09 ch8 arrays

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http://slidepdf.com/reader/full/09-ch8-arrays 1/122

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22

Introduction to arraysIntroduction to arrays

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3

Background Programmer often need the ability to represent a group of

values as a list

List may be one-dimensional or multidimensional

Java provides arrays and the collection classes The Vector class is an example of a collection class

Consider arrays first

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4

Example Definitions

char[] c;

int[] value = new int[10];

Causes

Array object variable c is un-initialized Array object variable value references a new ten element

list of integers

Each of the integers is default initialized to 0

value 0 0 0 0 0

-c

…

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8

Where we’ve seen arrays public static void main (String[] args)

Thus, the main() method takes in a String array as the

parameter

Note that you can also define it as:

public static void main (String args[])

or

public static void main (String[] foobar)

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9

Basic terminology List is composed of elements

Elements in a list have a common name

Example: a[3] = 5;

The common name is ‘a’

The list as a whole is referenced through the common name

List elements are of the same type — the base type

Elements of a list are referenced by subscripting (indexing) thecommon name

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Review of arrays Creating an array:

int[] foo = new int[10];

Accessing an array:

foo[3] = 7;

System.out.print (foo[1]);

Creating an array:

String[] bar = new String[10];

Accessing an array:

bar[3] = “qux”;

System.out.println (bar[1]);

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Explicit initialization Syntax

ElementType[] id = { exp0 , exp1 , ... expn-1 };

id references an array of n elements. id[0] hasvalue exp0, id[1] has value exp1, and so on.

Each expi is an expression thatevaluates to type ElementType

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Array members Member length

Size of the array

for (int i = 0; i < puppy.length; ++i) {

System.out.println(puppy[i]);

}

Note that length is a field, not a method!

I.e., it is not puppy.length()

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Chapter 2: Computer bugsChapter 2: Computer bugs

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Array members Member clone()

Produces a shallow copy

Point[] u = { new Point(0, 0), new Point(1, 1)};

Point[] v = u.clone();

v[1] = new Point(4, 30);

Point: (0, 0) Point: (1, 1)

u

u[0] u[1]

Point: (0, 0)

v

v[0] v[1]

Point: (1, 1)

u

u[0] u[1]

Point: (0, 0)

v

v[0] v[1]

Point: (1, 1)

u

u[0] u[1]

Point: (4, 30)



v[1] = new Point(4, 30);

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Member clone()

Produces a shallow copy



v[1].setX(10);



v[1].setX(10);

Array members

Point: (0, 0) Point: (1, 1)

u

u[0] u[1]

Point: (0, 0)

v

v[0] v[1]

Point: (1, 1)

u

u[0] u[1]

Point: (0, 0)

v

v[0] v[1]

Point: (10, 1)

u

u[0] u[1]

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Making a deep copy We want to copy the array and all the objects each element

of the array references

This is called a deep copy

Example

Point[] w = new Point[u.length];for (int i = 0; i < u.length; ++i) {

w[i] = (Point) u[i].clone();

}

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Making a deep copy

Poi nt : ( 0, 0)

w

w[ 0] w[ 1]

Poi nt : ( 2, 1) Poi nt : ( 2, 2)

w[ 2]

u

u[ 0] u[ 1] u[2]

Poi nt : ( 0, 0) Poi nt : ( 2, 1) Poi nt : ( 2, 2)

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How Java represents arrays Consider

int[] a = { 1, 2, 3, 4, 5 };

a 1 2 3 4 5

+ …

Array

- length = 5

- data = 1 2 3 4 5

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2525

TodayToday’ ’ ss demotivatorsdemotivators

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26

How are we doing with arrays?How are we doing with arrays?

V e r y w e

l l ! T h i

s s t u f f i

. . .

F a i r l y

w e l l

– w i t h

a l i t t .

.

O k a

y . I t ’ s

n o t

g r e a t

, b u .

. .

N o t

w e l l .

I ’ m

k i n

d a c . . .

N o t

a t a

l l . I ’ m

s o o

o o o .

. .

20% 20% 20%20%20%1.1. Very well! This stuff is Very well! This stuff is

easy!easy!

2.2. Fairly wellFairly well – – with a little with a littlereview, Ireview, I’’ll be goodll be good

3.3. Okay. ItOkay. It’’s not great,s not great,

but itbut it’’s not horrible,s not horrible,eithereither

4.4. Not well. INot well. I’’mm kindakinda

confusedconfused5.5. Not at all. INot at all. I’’mm

soooooosoooooo lost.lost.

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ArrayTools.java – outlinepublic class ArrayTools {

// class constantprivate static final int MAX_LIST_SIZE = 1000;

// sequentialSearch(): examine unsorted list for key

public static int sequentialSearch(int[] data, int key) { ...

// putList (): prints list to screenpublic static void putList(int[] data) { ...

// getList(): extract and return up to MAX_LIST_SIZE values

public static int[] getList() { ...

// reverse(): reverses the order of the element values

public static void reverse(int[] list) { ...

// binarySearch(): examine sorted list for a key

public static int binarySearch(char[] data, char key) { ...

}

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ArrayTools.java method putList() To print the array:

public static void putList(int[] data) {for (int i = 0; i < data.length; ++i) {

System.out.println(data[i]);

}}

Consider

int[] score = { 6, 9, 82, 11, 29, 85, 11, 28, 91 };

putList(score);

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ArrayTools.java method getList()public static int[] getList() {

Scanner stdin = new Scanner (System.in);

int[] buffer = new int[MAX_LIST_SIZE];int listSize = 0;

for (int i = 0; (i < MAX_LIST_SIZE) &&

stdin.hasNext(); ++i) {

buffer[i] = stdin.nextInt();++listSize;

}

int[] data = new int[listSize];for (int i = 0; i < listSize; ++i) {

data[i] = buffer[i];

}

return data;}

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How are we doing withHow are we doing with ArrayTools ArrayTools??

V e r y w

e l l !

T h i s

s t u f f i . .

.

F a i r l y

w e l l –

w i t h

a l i t t .

.

O k a

y . I t ’ s

n o t

g r e a t

, b u .

. .

N o t w e

l l . I ’ m

k i n

d a c . . .

N o t a t

a l l .

I ’ m s o o

o o o .

. .


easy!easy!







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Consider that main() method again

public static void main (String args[])

How does one pass in a parameter to the main method?

public class MainParameters {

public static void main (String args[]) {

System.out.println ("Number of paramters to “ +"main(): " + args.length);

if ( args.length > 0 ) {

for ( int i = 0; i < args.length; i++ )

System.out.println ("parameter " +

i + ": '" + args[i] + "'");

}

}}

P DP D

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Program DemoProgram Demo

MainParameters.javaMainParameters.java

ViaVia JCreator JCreator Via the command lineVia the command line

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Basic array searchingBasic array searching

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System.out.println("Enter search value (number): ");

int key = stdin.nextInt();

int i;for (i = 0; i < data.length; ++i) {

if (key == data[i]) {

break;

}

}

if (i != data.length) {

System.out.println(key + " is the " + i+ "-th element");

}

else {System.out.println(key + " is not in the list");

}

++i

System.out.println("Enter search value (number): ");

int key = stdin.nextInt();

int i;

if (key == data[i]) {

break;

if (i != data.length) {

System.out.println(key + " is the " + i+ "-th element");

}

i < data.lengthi = 0

Searching for a value

dat a 54 9

20 1

key 5

i -

dat a 54 9

20 1

key 5

i 0

dat a 54 9

20 1

key 5

i 0

dat a 54 9

20 1

key 5

i 0

dat a 54 9

20 1

key 5

i 1

dat a 54 9

20 1

key 5

i 1

dat a 54 9

20 1

key 5

i 1

dat a 54 9

20 1

key 5

i 2

dat a 54 9

20 1

key 5

i 2

dat a 54 9

20 1

key 5

i 2

dat a 54 9

20 1

key 5

i 2

dat a 54 9

20 1

key 5

i 2

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H d i i h hi ?H d i ith hi ?

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How are we doing with searching?How are we doing with searching?

V e r y w

e l l !

T h i s

s t u f f i . .

.

F a i r l y

w e l l –

w i t h

a l i t t .

.

O k a

y . I t ’

s n o t g

r e a t

, b u .

. .

N o t w

e l l .

I ’ m k i n

d a c . . .

N o t a t

a l l .

I ’ m

s o o

o o o .

. .


easy!easy!







FollowFollow--up on Knut the polar bearup on Knut the polar bear

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4646

p pp p

cubcub http://www.comedycentral.com/motherloahttp://www.comedycentral.com/motherloa

d/?ml_video=84023d/?ml_video=84023

http://www.comedycentral.com/motherload/?ml_video=84023






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public static int sequentialSearch(int[] data, int key) {

for (int i = 0; i < data.length; ++i) {

if (data[i] == key) {return i;

}

}return -1;

}

Consider

int[] score = { 6, 9, 82, 11, 29, 85 };

int i1 = sequentialSearch(score, 11);

sequentialSearch() finding an element

data 826 9 8511 29

20 1 53 4

key 11



}

}return -1;

}

int i = 0 i < data.length ++i

i 0123

i1 3

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for (int i = 0; i < data.length; ++i) {


}

}return -1;

}

Consider

int[] score = { 6, 9, 82, 11, 29, 85 };

int i1 = sequentialSearch(score, 30);

sequentialSearch() not finding an element

data 826 9 8511 29

20 1 53 4

key 11


if (data[i] == key) {

}

}return -1;

}

int i = 0 i < data.length ++i

i 0123

i1 -1

456


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V e r y w

e l l !

T h i s

s t u f f i . .

.

F a i r l

y w e

l l – w i t h

a l i t t .

.

O k a y .

I t ’ s n o t g

r e a t

, b u .

. .

N o t w

e l l .

I ’ m k i n

d a c . . .

N o t a t

a l l .

I ’ m

s o o

o o o .

. .


easy!easy!







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5050

SortingSorting

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l

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v 'Q''E' 'I' 'Y''R' 'T' 'W''U' 'P''O'

20 1 53 4 76 98

Selection sorting

Algorithm basis

On iteration i, a selection sorting method:

Finds the element containing the ith smallest value of its list v and exchanges that element with v[i]

Example – iteration 2

Swaps third smallest element with v[2]

This results in third smallest element being in the correctplace for a sorted result

v ‘O''E' 'I' 'Y''R' 'T' 'W''U' 'P'‘Q'

20 1 53 4 76 98

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How are we doing with sorting?How are we doing with sorting?

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59

How are we doing with sorting?How are we doing with sorting?

V e r y

w e l l !

T h i

s s t u f f i

. . .

F a i r l y

w e l l –

w i t h

a l i t t .

.

O k a y .

I t ’ s

n o t

g r e a t

, b u .

. .

N o t w

e l l .

I ’ m k i n

d a c . . .

N o t a

t a l l .

I ’ m

s o o

o o o .

. .


easy!easy!





confusedconfused5.5. Not at all. INot at all. I’’mmsoooooosoooooo lost.lost.

New 2008New 2008 demotivatorsdemotivators

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6060

New 2008New 2008 demotivatorsdemotivators

7/29/2019 09 Ch8 Arrays


Binary search

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Binary search

Given a list, find a specific element in the list

List MUST be sorted!

Each time it iterates through, it cuts the searchspace in half

A binary search is MUCH faster than a sequentialsearch

Binary search use

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Binary search use

The ‘BS’ in BSDemo is for Binary Search, mind you

public class BSDemo {

public static void main(String[] args) {int[] numbers = { 9, 3, 1, 8, 4, 6, 10, 2 };System.out.println ("The original list of numbers:");ArrayTools.putList(numbers);System.out.println();

ArrayTools.selectionSort(numbers);System.out.println ("The sorted list of numbers:");ArrayTools.putList(numbers);System.out.println();

System.out.println ("Searching for 0: " + ArrayTools.binarySearch(numbers, 0));System.out.println ("Searching for 1: " + ArrayTools.binarySearch(numbers, 1));System.out.println ("Searching for 4: " + ArrayTools.binarySearch(numbers, 4));System.out.println ("Searching for 5: " + ArrayTools.binarySearch(numbers, 5));System.out.println ("Searching for 6: " + ArrayTools.binarySearch(numbers, 6));System.out.println ("Searching for 10: " + ArrayTools.binarySearch(numbers, 10));System.out.println ("Searching for 11: " + ArrayTools.binarySearch(numbers, 11));

}}

Binary search use demoBinary search use demo……

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6464

yy

BSDemo.javaBSDemo.java

Binary search

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Binary search

public static int binarySearch (int[] data, int key) {

int i = 0; // left endpoint of search interval

int j = data.length-1; // right endpoint of search interval

while ( i < j ) {int m = (i+j)/2;

if ( key > data[m] ) {

i = m+1;

} else {j = m;

}

}

if ( key == data[i] ) {return i;

} else {

return -1;

}}

Binary search take 1

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if ( key == data[i] ) {

return i;

} else {

return -1;}


return i;

} else {

return -1;}

int i = 0;

int j = data.length-1;

int i = 0;


while ( i < j ) {

int m = (i+j)/2;


i = m+1;} else {

j = m;

}

}

while ( i < j ) {

int m = (i+j)/2;


i = m+1;} else {

j = m;

}

}

Binary search, take 1

2 4 6 8 10 12 14 16 18 20

a0 a1 a2 a3 a4 a5 a6 a7 a8 a9

i j m

0


key 14

945 7 76 656

returns:

6

data

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Binary search take 2

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68


return i;

} else {

return -1;}


return i;

} else {

return -1;}

int i = 0;


int i = 0;


while ( i < j ) {

int m = (i+j)/2;


i = m+1;} else {

j = m;

}

}

while ( i < j ) {

int m = (i+j)/2;


i = m+1;} else {

j = m;

}

}

Binary search, take 2

2 4 6 8 10 12 14 16 18 20

a0 a1 a2 a3 a4 a5 a6 a7 a8 a9

i j m

0


key 15

945 7 76

returns:

-1

data

7

How are we doing with binaryHow are we doing with binary

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69

search?search?

V e r y

w e l l !

T h i s

s t u f f i . .

.

F a i r l y

w e l l –

w i t h

a l i t t .

.

O k a y

. I t ’

s n o t g

r e a t

, b u .

. .

N o t w e l l .

I ’ m

k i n

d a c . . .

N o t a

t a l l .

I ’ m

s o o

o o o .

. .


easy!easy!




4.4. Not well. INot well. I’’mm kindakindaconfusedconfused

5.5. Not at all. INot at all. I’’mmsoooooosoooooo lost.lost.

7/29/2019 09 Ch8 Arrays


How long does a binary search take?

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71

How long does a binary search take?

Given a array of 64 elements

1st iteration cuts the array to 32

2nd iteration cuts the array to 16

3rd to 8

4th to 4

5th to 2

6th to 1

Given a array of 1024 elements

1st iteration cuts the array to 512

...

10th iteration cuts the list to 1 element

Thus, the binary search takes log2 n iterations! Where n is the size of the array

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Limitations of arrays

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75

y

You can’t change their size once created

This can be a big problem!

So we will create a new class that will operate like an array:

We can store and get elements by index number

It will automatically increase in size as needed

And other fancy features…

Let’s call the class Vector

As we are basically writing the java.util.Vector class

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Our first take on our Vector class

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public class Vector {

private Object array[];

private int size = 0;

Vector() {

array = new Object[100];

}

Vector(int length) {

array = new Object[length];

}

}

What does this mean?

We’ll see that a bit later…

But briefly, it means the array can store any object

Adding an element to our Vector

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public void add (Object o) {

array[size++] = o;}

Pretty easy!

But what if the array is full?

We need a way to increase the capacity of the array

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Methods can be private as well

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81

Notice that the increaseCapacity() method is called only bythe add() method when necessary

It’s not ever going to be called by whomever is using ourVector

Thus, we will make it private

That means that only other Vector methods can call it

Removing an element from a Vector

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public Object remove (int which) {

Object ret = array[which];for ( int i = which; i < array.length-1; i++ )

array[i] = array[i+1];

array[array.length-1] = null;

size--;

return ret;

}

Safety AwardsSafety Awards

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8383

safety_Awards_2006.ppssafety_Awards_2006.pps

Miscellaneous other methods

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public int size() {

return size;}

public Object get (int which) {

return array[which];

}

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Using our Vector

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86

This code is in a separate class called VectorUsage

public static void main (String[] args) {

Vector v = new Vector();for ( int i = 12; i < 30; i++ ) {

v.add (String.valueOf(i));

}

System.out.println (v);System.out.println (v.size());

String s = (String) v.get(5);

System.out.println (s);

v.remove (5);System.out.println (v);

v.remove (5);

System.out.println (v);

}


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8787

VectorUsage.javaVectorUsage.java

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8989

VectorUsage.javaVectorUsage.java

But usingBut using java.util.Vector java.util.Vector

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More on using the Vector class

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To add a String object s to the end of a Vector v

v.add(s);

To get the String object at the end of the Vector v

String s = (String) v.get(v.size()-1);

To remove a String object from the end of a Vector v

String s = (String) v.remove(v.size()-1);

This both removes the object from the Vector and stores

the removed value into s

How are we doing with Vectors?How are we doing with Vectors?

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V e r y w e l l !

T h i

s s t u f f i

. . .

F a i r l

y w e

l l – w i t h

a l i t t .

.

O k a y .

I t ’ s

n o t

g r e a t

, b u .

. .

N o t w

e l l .

I ’ m k i n

d a c . . .

N o t a t a

l l . I ’ m

s o o

o o o .

. .


easy!easy!



but itbut it

’’s not horrible,s not horrible,

eithereither



Stuff you donStuff you don’ ’ t see everydayt see everyday……

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9393

StuffYouDontSeeEveryDay.ppsStuffYouDontSeeEveryDay.pps

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Example

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97

Alternative

int[][] m = new int[3][4];

Produces

m

m[0] m[1] m[2]

0 0 0 0 0 0 0 0

00 0 0

m[2][0] m[2][1] m[2][2] m[2][3]

m[0][0] m[0][1] m[0][2] m[0][3] m[1][0] m[1][1] m[1][2] m[1][3]

Multidimensional array visualization

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A multi-dimensional array declaration (either one):

int[][] m = new int[3][4];

How we visualize it:

0 0 0

0 0 0

0 0 0

0 0 0

0

0

0

0

0

0

0

0

0

0

0

0

or

Example

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99

Segment

for (int c = 0; c < m.length; ++c) {

for (int r = 0; r < m[c].length; ++r) {

System.out.print("Enter a value: ");

m[c][r] = stdin.nextInt();

}

}

0

0

0

0

0

0

0

0

0

0

0

0

Rows by columns or columns by rows?

d i [][] i [ ][ ]

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Consider int[][] m = new int[3][4];

Is that 3 rows by 4 columns or 3 columns by 4 rows?

The answer is that it can be either

As long as you are consistent with your column/rowplacement

0 0 0

0 0 0

0 0 0

0 0 0

or

0 0 0

0 0 0

0 0 0 0

0

0

Rows by columns or columns by rows?

Thi k it 3 l b 4

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This makes it 3 columns by 4 rows:

for (int c = 0; c < m.length; ++c)

for (int r = 0; r < m[c].length; ++r) {System.out.print("Enter a value: ");

m[c][r] = stdin.nextInt();

}

This makes it 3 rows by 4 columns:

for (int r = 0; r < m.length; ++r)for (int c = 0; c < m[r].length; ++c) {

System.out.print("Enter a value: ");

m[r][c] = stdin.nextInt();

}

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Multidimensional array visualization

Segment

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103

Segment

String[][] s = new String[4][];

s[0] = new String[2];




Produces

Called a “ragged” array

0 0 0

0 0 0

0

0

0

0

0

0

0

0

0

0

0

0

0

or

0

0

0

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Fractal zoomingFractal zooming

htt // f t l t t t /2000/htt // f t l t t t /2000/

http://www.fractalartcontests.com/2000/en/entry-004-7.htm



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http://www.fractalartcontests.com/2000/ehttp://www.fractalartcontests.com/2000/e

n/entryn/entry--004004--7.htm7.htm

Explicit Initialization

Segment















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Segment

int c[][] = {{1, 2}, {3, 4}, {5, 6}, {7, 8, 9}};

Produces

c

c[ 0] c[ 1] c[ 2]

1 2

3 4

7 8 9

c[ 3] [ 0] c[ 3] [ 1] c[ 3] [ 2]

c[1] [ 0] c[ 1] [ 1]

c[ 0] [ 0] c[ 0] [ 1]

5 6

c[ 2] [ 0] c[ 2] [ 1]

c[ 3]

Matrices

A two-dimensional array is sometimes known as a matrix

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A two dimensional array is sometimes known as a matrixbecause it resembles that mathematical concept

A matrix a with m rows and n columns is representedmathematically in the following manner

a1 1, a1 2, … a1 n,

a2 1, a2 2, … a2 n,

… …

am 1, am 2, … am n,

Matrix addition

Definition C = A + B

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Definition C = A + B

cij = aij + bij

cij is sum of the elements in the same row and column of

A and B

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Homework 10

You will be creating a Map class

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You will be creating a Map class

The Map class contains a 2-D array

In each spot will be a Location object

(from a previous HW)

Lab 11 is (was?) going to be a MapPrinter class

Will print out the 2-D Map via text

How are we doing with 2How are we doing with 2--D arrays?D arrays?

20% 20% 20%20%20%1.1. Very well! This stuff isVery well! This stuff is

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111 V e r y

w e l l !

T h i s

s t u f f i . . .

F a i r l y

w e l l –

w i t h

a l i t t . .

O k a y

. I t ’

s n o t g

r e a t

, b u . . .

N o

t w e l l .

I ’ m k i n

d a c . . .

N o

t a t a

l l . I ’ m

s o o

o o o . . .


easy!easy!

2.2.

Fairly wellFairly well – – with a little with a littlereview, Ireview, I’’ll be goodll be good

3.3. Okay. ItOkay. It’’s not great,s not great,but itbut it’’s not horrible,s not horrible,eithereither



Driving in BoliviaDriving in Bolivia

DrivingInBolivia ppsDrivingInBolivia pps

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DrivingInBolivia.ppsDrivingInBolivia.pps

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Wrapper classesWrapper classes

But what about adding variables?

The add method takes an Object as a parameter

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public void add (Object o) {

Although we haven’t seen it yet, this means you can add

any object you want to the vector

Primitive types (i.e. variables) are not objects

How can they be added?

The solution: wrapper classes!

The Integer wrapper class

This is how you add an int variable to a Vector:

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int x = 5;

Integer i = new Integer(x);

vector.add (i);

//…

Integer j = (Integer) v.get(0);int y = j.intValue();

Pretty annoying syntax – we’ll see how to get around it in abit…

More on wrapper classes

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All the primitive types have wrapper classes

Usually, the names are just the capitalized version of the

type

I.e. Double for double, Byte for byte, etc.

Two exceptions: int and char

int has Integer char has Character

More on wrapper classes

Consider this code:

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int x = 5;

vector.add (x);

//…

int y = vector.get(0);

Does this code work?

It shouldn’t

As we are adding a variable (not an object) to a vector But it does work!

Why?

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More on auto-boxing

Consider the following code:

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Double d = 7.5;

Double e = 6.5;

Double f = d + e;

System.println (f);

This is doing a lot of auto-boxing (and auto-unboxing):

Double d = new Double(7.5);Double e = new Double(6.5);

Double f = newDouble(d.doubleValue() +e.doubleValue());

System.println (f);

How are we doing withHow are we doing with

Wrapper classes? Wrapper classes?


!!

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120 V e

r y w e l l !

T h i s

s t u f f i . . .

F

a i r l y

w e l l –

w i t h

a l i t t . .

O k

a y . I t ’

s n o t g

r e a t

, b u . . .

N o t

w e l l .

I ’ m k i n

d a c . . .

N o t

a t a

l l . I ’ m

s o o

o o o . . .

easy!easy!

2.2.

Fairly wellFairly well

– –

with a little with a little

review, Ireview, I’’ll be goodll be good

3.3. Okay. ItOkay. It’’s not great,s not great,but itbut it’’s not horrible,s not horrible,

eithereither4.4. Not well. INot well. I’’mm kindakinda

confusedconfused


Star Wars Episode 3 TrailerStar Wars Episode 3 Trailer

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Star Wars Episode 3 TrailerStar Wars Episode 3 Trailer

That was a edited versionThat was a edited version

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– – I changed the PGI changed the PG--rated trailer to a Grated trailer to a G--ratedrated

trailertrailer

The original one can be found atThe original one can be found athttp://http://www.sequentialpictures.comwww.sequentialpictures.com / /

– – Or Google forOr Google for “ “star wars parodystar wars parody” ”

09 ch8 arrays

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