Lec09 Arraysasd

7/17/2019 Lec09 Arraysasd

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CSCI 1530Computer Principles

and Java Programming

Arrays

2013


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Outlines

! Introduction to arrays (1-D arrays)

!

Declaring, creating, and initializing arrays

!

Accessing array elements

!

Passing arrays to methods

!

Returning arrays from methods

! 2-D arrays

!

Declaring, creating, and initializing 2-D arrays

!

2-D arrays as parameters/arguments and return values

! Array assignment


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import java.util.*;

classExample {

public static voidmain(String[] args) {

Scanner scanner = newScanner(System.in);

intgrade1, grade2, grade3;

System.out.print("Student 1: ");

grade1 = scanner.nextInt();





System.out.println("Average = "+

(grade1 + grade2 + grade3) / 3.0);

}

}

! The program works if there are only three students.

! What if there are 100 students?

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Arrayto the rescue!

Ordinary

VariableLike a box for storing

one value

Array

Like a cabinetcontaining many

drawers.

Each drawer stores

one value.

We can refer to each

drawer as 1stdrawer,2nddrawer, 3rddrawer,

etc.


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Array

!

Stores same type of data! Array size = # of elements in the array

! Array size remains unchanged throughoutprogram execution

!

To refer to an array element

arrayname[ index ]

! Index always starts from 0

! Index to last element is (array size 1)

grade[0]

grade[1]

grade[2]

grade[3]

grade[96]

grade[97]

grade[98]

grade[99]

Array name

.

..

.

..

Array Index

Storage


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Declaring Arrays

!

Syntax:

! type: Data type of each array element

!

arrayName: A valid identifier

! E.g.: int[] grade; // array of integers

float d[]; // array of floats

!

Declaring multiple arrays of the same type in one declarationint[] arrayA, arrayB;

int arrayA[], arrayB[];

type[] arrayName; typearrayName[];or


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Creating Arrays

! In Java, arrays are objects. So, we have to create themusing new.

! Syntax:

! arraySize: Number of elements in the array

! E.g.: grade = new int[12];

!

d = new float[100];

! Declaring and creating an array at once:

type[] arrayName; // or: typearrayName[];

arrayName= new type[ arraySize];

type[] arrayName= new type[ arraySize];


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Creating Arrays

! An array is an object, so it is of reference datatype.

! Note:

!

double/int/!

is a primitive type.

! But double[]/int[]/!is a reference type

double[] rainfall = newdouble[12];

0 1 2 3 4 5 6 7 8 9 10 11

rainfall

rainfall[2]


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Array Elements

! Each array element holds one value.

! Index (also called subscript) must be of type int,short,byte, or char(i.e., integral type, usually int).

!

Index can be an expression.

! E.g.: c[i 2]where iis an integer

// grade is an array of 100 integers

int[] grade = new int[100];

grade[0] = 10;

grade[1] = 3;

System.out.println(grade[0] + " "+ grade[1]);


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classExample {


intn[] = new int[10]; // an array of 10 integers

// Set all elements of array to 0

for( inti = 0; i < 10; i++ )

n[i] = 0;

System.out.println("Element Value");

// output contents of array n in tabular format

for( intj = 0; j < 10; j++ )

System.out.printf("%7d%13d\n", j, n[j]);

}

}

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Element Value

0 0

1 0

2 0

3 0

4 05 0

6 0

7 0

8 0

9 0


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Array Bounds

!

Indexes of an array of size N must range from 0 to N-1.

! An array index that is out of this range cause a run-timeerror (called array index out of bounds).

int[] c = new int[ 10 ];

int i = 4;c[ -1 ] = 5; // Index out of bound

c[ i+6 ] = 0; // Index out of bound

! It is the programmer's responsibility(not the compiler's) toensure correct array indexes.

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 10

at Example.main(Example.java:17)


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Initializing Arrays In Declaration

! Specify value of each element when array isdeclared.

int[] x = { 4, 2, 6, 1, 9 };

! The number of initializers determines the array

size.

! Size of xis automatically determined as 5.


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// Initializing an array in array declaration.

class Example {


// use initializer list to initialize array nint[] n = { 32, 27, 64, 18, 95, 14, 90, 70, 60, 37 };

System.out.println("Element Value");

// output contents of array n in tabular format

for( inti = 0; i < 10; i++ )

System.out.printf("%7d%13d\n", i, n[i]);

}

}

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Element Value

0 32

1 27

2 64

3 18

4 955 14

6 90

7 70

8 60

9 37


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Tips: Array Size

! An array is an object. Its public constant attribute

lengthreturns the size of the array.

! Make program more scalable and reduce the

risk of array out of bounds error.

System.out.print("How many students? ");

intn = scanner.nextInt();

int[] grade = new int[n];

for(inti = 0; i < grade.length; i++) {

System.out.print("Enter student score: ");

grade[i] = scanner.nextInt();

}


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// A program to count the # of 0's, 1's, 2's, and 3's// among the input values (Version 1).importjava.util.*;

classExample {public static voidmain(String[] args) {

Scanner scanner = newScanner(System.in);int[] count = new int[4];intnum;

for(inti = 0; i < count.length; i++)count[i] = 0; // Set all elements to 0

System.out.println("Enter 10 integers:");for(inti = 0; i < 10; i++) {

num = scanner.nextInt();if(num == 0) count[0]++;else if(num == 1) count[1]++;else if(num == 2) count[2]++;else if(num == 3) count[3]++;

}

System.out.println("No. Counts");for(inti = 0; i < count.length; i++)

System.out.printf("%3d%9d\n", i, count[i]);}

}

12345678910111213

1415161718192021222324252627

Enter 10 integers:

1 2 3 2 3 1 1 1 1 0

No. Counts

0 1

1 5

2 2

3 2


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// A program to count the # of 0's, 1's, 2's, and 3's

// among the input values (Version 2).

importjava.util.*;

classExample {

public static voidmain(String[] args) {Scanner scanner = newScanner(System.in);

int[] count = new int[4];

intnum;

for(inti = 0; i < count.length; i++)

count[i] = 0; // Set all elements to 0

System.out.println("Enter 10 integers:");

for(inti = 0; i < 10; i++) {

num = scanner.nextInt();

if(num >= 0 && num < count.length)

count[num]++; // Increase the count for "num"

}

System.out.println("No. Counts");

for(inti = 0; i < count.length; i++)

System.out.printf("%3d%9d\n", i, count[i]);

}

}

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Arrays and Methods

! Pass an array elementto a method

classFoo {

public voidprint(intx) {

System.out.println(x);

}}

// In class FooExample

int[] arr = new int[10];

Foo f = newFoo();

for(inti = 0; i < arr.length; i++)

f.print( arr[i]);

How about

passing an

entire arrayto a

method?


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public voidprint(int[] array) {

for(inti = 0; i < array.length; i++)System.out.print(array[i] + " ");

System.out.println();

}

Arrays as Parameters

State the type (e.g.,

int[]) and parameter

name (e.g., array) asusual.

Array size can be

accessed through

length.


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Arrays as Arguments

! Just specify array name (without [])

! Can accept an array of the same type of any

sizeas actual parameter


Foo f = newFoo();

int[] foo = newint[24];

int[] bar = newint[100];

// Print all elements in foo and bar

f.print( foo);

f.print(bar);


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Arrays and Methods

! Since arrays are of reference types, modifyingan array element in a method "can be seen"

from the caller.

classFoo {public voiddoubleElements(int[] num) {

for(inti = 0; i < num.length; i++)

num[i] *= 2;

}

}


Foo f = newFoo();

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

f.doubleElements(x);

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

0 1 2 3 4

3 2 2 4 1

num

x

X X X X X6 4 4 8 2


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Arrays as Method Return Values

! A method that returns an array:

! Calling a method that returns an array

// In class Foo

public double[] getRainfall() {

double[] rainfall = newdouble[12];

returnrainfall;

}


Foo f = newFoo();

double[] data;

data = f.getRainfall();

No need to create anarray before calling

the method.

Array as

return type


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Examples

! The following few slides are some examplesillustrating how we can process the data in anarray. The examples include:

!

Computing average of all the numbers in an array

! Finding the largest number in an array

! Locating a data in an array

! Using array elements as indexes to another array


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public doubleaverage(int[] num) {

intsum = 0;

for( inti = 0; i < num.length; i++ )

sum += num[ i ];

return(double)sum / num.length;

}

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Example: A method to compute average


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public intmax(int[] num) {

intlargest;

largest = num[0];


if(num[i] > largest)

largest = num[i];

returnlargest;

}

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Example: A method to find the largest number in an array


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// This method locates "target" in num.

// Return value:

// -1 if "target is not found in num.

// Otherwise location of "target" in num.

public int search(int[] num, inttarget) {


if(num[ i ] == target)

returni;

return-1;

}

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! A search method typically returns the location where theitem is found.


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// Counting the numbers in num

public void countZeroToNine(int[] num) {

int[] count = new int[10];

for( inti = 0; i < count.length; i++ )

count[i] = 0; // set all elements to 0


if ( num[i] >= 0 && num[i]


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2-D Arrays

! A table that consists of rows and columns

! Conceptually, rectangular in shape

! Store values of the same type

!

Need two indexes to identify each element

Column 0 Column 1 Column 2 Column 3

Row 0

Row 1

Row 2


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Declaring and Creating 2-D Arrays

! Declaration:

! type[][] arrayName; // variation 1

! typearrayName[][]; // variation 2

! Creation:

! arrayName= new type[size1][size2];

! Example:

Column 0 Column 1 Column 2 Column 3

Row 0 a[0][0] a[0][1] a[0][2] a[0][3]

Row 1 a[1][0] a[1][1] a[1][2] a[1][3]

Row 2 a[2][0] a[2][1] a[2][2] a[2][3]

1stindex to row

2ndindex tocolumn

int[][] a;

a = new int[3][4];


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2-D Array Initialization

! Declaration and initialization// As array of arrays

int[][] a = {{1, 2, 3}, {4, 5, 6}};

! The size of each row can be different.// row 0: size 3, row 1: size 2

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

a[0] 1 2 3

a[1] 4 5 6

Automaticallydetermines that a

has 2 rows and 3columns

b[0] 1 2 3

b[1] 4 5


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2-D Array Size of Each Dimension

! Obtaining the size of each dimension of an array

! E.g.:

int[][] array1 = new int[5][9];

System.out.println(array1.length);System.out.println(array1[0].length);

System.out.println(array1[1].length);

int[][] array2 = {{3, 2, 4}, {5, 9, 7, 1}};

System.out.println(array2.length);System.out.println(array2[0].length);

System.out.println(array2[1].length);

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// arrA, arrB, and arrC are 3x3 arrays

int[][] arrA = {{ 1, 2, 3}, { 0, -1, 2}, { 0, 0,

1}};

int[][] arrB = {{ 1, 0, 0}, { 2, -1, 0}, { 3, 2,

1}};

int[][] arrC = new int[3][3];

// Adds arrA and arrB, element by element

for (inti = 0; i < arrA.length; i++)

for (intj = 0; j < arrA[i].length; j++)

arrC[i][j] = arrA[i][j] + arrB[i][j];

// Print array arrC

for (inti = 0; i < arrC.length; i++) {

for (intj = 0; j < arrC[i].length; j++)

System.out.printf("%4d", arrC[i][j]);

System.out.println();

}

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Example: adding two 2-Darrays element by element

2 2 3

2 -2 2

3 2 2

1 2 3

0 -1 2

0 0 1

1 0 0

2 -1 0

3 2 1

arrA

arrB


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2-D Arrays as Parameters

! State the array type type[][]and parameter

name as usual. (E.g., int[][]array)

! The size of each dimension of the array can be

obtained using array.lengthand

array[i].lengthrespectively.

! Parameter can accept any2-D array of the same

type

public voidfoo(int[][]array) {

}


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2-D Arrays as Parameters

classFoo {public voidfoo( int[][] array) {

}

}


Foo f = newFoo();

int[][] a1 = newint[24][4];

int[][] a2 = newint[100][4];

int[][] a3 = newint[10][2];

int[] a4 = newint[20];

f.foo( a1 ); // OK

f.foo( a2 ); // OK

f.foo( a3 ); // OK

f.foo( a4 ); // Compilation error


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classFoo {

public voidprint(int[] a) {

for(inti = 0; i < a.length; i++)

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

}}

A 2-D array is a 1-D array of 1-D arrays

Note: Each row of a 2-D array is a 1-D array


Foo f = newFoo();

int[][] b = new int[10][4];

int[][] c = new int[2][10];

int[] d = new int[3];

f.print( d ); // Print all 3 elements in d

f.print(b[1]); // Print all 4 elements in 2nd row of b

f.print( c[0]); // Print all 10 elements in 1st row of c


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Returning a 2-D Array

classFoo {

// A method that returns a 2-D array

public int[][] foo() {int[][] array = ;

returnarray;

}

}


Foo f = newFoo();

int[][] x;

x = f.foo();

Return type stated as:type[][]

Receiving a referenceof the array from the

method call


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Applications of 2-D arrays

!

Digital Images (2-D array of pixels)!Assignment scores of students

! Each row represents a student

! Each column represents the student's scores from

different components

!

Game board (Chess, Minesweeper, etc.)

!

Spreadsheet

!

etc.


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Array Assignment

! Two array variables can be assigned from one toanother.

! Only a reference of the array is copied.

!

Arrays are objects anyway.

int[] array1 = ;

int[] array2;

array2 = array1; // ok

0 1 2 !array1

array2


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Array Assignment

int[] a;

int[] b = { 11, 22, 33, 44, 55 };

int[] c = { 10, 9, 8 };

a = b; // a and b refers to the same array

System.out.println( a[0] ); // Prints 11


a[4] = 66; // b[4] is also changed to 66

System.out.println( b[4] ); // Prints 66

a = c; // Now, a and c refers to the same array



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int[] array, tempArray;

// Create an array of 5 integers

array = newint[5];

for(inti = 0; i < array.length; i++)

array[i] = i;

// Want to enlarge array to hold 100 integers,

// but keeping its previous contents intact.

// Solution: create another new array (tempArray) and copy

// the contents of old array to the new one, and makes

// array point to the new array

tempArray = newint[100];

for(inti = 0; i < array.length; i++)

tempArray[i] = array[i];

array = tempArray;

An example to illustrate how you can enlarge an array.


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To Wrap Up!

!

Recall our GradeBookexample. An object ofclass GradeBookis used to store the examscores of the students in one course.

!

A straightforward choice of attributes of classGradeBookis:! Name of the course (a string)

!An array of the students scores (an array of integers)

!

Lets enrich our class GradeBookto includemore functionalities.


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classGradeBook {

privateString courseName;

privateint[] grades;

publicGradeBook(String name, int[] scores) {

setCourseName(name);

grades = new int[scores.length];

for(inti = 0; i < grades.length; i++)

if(scores[i] > 100)

grades[i] = 100;else

grades[i] = scores[i];

}

public voidsetCourseName(String name) {

courseName = name;

}

publicString getCourseName() {

returncourseName;

}

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GradeBook.java


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public voiddisplayMessage() {

System.out.println("Welcome to the grade book for "

+ getCourseName() + "!");}

public voidprocessGrades() {

outputGrades();

System.out.println("Class average is "

+ getAverage());System.out.println("Lowest grade is "+ getMin());

System.out.println("Highest grade is "+ getMax());

}

public intgetMax() {

intlargest = 0;

for(inti = 0; i < grades.length; i++)if(grades[i] > largest)

largest = grades[i];

returnlargest;

}

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GradeBook.java(continue)


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public intgetMin() {

intsmallest = 100;


if(grades[i] < smallest)smallest = grades[i];

returnsmallest;

}

public doublegetAverage() {

intsum = 0;


sum += grades[i];

return(double)sum / grades.length;

}

public voidoutputGrades() {

System.out.println("The grades are:");for(inti = 0; i < grades.length; i++)

System.out.printf("Student %3d:%10d\n",

i + 1, grades[i]);

}

}

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GradeBook.java(continue)


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classGradeBookExample {public static voidmain(String[] args) {

int[] scores = {87, 68, 94, 100, 83,78, 85, 91, 76, 87};

GradeBook book = newGradeBook("CSC1530", scores);

book.displayMessage();book.processGrades();

}}

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GradeBookExample.javaWelcome to the grade book for CSC1530!

The grades are:Student 1: 87Student 2: 68Student 3: 94Student 4: 100Student 5: 83Student 6: 78Student 7: 85

Student 8: 91Student 9: 76Student 10: 87Class average is 84.9Lowest grade is 68Highest grade is 100

GradeBookExample.java

GradeBook.java


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Summary

! Understanding the characteristics of 1-D and 2-D arrays

! Knowing how to declare, create, and initialize 1-D and 2-

D arrays

! Knowing how to process 1-D and 2-D arrays

! Knowing how to pass arrays to methods and return

arrays from methods

! Understand the consequences of array assignment

Documents

Lec09 Arraysasd