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Chapter 6 Arrays
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
1
Objectives















To describe why arrays are necessary in programming (§6.1).
To declare array reference variables and create arrays (§§6.2.1-6.2.2).
To initialize the values in an array (§6.2.3).
To access array elements using indexed variables (§6.2.4).
To declare, create, and initialize an array using an array initializer (§6.2.5).
To program common array operations (displaying arrays, summing all elements,
finding min and max elements, random shuffling, shifting elements) (§6.2.6).
To simplify programming using the for-each loops (§6.2.7).
To apply arrays in the LottoNumbers and DeckOfCards problems (§§6.3-6.4).
To copy contents from one array to another (§6.5).
To develop and invoke methods with array arguments and return value (§6.6–6.7).
To define a method with variable-length argument list (§6.8).
To search elements using the linear (§6.9.1) or binary (§6.9.2) search algorithm.
To sort an array using the selection sort (§6.10.1)
To sort an array using the insertion sort algorithm (§6.10.2).
To use the methods in the Arrays class (§6.11).
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
2
Opening Problem
Read one ten numbers, compute their average,
and find out how many numbers are above the
average. How would you compute this
problem?
An efficient and organized manner is to use
data structure, the array, which stores a fixed
sequential collection of elements of the same
type. By store all 10 numbers into an array and
access them through single array variable.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
3
Solution
public class AnalyzeNumbers {
public static void main(String[] args) {
final int NUMBER_OF_ELEMENTS = 10;
double[] numbers = new double[NUMBER_OF_ELEMENTS]; // Creates & Declare a array of 10 elements
double sum = 0;
java.util.Scanner input = new java.util.Scanner(System.in);
for (int i = 0; i < NUMBER_OF_ELEMENTS; i++) {
System.out.print("Enter a new number: ");
numbers[i] = input.nextDouble(); // store numbers into the array
sum += numbers[i]; //add each number to sum
}
double average = sum / NUMBER_OF_ELEMENTS; //obtain the average
int count = 0; // The number of elements above average
for (int i = 0; i < NUMBER_OF_ELEMENTS; i++)
// compares each number in the array
if (numbers[i] > average)
// compares each number in the array
count++;
System.out.println("Average is " + average);
System.out.println("Number of elements above the average "
+ count);
}
}
Run with prepared input
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
4
Introducing Arrays
Array is a data structure that represents a collection of
the same types of data.
double[] myList = new double[10];
myList
reference
Array reference
variable
Array element at
index 5
myList[0]
5.6
myList[1]
4.5
myList[2]
3.3
myList[3]
13.2
myList[4]
4
myList[5]
34.33
myList[6]
34
myList[7]
45.45
myList[8]
99.993
myList[9]
11123
Element value
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
5
Declaring Array Variables

To use an array in a program, you must declare a
variable to reference that array and specify the
array’s element type.
datatype[] arrayRefVar;
Example for declaring an array variable:
double[] myList;
datatype arrayRefVar[]; // This style is
allowed, but not preferred
Example:
double myList[];
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
6
Creating Arrays
The declaration of an array variables does not allocate
any space in memory for the array, it creates a storage
location for the reference to an array.
arrayRefVar = new datatype[arraySize];
Example:
myList = new double[10];
myList[0] //references the first element in the array.
myList[9] //references the last element in the array.
Note: if a variable does not contain references to an
array, then the value of the variables is null
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
7
Declaring and Creating
in One Step

datatype[]arrayRefVar = new
datatype[arraySize];
double[] myList = new double[10];
//declare array name myList as double

datatype arrayRefVar[] = new
datatype[arraySize];//create and
create Arrays
double myList[] = new double[10];
//declare and create array called myList[]
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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8
The Length of an Array
Once an array is created, its size is fixed. It cannot be
changed. You can find its size using
arrayRefVar.length
For example,
myList.length returns 10
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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9
Array size & Default Values
When the space of an array allocated, the array
size must be defined the number of elements that
can be stored in it.
When an array is created, its elements are
assigned the default value of
0 for the numeric primitive data types,
'\u0000' for char types, and
false for boolean types.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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10
Arrays
 An
array is an ordered list of values:
Each value has a numeric index
The entire array
has a single name
0
scores
1
2
3
4
5
6
7
8
9
79 87 94 82 67 98 87 81 74 91
An array of size N is indexed from zero to N-1
This array holds 10 values that are indexed from 0 to 9
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
Copyright © 2012
Pearson Education, Inc.
rights reserved. 0132130807
Arrays
A
particular value in an array is referenced using
the array name followed by the index in brackets
 For
example, the expression
scores[2]
refers to the value 94 (the 3rd value in the array)
 That
expression represents a place to store a
single integer and can be used wherever an integer
variable can be used
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
Copyright © 2012
Pearson Education, Inc.
rights reserved. 0132130807
Arrays
 For
example, an array element can be assigned a
value, printed, or used in a calculation:
scores[2] = 89;
scores[first] = scores[first] + 2;
mean = (scores[0] + scores[1])/2;
System.out.println ("Top = " +
scores[5]);
pick = scores[rand.nextInt(11)];
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
Copyright © 2012
Pearson Education, Inc.
rights reserved. 0132130807
Arrays
 The
values held in an array are called array
elements
array stores multiple values of the same type –
the element type
 An
 The
element type can be a primitive type or an
object reference
 Therefore,
we can create an array of integers, an
array of characters, an array of String objects,
an array of Coin objects, etc.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
Copyright © 2012
Pearson Education, Inc.
rights reserved. 0132130807
Arrays
 In
Java, the array itself is an object that must be
instantiated
 Another way to depict the scores array:
scores
The name of the array
is an object reference
variable
79
87
94
82
67
98
87
81
74
91
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
Copyright © 2012
Pearson Education, Inc.
rights reserved. 0132130807
Indexed Variables
The array elements are accessed through the index.
The array indices are 0-based, i.e., it starts from 0 to
arrayRefVar.length-1. In the example in Figure 6.1,
myList holds ten double values and the indices are
from 0 to 9.
Each element in the array is represented using the
following syntax, known as an indexed variable:
arrayRefVar[index];
myList[9] represents the last element in the array myList.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
16
Using Indexed Variables
After an array is created, an indexed variable can
be used in the same way as a regular variable.
For example, the following code adds the value
in myList[0] and myList[1] to myList[2].
myList[2] = myList[0] + myList[1];
Loop assigns 0 to myList[0], 1 to myList[1],… and 9 to myList[9]
For (int i=0; i< myList.lenght; i++) {
myList[i] = 1;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
17
Array Initializers
double[] myList; //declare
myList = new double[4]; //create
Declaring, creating, initializing in one step:
double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand syntax must be in one
statement.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
18
Declaring, creating, initializing
Using the Shorthand Notation
double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand notation is equivalent to the
following statements:
double[] myList = new double[4];
myList[0] = 1.9;
myList[1] = 2.9;
myList[2] = 3.4;
myList[3] = 3.5;
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
19
CAUTION
Using the shorthand notation, you
have to declare, create, and initialize
the array all in one statement.
Splitting it would cause a syntax
error. For example, the following is
wrong:
double[] myList;
myList = {1.9, 2.9, 3.4, 3.5};
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
20
//read 10 numbers;
//computer their average;
// find out how many numbers are above the average.
public class AnalyzeNumbers {
public static void main(String[] args) {
final int NUMBER_OF_ELEMENTS = 10;
double[] numbers = new double[NUMBER_OF_ELEMENTS]; // this line creates an array of 10
elements
double sum = 0;
java.util.Scanner input = new java.util.Scanner(System.in);
for (int i = 0; i < NUMBER_OF_ELEMENTS; i++) {
System.out.print("Enter a new number: ");
numbers[i] = input.nextDouble(); //store numbers into the array
sum += numbers[i]; // add each number to sum
}
double average = sum / NUMBER_OF_ELEMENTS; // obtain the average
int count = 0; // The number of elements above average
for (int i = 0; i < NUMBER_OF_ELEMENTS; i++) // then compares each number in the array with
average
if (numbers[i] > average) // to count the number if values above the average
count++;
System.out.println("Average is " + average);
System.out.println("Number of elements above the average "
+ count);
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
}
21
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animation
Trace Program with Arrays
Declare array variable values, create an
array, and assign its reference to values
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i= 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the array is created
0
0
1
0
2
0
3
0
4
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
22
animation
Trace Program with Arrays
i becomes 1
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the array is created
0
0
1
0
2
0
3
0
4
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
23
animation
Trace Program with Arrays
i (=1) is less than 5
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the array is created
0
0
1
0
2
0
3
0
4
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
24
animation
Trace Program with Arrays
After this line is executed, value[1] is 1
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the first iteration
0
0
1
1
2
0
3
0
4
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
25
animation
Trace Program with Arrays
After i++, i becomes 2
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the first iteration
0
0
1
1
2
0
3
0
4
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
26
animation
Trace Program with Arrays
public class Test {
public static void main(String[]
args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] +
values[4];
}
}
i (= 2) is less than 5
After the first iteration
0
0
1
1
2
0
3
0
4
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
27
animation
Trace Program with Arrays
After this line is executed,
values[2] is 3 (2 + 1)
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the second iteration
0
0
1
1
2
3
3
0
4
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
28
animation
Trace Program with Arrays
After this, i becomes 3.
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the second iteration
0
0
1
1
2
3
3
0
4
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
29
animation
Trace Program with Arrays
i (=3) is still less than 5.
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the second iteration
0
0
1
1
2
3
3
0
4
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
30
animation
Trace Program with Arrays
After this line, values[3] becomes 6 (3 + 3)
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the third iteration
0
0
1
1
2
3
3
6
4
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
31
animation
Trace Program with Arrays
After this, i becomes 4
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the third iteration
0
0
1
1
2
3
3
6
4
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
32
animation
Trace Program with Arrays
i (=4) is still less than 5
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the third iteration
0
0
1
1
2
3
3
6
4
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
33
animation
Trace Program with Arrays
After this, values[4] becomes 10 (4 + 6)
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the fourth iteration
0
0
1
1
2
3
3
6
4
10
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
34
animation
Trace Program with Arrays
After i++, i becomes 5
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the fourth iteration
0
0
1
1
2
3
3
6
4
10
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
35
animation
Trace Program with Arrays
i ( =5) < 5 is false. Exit the loop
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After the fourth iteration
0
0
1
1
2
3
3
6
4
10
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
36
animation
Trace Program with Arrays
After this line, values[0] is 11 (1 + 10)
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
0
11
1
1
2
3
3
6
4
10
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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37
Processing Arrays
See the examples in the text.
1.
(Initializing arrays with input values)
2.
(Initializing arrays with random values)
3.
(Printing arrays)
4.
(Summing all elements)
5.
(Finding the largest element)
6.
(Finding the smallest index of the largest element)
7.
(Random shuffling)
8.
(Shifting elements)
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
38
Initializing arrays with input values
Java has a shorthand notation, known as the array initializer, which
combines in one statement declaring an array.
elementType[] arrayRefVar = [value0,value1, …, valuek};
For example:
double [] myList = {1.9, 2.9, 3.4, 3.5};
The statement below declares, creates, and initializing the array
myList with four elements:
double[] myList = new double[4];
myList[0] = 1.9;
myList[1] = 2.9;
myList[2] = 3.4;
myList[3] = 3.5;
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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39
Printing arrays
Display arrays, you need to print each element in
the array using a loop like following:
for (int i = 0; i < myList.length; i++) {
System.out.print(myList[i] + " ");
}
For string:
char[] city = {‘D’, ‘a’, ‘l’, ‘l’, ‘a’, ‘s’};
System.out.println(city);
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
40
Summing all elements
By using a variable named total to store the sum, initially total is
0. add each element in the array to total using a loop like below:
double total = 0;
for (int i = 0; i < myList.length; i++) {
total += myList[i];
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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41
Finding the largest element
Use a variable named max to store the largest element. Initially max
is myList[0]. To find the largest element in the array myList,
compare each element with max, and update max if the element is
greater than max
double max = myList[0];
for (int i = 1; i < myList.length; i++) {
if (myList[i] > max) max = myList[i];
}
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42
Random shuffling
In many applicaition, you need to randomly reorder
the elements in an array. This is called a shuffling.
Therefore, for each element myList[i], randomly
generate an index j and swap myList[i] as follow:
for (int i = 0; i < myList.length; i++) {
// Generate an index j randomly
int index = (int)(Math.random()
* myList.length);
// Swap myList[i] with myList[j]
double temp = myList[i];
myList[i] = myList[index];
myList[index] = temp;
}
myList
i
[0]
[1]
.
.
.
swap
[index]
A random index
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43
Shifting Elements
Sometimes you need to shift the elements left or right. Example to
shift the elements one position to the left and fill the last element
with first element:
double temp = myList[0]; // Retain the first element
// Shift elements left
for (int i = 1; i < myList.length; i++) {
myList[i - 1] = myList[i];
}
myList
// Move the first element to fill in the last position
myList[myList.length - 1] = temp;
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Enhanced for Loop (for-each loop)
JDK 1.5 introduced a new for loop that enables you to traverse the complete
array sequentially without using an index variable. For example, the following
code displays all elements in the array myList:
for (double value: myList)
System.out.println(value);
In general, the syntax is
for (elementType value: arrayRefVar) {
// Process the value
}
Note: You still have to use an index variable if you wish to traverse the array in a
different order or change the elements in the array.
for (int i = 0; i < list.length; i++)
System.out.print(list[i] + “ “);
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45
Enhanced For Loop

class EnhancedForTest{
4 public static void main(String[] args)
5 {
6 int[] arrayList = {1, 12, 3, 4, 15, 6, 67};
8 int total = 0;
9
10 // add each element's value to total

11 for (int number : arrayList )
12
total += number;

13

//line List 11 & 12 equivalent to the following counter controlled repetition used in
// line 14 & 15
14 //for (int counter =0; counter <arrayList.length; counter++)
15 // total += array[counter];
16
17 System.out.printf("Total of array elements: %d\n" , total);
18 }
19 }
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Problem: Lotto Numbers
Suppose you play the Pick-10 lotto. Each ticket has
10 unique numbers ranging from 1 to 99. You buy
a lot of tickets. You like to have your tickets to
cover all numbers from 1 to 99. Write a program
that reads the ticket numbers from a file and checks
whether all numbers are covered. Assume the last
number in the file is 0.
Lotto Numbers Sample Data
LottoNumbers
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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47
import java.util.Scanner;
public class LottoNumbers {
public static void main(String args[]) {
Scanner input = new Scanner(System.in);
boolean[] isCovered = new boolean[99]; // default false -- create and initialize array
// Read all numbers and mark corresponding element covered -- read number
int number = input.nextInt(); // read the number
while (number != 0) {
isCovered[number - 1] = true; // mark number covered – “true”
number = input.nextInt();
// read the number
}
// Check if all covered
boolean allCovered = true; // Assume all covered
for (int i = 0; i < 99; i++)
if (!isCovered[i]) {
allCovered = false;
// Find one number is not covered “false”
break;
}
// Display result
if (allCovered)
// check allCovered?
System.out.println("The tickets cover all numbers");
else
System.out.println("The tickets don’t cover all numbers");
}
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
48
Problem: Deck of Cards
The problem is to write a program that picks four cards
randomly from a deck of 52 cards. All the cards can be
represented using an array named deck, filled with initial
values 0 to 52, as follows:
int[] deck = new int[52]; //declared and create cards
// initialize cards
for (int i = 0; i < deck.length; i++)
deck[i] = i;
DeckOfCards
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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49
public class DeckOfCards {
public static void main(String[] args) {
int[] deck = new int[52];
String[] suits = {"Spades", "Hearts", "Clubs", "Diamonds"}; // create array deck – array of string
String[] ranks = {"Ace", "2", "3", "4", "5", "6", "7", "8", "9", // array of strings
"10", "Jack", "Queen", "King"};
// Initialize deck
for (int i = 0; i < deck.length; i++)
deck[i] = i;
// Shuffle the deck
for (int i = 0; i < deck.length; i++) {
// Generate an index randomly
int index = (int)(Math.random() * deck.length);
int temp = deck[i];
deck[i] = deck[index];
deck[index] = temp;
}
// Display the first four cards
for (int i = 0; i < 4; i++) {
String suit = suits[deck[i] / 13];
// suite of card
String rank = ranks[deck[i] % 13]; // rank of a card
System.out.println("Card number " + deck[i] + ": "
+ rank + " of " + suit);
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
}
rights reserved. 0132130807
}
50
Problem: Deck of Cards
This problem builds a foundation for future more interesting and
realistic applications:
See Exercise 25.9.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
51
Problem: Deck of Cards, cont.
0
.
.
.
12
13
.
.
.
25
26
.
.
.
38
39
.
.
.
51
13 Spades (? )
13 Hearts (? )
13 Diamonds (? )
13 Clubs (? )
deck
[0] 0
.
.
.
.
.
.
[12] 12
[13] 13
.
.
.
.
.
.
[25] 25
[26] 26
.
.
.
.
.
.
[38] 38
[39] 39
.
.
.
.
.
.
[51] 51
Random shuffle
GUI Demo (picking four cards)
deck
[0] 6
[1] 48
[2] 11
[3] 24
[4] .
[5] .
.
.
.
.
.
.
[25] .
[26] .
.
.
.
.
.
.
[38] .
[39] .
.
.
.
.
.
.
[51] .
Card number 6 is
7 of Spades
Card number 48 is
10 of Clubs
Card number 11 is
Queen of Spades
Card number 24 is
Queen of Hearts
DeckOfCards
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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52
Copying Arrays
Often, in a program, you need to duplicate an array or a part of an
array. In such cases you could attempt to use the assignment
statement (=), as follows:
list2 = list1;
Before the assignment
list2 = list1;
list1
Note: You can copy primitive data
Type in Java, but you cannot copy
one array variable to another array
After the assignment
list2 = list1;
Contents
of list1
list2
list1
Contents
of list1
list2
Contents
of list2
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Garbage
Contents
of list2
53
Copying Arrays
Three ways to copy arrays
 Use a loop to copy individual elements one by one.
 Use the static araycopy method in the System class.
 Use the clone method to copy arrays;
Using a loop:
int[] sourceArray = {2, 3, 1, 5, 10};
// initial array
int[] targetArray = new int[sourceArray.length];
for (int i = 0; i < sourceArrays.length; i++)
targetArray[i] = sourceArray[i];
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
54
Passing Arrays to Methods
public static void printArray(int[] array) {
for (int i = 0; i < array.length; i++) {
System.out.print(array[i] + " ");
}
}
//Invoke the method
int[] list = {3, 1, 2, 6, 4, 2};
printArray(list);
//initialize array
//Invoke the method
printArray(new int[]{3, 1, 2, 6, 4, 2});
Anonymous array
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
55
Anonymous Array
The statement
printArray(new int[]{3, 1, 2, 6, 4, 2});
creates an array using the following syntax:
new dataType[]{literal0, literal1, ..., literalk};
There is no explicit reference variable for the array.
Such array is called an anonymous array.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
56
1 public class TestPassArray {
2 /** Main method */
3 public static void main(String[] args) {
4 int[] a = {1, 2};
5 // Swap elements using the swap method
6 System.out.println("Before invoking swap");
7 System.out.println("array is {" + a[0] + ", " + a[1] + "}");
8 swap(a[0], a[1]);
9 System.out.println("After invoking swap");
10 System.out.println("array is {" + a[0] + ", " + a[1] + "}");
11 // Swap elements using the swapFirstTwoInArray method
12 System.out.println("Before invoking swapFirstTwoInArray");
13 System.out.println("array is {" + a[0] + ", " + a[1] + "}");
14 swapFirstTwoInArray(a);
15 System.out.println("After invoking swapFirstTwoInArray");
16 System.out.println("array is {" + a[0] + ", " + a[1] + "}");
17 }
18 /** Swap two variables */
19 public static void swap(int n1, int n2) {
20 int temp = n1;
21 n1 = n2;
22 n2 = temp;
23 }
24 /** Swap the first two elements in the array */
25 public static void swapFirstTwoInArray(int[] array) {
26 int temp = array[0];
27 array[0] = array[1];
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
28 array[1] = temp;
57
Pass By Value – Method Call
Java uses pass by value to pass arguments to a method. There
are important differences between passing a value of variables
of primitive data types and passing arrays.
For a parameter of a primitive type value, the actual value is
passed. Changing the value of the local parameter inside the
method does not affect the value of the variable outside the
method.

For a parameter of an array type, the value of the parameter
contains a reference to an array; this reference is passed to the
method. Any changes to the array that occur inside the method
body will affect the original array that was passed as the
argument.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
58
Simple Example
public class Test {
public static void main(String[] args) {
int x = 1; // x represents an int value
int[] y = new int[10]; // y represents an array of int
values
m(x, y); // Invoke m with arguments x and y
System.out.println("x is " + x);
System.out.println("y[0] is " + y[0]);
}
public static void m(int number, int[] numbers) {
number = 1001; // Assign a new value to number
numbers[0] = 5555; // Assign a new value to numbers[0]
}
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
59
Call Stack
Stack
Heap
Space required for
method m
int[] numbers:reference
int number: 1
Space required for the
main method
int[] y: reference
int x: 1
0
0
The arrays are
stored in a
heap.
0
Array of
ten int
values is
When invoking m(x, y), the values of x and y are
passed to number and numbers. Since y contains the
reference value to the array, numbers now contains
the same reference value to the same array.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
60
Call Stack
Stack
Heap
Space required for
method m
int[] numbers:reference
int number: 1001
Space required for the
main method
int[] y: reference
int x: 1
The arrays are
stored in a
heap.
5555
0
0
Array of ten int
values is stored here
When invoking m(x, y), the values of x and y are
passed to number and numbers. Since y contains the
reference value to the array, numbers now contains
the same reference value to the same array.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
61
Heap
Heap
Space required for the
main method
int[] y: reference
int x: 1
5555
0
The arrays are
stored in a
heap.
0
The JVM stores the array in an area of memory,
called heap, which is used for dynamic memory
allocation where blocks of memory are allocated and
freed in an arbitrary order. Web Link on Stack and Heap
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
62
Passing Arrays as Arguments
 Objective:
Demonstrate differences of
passing primitive data type variables
and array variables.
TestPassArray
Run
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
63
Example, cont.
Stack
Heap
Space required for the
swap method
n2: 2
n1: 1
Space required for the
main method
int[] a reference
Invoke swap(int n1, int n2).
The primitive type values in
a[0] and a[1] are passed to the
swap method.
a[1]: 2
a[0]: 1
The arrays are
stored in a
heap.
Stack
Space required for the
swapFirstTwoInArray
method
int[] array reference
Space required for the
main method
int[] a reference
Invoke swapFirstTwoInArray(int[] array).
The reference value in a is passed to the
swapFirstTwoInArray method.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
64
Returning an Array from a Method
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
list
return result;
}
result
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
65
animation
Trace the reverse Method
int[] list1 = {1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
Declare result and create array
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
0
0
0
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
66
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i = 0 and j = 5
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
0
0
0
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
67
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i (= 0) is less than 6
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
0
0
0
0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
68
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i = 0 and j = 5
Assign list[0] to result[5]
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
0
0
0
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
69
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
After this, i becomes 1 and j
becomes 4
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
0
0
0
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
70
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i (=1) is less than 6
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
0
0
0
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
71
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i = 1 and j = 4
Assign list[1] to result[4]
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
0
0
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
72
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
After this, i becomes 2 and
j becomes 3
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
0
0
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
73
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i (=2) is still less than 6
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
0
0
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
74
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i = 2 and j = 3
Assign list[i] to result[j]
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
0
3
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
75
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
After this, i becomes 3 and
j becomes 2
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
0
3
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
76
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i (=3) is still less than 6
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
0
3
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
77
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i = 3 and j = 2
Assign list[i] to result[j]
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
4
3
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
78
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
After this, i becomes 4 and
j becomes 1
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
4
3
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
79
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i (=4) is still less than 6
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
0
4
3
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
80
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i = 4 and j = 1
Assign list[i] to result[j]
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
5
4
3
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
81
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
After this, i becomes 5 and
j becomes 0
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
5
4
3
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
82
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i (=5) is still less than 6
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
0
5
4
3
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
83
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i = 5 and j = 0
Assign list[i] to result[j]
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
6
5
4
3
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
84
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
After this, i becomes 6 and
j becomes -1
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
6
5
4
3
2
1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
85
animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
i (=6) < 6 is false. So exit
the loop.
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
result
1
2
3
4
5
6
6
5
4
3
2
1
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animation
Trace the reverse Method, cont.
int[] list1 = new int[]{1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
Return result
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
list
1
2
3
4
5
6
6
5
4
3
2
1
list2
result
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Problem: Counting Occurrence of Each
Letter


Generate 100 lowercase
letters randomly and assign
to an array of characters.
Count the occurrence of each
letter in the array.
CountLettersInArray
(a) Executing
createArray in Line 6
(b) After exiting
createArray in Line 6
Stack
Heap
Space required for the
createArray method
char[] chars: ref
Array of 100
characters
Space required for the
main method
char[] chars: ref
Stack
Heap
Array of 100
characters
Space required for the
main method
char[] chars: ref
Run
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88
Searching Arrays
Searching is the process of looking for a specific element in
an array; for example, discovering whether a certain score is
included in a list of scores. Searching is a common task in
computer programming. There are many algorithms and
data structures devoted to searching.
In this section, two commonly used approaches are
discussed, linear search and binary search.
public class LinearSearch {
/** The method for finding a key in the list */
public static int linearSearch(int[] list, int key) {
for (int i = 0; i < list.length; i++)
[0] [1] [2] …
if (key == list[i])
return i;
list
return -1;
key Compare key with list[i] for i = 0, 1, …
}
}
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89
Linear Search
The linear search approach compares the key
element, key, sequentially with each element in
the array list.
The method continues to do so until the key
matches an element in the list or the list is
exhausted without a match being found.
If a match is made, the linear search returns the
index of the element in the array that matches the
key. If no match is found, the search returns -1.
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90
animation
Linear Search Animation
Key
List
3
6
4
1
9
7
3
2
8
3
6
4
1
9
7
3
2
8
3
6
4
1
9
7
3
2
8
3
6
4
1
9
7
3
2
8
3
6
4
1
9
7
3
2
8
3
6
4
1
9
7
3
2
8
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91
From Idea to Solution
/** The method for finding a key in the list */
public static int linearSearch(int[] list, int key) {
for (int i = 0; i < list.length; i++)
if (key == list[i])
return i;
return -1;
}
Trace the method
int[]
int i
int j
int k
list = {1, 4, 4, 2, 5, -3, 6, 2};
= linearSearch(list, 4); //returns 1 located at[1]
= linearSearch(list, -4); //returns -1 No match
= linearSearch(list, -3); //returns 5 located at[5]
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Binary Search
For binary search to work, the elements in the
array must already be ordered. Without loss of
generality, assume that the array is in ascending
order.
e.g., 2 4 7 10 11 45 50 59 60 66 69 70 79
The binary search first compares the key with the
element in the middle of the array.
Video Binary Search Simulation
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93
Binary Search, cont.
Consider the following three cases:



If the key is less than the middle element,
you only need to search the key in the first
half of the array.
If the key is equal to the middle element,
the search ends with a match.
If the key is greater than the middle
element, you only need to search the key in
the second half of the array.
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94
animation
Binary Search
Key
List
8
1
2
3
4
6
7
8
9
8
1
2
3
4
6
7
8
9
8
1
2
3
4
6
7
8
9
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95
Binary Search, cont.
key is 11
low
key < 50
[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
list
2
low
mid
high
4 7 10 11 45 50 59 60 66 69 70 79
mid
high
[0] [1] [2] [3] [4] [5]
list 2 4 7 10 11 45
key > 7
low
mid
high
[3] [4] [5]
key == 11
list
10 11 45
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96
key is 54
low
Binary
Search,midcont.
[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
key > 50
list
2
4 7 10 11 45
50 59 60 66 69 70 79
low
key < 66
high
mid
high
[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
list
59 60 66 69 70 79
low mid
high
[7] [8]
key < 59
list
59 60
low
high
[6] [7] [8]
59 60
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Binary Search, cont.
The binarySearch method returns the index of the
element in the list that matches the search key if it
is contained in the list. Otherwise, it returns
-insertion point - 1.
The insertion point is the point at which the key
would be inserted into the list.
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From Idea to Solution
/** Use binary search to find the key in the list */
public static int binarySearch(int[] list, int key) {
int low = 0;
int high = list.length - 1;
while (high >= low) {
int mid = (low + high) / 2;
if (key < list[mid])
high = mid - 1;
else if (key == list[mid])
return mid;
else
low = mid + 1;
}
return -1 - low;
}
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99
The Arrays.binarySearch Method
Since binary search is frequently used in programming, Java provides several
overloaded binarySearch methods for searching a key in an array of int, double,
char, short, long, and float in the java.util.Arrays class. For example, the
following code searches the keys in an array of numbers and an array of
characters.
int[] list = {2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79};
System.out.println("Index is " +
java.util.Arrays.binarySearch(list, 11));
Return is 4
char[] chars = {'a', 'c', 'g', 'x', 'y', 'z'};
System.out.println("Index is " +
java.util.Arrays.binarySearch(chars, 't'));
Return is –4 (insertion point is
3, so return is -3-1)
For the binarySearch method to work, the array must be pre-sorted in increasing
order.
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100
Sorting Arrays
Sorting, like searching, is also a common task in
computer programming. Many different algorithms
have been developed for sorting. This section
introduces two simple, intuitive sorting algorithms:
selection sort and insertion sort.
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101
Selection Sort
Selection sort finds the largest number in the list and places it last. It then finds the largest
number remaining and places it next to last, and so on until the list contains only a single
number. Figure 6.17 shows how to sort the list {2, 9, 5, 4, 8, 1, 6} using selection sort.
swap
Select 1 (the smallest) and swap it
with 2 (the first) in the list
2
9
5
4
8
1
6
8
2
6
The number 1 is now in the
correct position and thus no
longer needs to be considered.
swap
Select 2 (the smallest) and swap it
with 9 (the first) in the remaining
list
1
Select 4 (the smallest) and swap it
with 5 (the first) in the remaining
list
1
2
5
4
8
9
6
The number 2 is now in the
correct position and thus no
longer needs to be considered.
5 is the smallest and in the right
position. No swap is necessary
1
2
4
5
8
9
6
The number 6 is now in the
correct position and thus no
longer needs to be considered.
Select 6 (the smallest) and swap it
with 8 (the first) in the remaining
list
1
6
The number 5 is now in the
correct position and thus no
longer needs to be considered.
Select 8 (the smallest) and swap it
with 9 (the first) in the remaining
list
1
2
4
5
6
9
8
The number 6 is now in the
correct position and thus no
longer needs to be considered.
Since there is only one element
remaining in the list, sort is
completed
1
2
4
5
6
8
9
The number 8 is now in the
correct position and thus no
longer needs to be considered.
9
5
4
swap
swap
2
4
5
8
9
swap
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From Idea to Solution
for (int i = 0; i < list.length; i++)
{
select the smallest element in list[i..listSize-1];
swap the smallest with list[i], if necessary;
// list[i] is in its correct position.
// The next iteration apply on list[i..listSize-1]
}
list[0] list[1] list[2] list[3] ...
list[10]
list[0] list[1] list[2] list[3] ...
list[10]
list[0] list[1] list[2] list[3] ...
list[10]
list[0] list[1] list[2] list[3] ...
list[10]
list[0] list[1] list[2] list[3] ...
list[10]
...
list[0] list[1] list[2] list[3] ...
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list[10]
103
for (int i = 0; i < listSize; i++)
{
select the smallest element in list[i..listSize-1];
swap the smallest with list[i], if necessary;
// list[i] is in its correct position.
// The next iteration apply on list[i..listSize-1]
}
Expand
double currentMin = list[i];
int currentMinIndex = i;
for (int j = i; j < list.length; j++) {
if (currentMin > list[j]) {
currentMin = list[j];
currentMinIndex = j;
}
}
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104
for (int i = 0; i < listSize; i++)
{
select the smallest element in list[i..listSize-1];
swap the smallest with list[i], if necessary;
// list[i] is in its correct position.
// The next iteration apply on list[i..listSize-1]
}
Expand
double currentMin = list[i];
int currentMinIndex = i;
for (int j = i; j < list.length; j++) {
if (currentMin > list[j]) {
currentMin = list[j];
currentMinIndex = j;
}
}
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105
for (int i = 0; i < listSize; i++)
{
select the smallest element in list[i..listSize-1];
swap the smallest with list[i], if necessary;
// list[i] is in its correct position.
// The next iteration apply on list[i..listSize-1]
}
Expand
if (currentMinIndex != i) {
list[currentMinIndex] = list[i];
list[i] = currentMin;
}
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106
Wrap it in a Method
/** The method for sorting the numbers */
public static void selectionSort(double[] list) {
for (int i = 0; i < list.length; i++) {
// Find the minimum in the list[i..list.length-1]
double currentMin = list[i];
int currentMinIndex = i;
for (int j = i + 1; j < list.length; j++) {
if (currentMin > list[j]) {
currentMin = list[j];
currentMinIndex = j;
}
}
// Swap list[i] with list[currentMinIndex] if necessary;
if (currentMinIndex != i) {
list[currentMinIndex] = list[i];
list[i] = currentMin;
Invoke it
}
}
selectionSort(yourList)
}
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107
Insertion Sort
int[] myList = {2, 9, 5, 4, 8, 1, 6}; // Unsorted
The insertion sort
algorithm sorts a list
of values by
repeatedly inserting
an unsorted element
into a sorted sublist
until the whole list
is sorted.
Step 1: Initially, the sorted sublist contains the
first element in the list. Insert 9 to the sublist.
2
9
5
4
8
1
6
Step2: The sorted sublist is {2, 9}. Insert 5 to the
sublist.
2
9
5
4
8
1
6
Step 3: The sorted sublist is {2, 5, 9}. Insert 4 to
the sublist.
2
5
9
4
8
1
6
Step 4: The sorted sublist is {2, 4, 5, 9}. Insert 8
to the sublist.
2
4
5
9
8
1
6
Step 5: The sorted sublist is {2, 4, 5, 8, 9}. Insert
1 to the sublist.
2
4
5
8
9
1
6
Step 6: The sorted sublist is {1, 2, 4, 5, 8, 9}.
Insert 6 to the sublist.
1
2
4
5
8
9
6
Step 7: The entire list is now sorted
1
2
4
5
6
8
9
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108
animation
Insertion Sort
int[] myList = {2, 9, 5, 4, 8, 1, 6}; // Unsorted
2
2
2
1
9
5
4
2
5
9
4
4
5
4
8
5
8
8
9
6
1
1
1
8
6
2
9
5
4
8
1
6
2
4
5
9
8
1
6
1
2
4
5
8
9
6
6
6
9
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109
How to Insert?
The insertion sort
algorithm sorts a list
of values by
repeatedly inserting
an unsorted element
into a sorted sublist
until the whole list
is sorted.
[0] [1] [2] [3] [4] [5] [6]
list
2
5 9
4
Step 1: Save 4 to a temporary variable currentElement
[0] [1] [2] [3] [4] [5] [6]
list
2
5
9
Step 2: Move list[2] to list[3]
[0] [1] [2] [3] [4] [5] [6]
list
2
5 9
Step 3: Move list[1] to list[2]
[0] [1] [2] [3] [4] [5] [6]
list
2
4
5 9
Step 4: Assign currentElement to list[1]
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110
From Idea to Solution
for (int i = 1; 1; i < list,length; i++) {
insert list[i] into a sorted sublist list[0..i-1] so that
list[0..i] is sorted
}
list[0]
list[0] list[1]
list[0] list[1] list[2]
list[0] list[1] list[2] list[3]
list[0] list[1] list[2] list[3] ...
InsertSort
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111
The Arrays.sort Method
Since sorting is frequently used in programming, Java provides several
overloaded sort methods for sorting an array of int, double, char, short,
long, and float in the java.util.Arrays class. For example, the following
code sorts an array of numbers and an array of characters.
double[] numbers = {6.0, 4.4, 1.9, 2.9, 3.4, 3.5};
java.util.Arrays.sort(numbers);
char[] chars = {'a', 'A', '4', 'F', 'D', 'P'};
java.util.Arrays.sort(chars);
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112
JVA JDK
The Java Development Kit (JDK) is an implementation of either
one of the Java SE, Java EE or Java ME platforms[1] released by
Oracle Corporation in the form of a binary product aimed at Java
developers on Solaris, Linux, Mac OS X or Windows.[2] Since the
introduction of Java platform, it has been by far the most widely
used Software Development Kit (SDK).[citation needed] On 17
November 2006, Sun announced that it would be released under the
GNU General Public License (GPL), thus making it free software.
This happened in large part on 8 May 2007, when Sun contributed
the source code to the OpenJDK.[3]
JDK contents
The JDK has as its primary components a collection of
programming tools, including:
appletviewer – this tool can be used to run and debug Java applets
without a web browser
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