end of word to make plural

But for 1 special word, adding an “s” at the end: Makes word go from plural to singular Makes masculine word into a feminine one

What is this special word?

English Conundrum

In English, add “s” to end of word to make plural

But for 1 special word, adding an “s” at the end: Makes word go from plural to singular Makes masculine word into a feminine one

What is this special word?

princes princess

LECTURE 3:ARRAYS

Announcements

If you need more review of Java… I have lots of good resources – talk to me Use “Additional Help” link on webpage

Weekly assignments problems due before class

Problems NOT optional Get back into coding & remove rust from

summer If cannot solve in 10 minutes, talk to me

Primitives vs. References

Primitive Variables Reference Variables

Variables hold actual value Assignment copies

value Update variable being

assigned only

Aliased with assignments Both refer to same

instance Both see updates to

object Re-assigning variable

does not update aliases

Opening Question

Are array variables primitives or references?

Array Variables

Can be set to null to mark there is no array

Cannot use until after have instantiated arrayint[] bob = new int[30];Car[] parkingLot = new Car[300];

Variables aliased by any assignments Can deduce, array variables always

references May hold primitive data, but these are

separate issue

Array Variables

Can be set to null to mark there is no array

Cannot use until after have instantiated arrayint[] bob = new int[30];Car[] parkingLot = new Car[300];

Variables aliased by any assignments Can deduce, array variables always

references May hold primitive data, but these are

separate issue

Memory Trace Example

int[] data = new int[3];int[] al = data;for (int i = 0; i < data.length; i++) { data[i] = i * 2;}al[0] = 5;al = new int[1];al[0] = 7;

Entries Are Variables, Too!

Within an array entries like individual variablesint[] bob = new int[30];Car[] parkingLot = new Car[300];

bob’s entries would be primitive variables parkingLot’s entries refer to instances

Entries initialized like variable when array created bob’s entries would all be set to 0 null stored in all entries in parkingLot

Memory Trace Example

Car[] data = new Car[3];Car[] al = data;for (int i = 0; i < data.length; i++) { data[i] = new Car(); }al[0] = new Car();al = new Car[1];al[0] = data[0];data[0] = data[1];

Higher-Dimension Arrays

Arrays instantiated with any dimension desired 2-d array is table

Arrays instantiated with any dimension desired 2-d array is table identifying entries by row

& columnint[][] pixels = new int[20][100];pixels[1][18] = 32;

Entries stored in virtual cube with 3-d arrayint[][][] bert = new int[60][4][10];bert[45][3][7] = 42;

Rare to use higher dimensions, but is possible

Computer Professor

Memory Huge (> 250GB) Can’t remember 18 names

Computingspeed

Fast (> 2 billion/second)

Slow (needs fingers & toes)

Takes direction

Does exactly as told Leaves toilet seat up

Speed of updates

Nearly instantaneous Wears t-shirts from 1989

Ability to plan & reason

Cannot make plans; No reasoning skills;Lacks common sense

Makes (semi-)complicated plans; Good reasoning skills;Lacks common sense

Computer vs. Professor

People are very good at reasoning & analyzing Slow computing outcome of each potential decision Considering every possible data combinations hard Trouble when lots of data available to be examined

Computers can perform simple tasks quickly Maintain billions of data items within memory Very good when at performing simple operations Cannot create plan since lacks concept of future To do any task, must be given specific instructions

People vs. Computers

Computers best processing huge data sets Perform specific tasks and output results:

College’s students’ transcripts availableCompute & print receipt for groceries

Filter & provide few items for user to choose fromPersonalized recommendations from AmazonGoogle’s list of matching web pages

Most of year examines how to organize data How data used determines structures selected

Computers Use Collections

How Could We Do This?

Know simple way to organize data: array Entries refer to another array to create

matrices Many limitations arise when using

arrays Must specify unchangeable size when

createdPirate[] rum = new Pirate[1];Pirate[] h2o = new Pirate[variable];rum = new Pirate[60]; // old array was lost!h2o = rum; // h20 now alias to rum’s instance

Waste memory requiring maximum size for array/* Each Amazon.com customer uses 400+MB of RAM! */Rating[] hertz = new Rating[100000000];

Often need to keep values ordered or organized Alphabetical list of names & phone

numbers Keep list of job bids from smallest to largest Web pages ordered from most to least

important Desired index’s not just assigned new

Inserting into an Array

value Lose current value stored at that index

value Lose current value stored at that index Instead, must first shift values within array

Still need to keep values ordered or organized

Removing a value presents own set of problems Could simply assign null

Removing From an Array

0 1 2 n

Removing a value presents own set of problems Could simply assign null, but that creates

a gap Finding start & end of data hard once gaps

allowed

0 1 2 n

allowed Instead, must shift values to fill in all gaps

that exist

0 1 2 n

that exist

0 1 2 n

that exist

0 1 2 n

that exist

0 1 2 n

that exist

0 1 2 n

that exist

0 1 2 n

that exist

0 1 2 n

that exist

0 1 2 n

that exist

0 1 2 n

Your Turn

Get into your groups and complete activity

For Next Lecture

Moving to review OO concerns next week How do we write & use classes on our own? Instances can differ, but how is this

expressed? What is an instance variable & how is it

used? What does static mean? When would we

want it?

There is weekly assignment problem on Angel Due before Wednesday’s lecture (via

Submitter) Get back into the swing of writing Java code

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