Chapter 9 – Arrays

Array Basics Array Declaration Array Creation Array Initializer Array Default Values Array length Property Partially Filled Arrays Copying an Array Histograms Searching an Array Sorting an Array Selection Sort Two-Dimensional Arrays Arrays of Objects For-Each Loop

1

Array Basics

A class stores a group of related data, and it stores the methods that operate on that data.

An array is a limited version of a class. Like a class, an array also stores a group of related data, but an array does

not store methods. Another difference between an array and a class is that an array's data must

all be of the same type. Here's a picture of an array that holds a list of phone numbers. Each of

the five boxes is called an array element and each box stores one phone number.

phoneList

8167412000

2024561111

7852963232

8008675309

0035318842133

first phone number

last phone number

2

Array Basics

A class uses dot notation to access one of its members. On the other hand, an array uses square brackets around an

index to access one of its elements. The rightmost column shows how to access each of the 5

elements in the phoneList array. Note that the index values start at 0 instead of 1 and the last

index value is one less than the number of elements in the array.

index phoneList

how to access each

element

0 8167412000 phoneList[0]

1 2024561111 phoneList[1]

2 7852963232 phoneList[2]

3 8008675309 phoneList[3]

4 0035318842133

phoneList[4]

5 elements

3

Array Basics

Here's how you can change the first phone number to 2013434:phoneList[0] = 2013434;

And here's how you can print the second phone number:System.out.println(phoneList[1]);

4

Array Basics

/************************************************************** ContactList.java* Dean & Dean** This program creates a cell phone contacts phone number* list and prints the created list.*************************************************************/

import java.util.Scanner;

public class ContactList{ public static void main(String[] args) { Scanner stdIn = new Scanner(System.in); long[] phoneList; // list of phone numbers int sizeOfList; // number of phone numbers long phoneNum; // an entered phone number

System.out.print( "How many contact numbers would you like to enter? "); sizeOfList = stdIn.nextInt(); phoneList = new long[sizeOfList];

5

Array Basics

for (int i=0; i<sizeOfList; i++) { System.out.print("Enter phone number: "); phoneNum = stdIn.nextLong(); phoneList[i] = phoneNum; } // end for

System.out.println("\nContacts List:"); for (int i=0; i<sizeOfList; i++) { System.out.println((i + 1) + ". " + phoneList[i]); } // end for } // end main} // end class ContactList

6

Array Declaration

An array is a variable and, as such, it must be declared before you can use it.

Array declaration syntax:<element-type>[] <array-variable>;

Array declaration examples:int[] ids;

double[] workHours;

String[] names;

8

Array Creation

In Java, arrays are objects. As with all objects, arrays are

created/instantiated with the new operator. Syntax for creating and assigning an array

object:<array-variable> = new <element-type>[<array-size>];

Example:long[] phoneList;

phoneList = new long[10];

<element-type> indicates the type of each element in the array

<array-size> indicates the number of elements in the array

array creation

9

Array Creation

It's legal to combine an array's declaration, creation, and assignment operations. Here's an example:long[] phoneList = new long[10];

Provide a single statement that declares, creates, and assigns a 100-element array that stores book titles.

10

Array Initializer

An array initializer is a single statement made up of an array declaration, creation, and {} assignment.

Array element initialization syntax:<element-type>[] <array-name> = {<element-values-list>};

Array element initialization example:String[] students = {"Eric", "Curtis", "John Robert"};

When an array initializer is used, the size of the array equals the number of elements in the initialization list.

Note that with an array initializer, you create an array object without using the new operator.

11

Array Default Values

An array is an object and an array's elements are the instance variables for an array object. As such, an array's elements get default values when the array is instantiated, the same as any other instance variables get default values.

Here are the default values for array elements (they're also the default values for instance variables and class variables):

Array element's type Default value

integer 0

floating point 0.0

boolean false

reference null

For example, what are the default values below?float[] gpas = new float[1000];

String[] states = new String[50];

12

Array length Property

Suppose you have a five-element colors array that's been initialized like this: String[] colors = {"blue", "gray", "lime", "teal", "yellow"};

Here's how to print such an array:for (int i=0; i<colors.length; i++){ System.out.println(colors[i]);}

To obtain an array's length, specify array name, dot, and then length.

Note that length is used in two different ways: length is a String method length is an array property

Mnemonic acronym to help you remember when to use parentheses with length:

ANSY (arrays no, strings yes)

Note how an array object's length property gets the array's size.

13

Array length Property and Partially Filled Arrays

import java.util.Scanner;

public class ContactList2{ public static void main(String[] args) { Scanner stdIn = new Scanner(System.in); String[] phoneList = new String[100]; // phone numbers int filledElements = 0; // number of phone numbers String phoneNum; // an entered phone number

System.out.print("Enter phone number (or q to quit): "); phoneNum = stdIn.nextLine(); while (!phoneNum.equalsIgnoreCase("q") && filledElements < phoneList.length) { if (phoneNum.length() < 1 || phoneNum.length() > 16) { System.out.println("Invalid entry." + " Must enter between 1 and 16 characters."); }

Array length property does not use ( )'s.

String length method uses ( )'s.

14

Array length Property and Partially Filled Arrays

else

{

phoneList[filledElements] = phoneNum;

filledElements++;

}

System.out.print("Enter phone number (or q to quit): ");

phoneNum = stdIn.nextLine();

} // end while

System.out.println("\nContact List:");

for (int i=0; i<filledElements; i++)

{

System.out.println((i + 1) + ". " + phoneList[i]);

} // end for

} // end main

} // end class ContactList2

15

Partially Filled Arrays

A partially filled array is when you use some of an array's elements, but not all.

If you have a partially filled array, you have to keep track of the number of filled elements in the array so you can process the filled elements differently from the non-filled elements.

In the ContactList2 program, note how the filledElements variable keeps track of the number of phone numbers in the array.

16

Copying an Array

As with all objects and their associated reference variables, if you assign one array reference variable to another array reference variable, both array reference variables then point to the same single array object.

What's the problem with that scenario? More specifically, what's wrong with the following code if the goal is to make a copy of arr1?arr2 = arr1;

17

Copying an Array

Usually when you make a copy of an array, you'll want the copy and the original to point to different array objects. To do that, assign array elements one at a time.

For example, suppose you use arrays to hold a store's prices, one array for each month's prices. And you'd like to copy January's price array into February's price array and make a change in February's second price. The program on the next slide does that by assigning array elements one at a time. Here's the program's output: Jan Feb 1.29 1.29 9.99 10.99 22.50 22.50 4.55 4.55 7.35 7.35 6.49 6.49

18

Copying an Array

public class ArrayCopy{ public static void main(String[] args) { double[] pricesJanuary = {1.29, 9.99, 22.50, 4.55, 7.35, 6.49}; double[] pricesFebruary = new double[pricesJanuary.length];

for (int i=0; i<pricesJanuary.length; i++) { pricesFebruary[i] = pricesJanuary[i]; } pricesFebruary[1] = 10.99;

System.out.printf("%7s%7s\n", "Jan", "Feb"); for (int i=0; i<pricesJanuary.length; i++) { System.out.printf("%7.2f%7.2f\n", pricesJanuary[i], pricesFebruary[i]); } } // end main} // end class ArrayCopy

19

Histograms

A histogram is a graph that displays quantities for a set of categories. It indicates category quantities with bars - shorter bars equate to smaller quantities, longer bars equate to larger quantities.

For example, the histogram on the next slide shows the average number of deaths by age 5 per 1000 live births.

20

Histograms21

Histograms

Problem Description: Suppose you have three coins. When you flip all three,

you're curious how likely it is you'll get zero heads, how likely it is you'll get one head, how likely it is you'll get two heads, and how likely it is you'll get three heads. In other words, you're curious about the frequency distribution for the number of heads.

Solution: Write a main method that simulates throwing the

three coins a million times. Print the simulation results in the form of a histogram:

For each of the four cases (zero heads, one head, etc.), print a bar that represents the number of times the case occurred.

To mimic a bar, print a certain number of *'s where each * represents 1% of the total number of simulation iterations.

22

Histograms

Output for the coin-flip program:Number of times each head count occurred:

0 124685 ************

1 374759 *************************************

2 375420 **************************************

3 125136 *************

Program implementation tips: Use a four-element frequency array to keep track of the

number of times each head-count value occurs. The frequency[0] element holds the number of times none of the three coins lands heads up. The frequency[1] element holds the number of times one of the three coins lands heads up.

Each element in the frequency array is called a "bin." After each simulation iteration, add 1 to the appropriate bin.

For example, increment the frequency[3] bin if all three of the coins land heads up.

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Histogramspublic class CoinFlips{ public static void main(String[] args) { final int NUM_OF_COINS = 3; // number of coins that are flipped final int NUM_OF_REPS = 1000000; // total number of repetitions

// The frequency array holds the number of times a particular number // of heads occurred. int[] frequency; int heads; // number of heads in the current group of flips float fractionOfReps; // Number of times a particular head count occurred // divided by total number of repetitions. int numOfAsterisks; // Number of asterisks in histogram for a // particular head count.

frequency = new int[NUM_OF_COINS + 1]; for (int rep=0; rep<NUM_OF_REPS; rep++) { // perform a group of flips heads = 0; for (int i=0; i<NUM_OF_COINS; i++) { heads += (int) (Math.random() * 2); } frequency[heads]++; // update appropriate bin } // end for

24

Histograms

heads frequency

1st group of flips0 0 0

1 1 0 1 2

2nd group of flips

0 2 0 1

1 3 0

2

3rd group of flips0

1

0···

25

Histograms

System.out.println( "Number of times each head count occurred:"); for (heads=0; heads<=NUM_OF_COINS; heads++) { System.out.print( " " + heads + " " + frequency[heads] + " "); fractionOfReps = (float) frequency[heads] / NUM_OF_REPS; numOfAsterisks = Math.round(fractionOfReps * 100);

for (int i=0; i<numOfAsterisks; i++) { System.out.print("*"); } System.out.println(); } // end for } // end main} // end class CoinFlips

26

Searching an array

Often, you'll need to determine whether an array contains a particular value. Here's the pseudocode algorithm for conducting a sequential search for a particular value within a list array:

i ← 0while i < number of filled elements{ if list[i] equals the searched-for value <do something and stop the loop> increment i}

27

Searching an array

Problem description: Write a helper method named findStudent that

searches for an id value within an array of student id's. The findStudent method should receive an id

parameter and return the index value of id's location within an ids array instance variable.

If id's value is not found, then return -1. As always, use appropriate access modifiers

(public vs. private, class method vs. instance method).

28

Sorting an Array

Sorting is a very common task in programming. Examples:

Sort emails in an inbox – by date, by sender Sort songs – by title, by author Sort student records – by student ID

30

There are many different sorting algorithms with varying degrees of complexity and efficiency. Since this is your first exposure to sorting, we'll cover a simple algorithm - the selection sort algorithm. Here it is:for (i0; i<list's_length; i++)

{ find the smallest value in list from list[i] down to the bottom of

the list swap the found value with list[i]}

5

10

-3

20

2

-3

10

5

20

2

list (original)

-3

2

5

20

10

-3

2

5

20

10

-3

2

5

10

20

list (sorted)

0

1

2

3

4

Selection Sort31

Sorting an Array

You could include a sorting method in every class that needs sorting functionality.

For example, you could include: A studentSort method in a Students class that sorts

students by student id. A bookSort method in a Books class that sorts books

by ISBN number. A productSort method in a Products class that sorts

products by product id. But suppose that you want to make a generic

sorting method that receives an array as a parameter (e.g., studentIds, bookISBNs, or productIds) and sorts it.

32

To make the sorting method generic so that it can be used by multiple classes, put the sort method in a utility class.

A utility class is a class with general-purpose methods that other classes can easily use. To make the methods easy to use, use class methods (as opposed to instance methods).

Why would it be easy for other classes to use the sort method if the sort method is implemented as a class method?

Sorting an Array33

If you make the sort method an instance method, then you'll be required to instantiate the sort method's enclosing class prior to calling the sort method. For example, assuming the sort method's enclosing class is named Sort:Sort s = new Sort();

s.sort(studentIds);

On the other hand, if you make the sort method a class method, then you are not required to instantiate the sort method's enclosing class prior to calling the sort method. Instead, you simply need to prefix the sort method call with the class name and then a dot. For example:Sort.sort(studentIds);

Thus, in the interest of simplicity and elegance, let's make the sort method a class method.

Sorting an Array34

Selection Sort

public class Sort{ public static void sort(int[] list) { int j; for (int i=0; i<list.length-1; i++) { j = indexOfNextSmallest(list, i); swap(list, i, j); } } // end sort

private static int indexOfNextSmallest(int[] list, int startIndex) { int min = list[startIndex]; int minIndex = startIndex;

for (int i=startIndex+1; i<list.length; i++) { if (list[i] < min) { min = list[i]; minIndex = i; } } // end for return minIndex; } // end indexOfNextSmallest

35

Selection Sort

private static void swap(int[] list, int i, int j) { int temp; temp = list[i]; list[i] = list[j]; list[j] = temp; } // end swap} // end Sort

public class SortDriver{ public static void main(String[] args) { int[] studentIds = {3333, 1234, 2222, 1000}; Sort.sort(studentIds);

for (int i=0; i<studentIds.length; i++) { System.out.print(studentIds[i] + " "); } } // end main} // end SortDriver

36

Two-Dimensional Arrays

If you have a group of related data that's organized in a table format, consider using a two-dimensional array.

Two-dimensional arrays use the same basic syntax as one-dimensional arrays except for a second pair of []'s.

The first index identifies the row and the second index identifies the column position within a row.

For example, here's a two-row by three-column array named x:

xhow to access each

element

0 1 2

0 8 -2 4 x[0][0] x[0][1]x[0][2]

1 1 0 5 x[1][0] x[1][1]x[1][2]

column indexes

row indexes

37

Two-Dimensional Arrays

As with one-dimensional arrays, there are two ways to assign values into a two-dimensional array’s elements: (1) an array initializer, and (2) assignment statements.

Here’s how you can declare the previous slide's x array and assign values into its elements, using an array initializer:int[][] x = {{8,-2,4}, {1,0,5}};

You can use the array initializer technique only if you know the assigned values when you first declare the array. Otherwise, you need to provide array element assignment statements that are separate from the array's declaration and creation.

initializer for a 2-row by 3-column array

38

Two-Dimensional Arrays

The following code fragment declares and creates the x array in one statement, and assigns values to x's elements in a separate statement (inside a nested loop).

int[][] x = new int[2][3];

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

{

for (int j=0; j<x[0].length; j++)

{

System.out.print(

"Enter value for row " + i + ", col " + j + ": ");

x[i][j] = stdIn.nextInt();

} // end for j

} // end for i

Assign a value to the element at row i column j.

Declare and create a 2-row by 3-column array.

39

Two-Dimensional Arrays

Bottom line: To loop through the rows in a two-dimensional array, use <array-name>.length . And to loop through the elements within a particular row, use <array-name>[0].length. For example:for (int i=0; i<x.length; i++){ for (int j=0; j<x[0].length; j++) { ...

40

Two-Dimensional Arrays

The upcoming FlightTimes program: Uses a two-dimensional array to store this table of

flight times between cities:

Wch Top KC Col StL

Wch 0 22 30 42 55

Top 23 0 9 25 37

KC 31 10 0 11 28

Col 44 27 12 0 12

StL 59 41 30 14 0

Contains a promptForFlightTime method that prompts the user for a departure city and a destination city and prints the flight time for that flight.

Contains a displayFlightTimesTable method that prints the table.

It takes 25 minutes to fly from Topeka, KS to Columbia, MO.

41

Two-Dimensional Arrays

public class FlightTimesDriver{ public static void main(String[] args) { int[][] flightTimes = { {0, 22, 30, 42, 55}, {23, 0, 9, 25, 37}, {31, 10, 0, 11, 28}, {44, 27, 12, 0, 12}, {59, 41, 30, 14, 0} }; // Define terminals in the Kansas-Missouri region. String[] cities = {"Wch", "Top", "KC", "Col", "StL"}; FlightTimes ft = new FlightTimes(flightTimes, cities);

System.out.println("\nFlight times for KansMo Airlines:\n"); ft.displayFlightTimesTable(); System.out.println(); ft.promptForFlightTime(); } // end main} // end class FlightTimesDriver

42

Two-Dimensional Arrays

import java.util.Scanner;

public class FlightTimes{ private int[][] flightTimes; // table of flight times private String[] cities; // names of cities in flightTimes table

public FlightTimes(int[][] ft, String[] c) { this.flightTimes = ft; this.cities = c; } //******************************************** // This method prompts the user for departure and destination cities // and prints the associated flight time. public void promptForFlightTime() { Scanner stdIn = new Scanner(System.in); int departure; // index for departure city int destination; // index for destination city

43

Two-Dimensional Arrays

for (int i=0; i<cities.length; i++) { System.out.println(i+1 + " = " + cities[i]); } System.out.print("Enter departure city's number: "); departure = stdIn.nextInt() - 1; System.out.print("Enter destination city's number: "); destination = stdIn.nextInt() - 1; System.out.println("Flight time = " + flightTimes[departure][destination] + " minutes."); } // end promptForFlightTime //******************************************** // This method prints a table of all flight times.

<Insert displayFlightTimesTable method here.>

} // end class FlightTimes

44

Arrays of Objects

Suppose you need to keep track of total sales for each sales clerk in a department store.

In the following clerks array, each array element holds a reference for a SalesClerk object.

Each SalesClerk object holds a sales clerk's name and a total-sales value for the sales clerk.

If sales clerk Abe sells two items for $55.45 and $22.01, then you'd like to store 77.46 for his total-sales value.

clerks

clerks[0]

clerks[1]

clerks[2]

clerks[3] null

Jordan, 6.25

Prashuv, 58.12

Abe, 77.46

46

Arrays of Objects

Using the input shown below, how would the clerks array get filled?

clerks

null

null

null

null

input filledElements

Jordan 0

6.25

Prashuv

58.12

Abe

40

Jordan

-6.25

Prashuv

12.88

47

Arrays of Objects

import java.util.Scanner;

public class SalesClerksDriver{ public static void main(String[] args) { Scanner stdIn = new Scanner(System.in); SalesClerks clerks = new SalesClerks(2); String name;

System.out.print("Enter clerk's name (q to quit): "); name = stdIn.nextLine(); while (!name.equals("q")) { System.out.print("Enter sale amount: "); clerks.addSale(name, stdIn.nextDouble()); stdIn.nextLine(); // flush newline System.out.print("Enter clerk's name (q to quit): "); name = stdIn.nextLine(); } // end while clerks.dumpData(); } // end main} // end SalesClerksDriver

48

Arrays of Objectsclass SalesClerks{ private SalesClerk[] clerks; // contains names and sales private int filledElements = 0; // number of elements filled

//***********************************************************

public SalesClerks(int initialSize) { clerks = new SalesClerk[initialSize]; } // end SalesClerks constructor

//***********************************************************

// Process a sale for the clerk whose name is passed in. // If the name is not already in the clerks array, // create a new object and insert a reference to it in the // next array element, doubling array length if necessary.

public void addSale(String name, double amount) { int clerkIndex = findClerk(name);

if (clerkIndex == -1) // add a new clerk { if (filledElements == clerks.length) { doubleLength(); }

49

Arrays of Objects

clerkIndex = filledElements; clerks[clerkIndex] = new SalesClerk(name); filledElements++; } // end if

clerks[clerkIndex].adjustSales(amount); } // end addSale

//**********************************************************

// Print all the data - sales clerk names and sales.

public void dumpData() { for (int i=0; i<filledElements; i++) { System.out.printf("%s: %6.2f\n", clerks[i].getName(), clerks[i].getSales()); } } // end dumpData

//**********************************************************

50

Arrays of Objects

// Search for the given name. If found, return the index. // Otherwise, return -1.

private int findClerk(String name) { for (int i=0; i<filledElements; i++) { if (clerks[i].getName().equals(name)) { return i; } } // end for return -1; } // end findClerk

//**********************************************************

// Double the length of the array. private void doubleLength() { SalesClerk[] clerks2 = new SalesClerk[2 * clerks.length]; System.arraycopy(clerks, 0, clerks2, 0, clerks.length); clerks = clerks2; } // end doubleLength} // end class SalesClerks

The arraycopy method copies the first argument's array (starting at the second argument's position) to the third argument's array (starting at the fourth argument's position). The fifth argument specifies the number of elements that are to be copied.

51

Arrays of Objectspublic class SalesClerk{ private String name; // sales clerk's name private double sales = 0.0; // total sales for clerk

//*******************************************************

public SalesClerk(String name) { this.name = name; }

//*******************************************************

public String getName() { return name; }

public double getSales() { return sales; }

//*******************************************************

// Adjust clerk's total sales by adding the passed-in sale.

public void adjustSales(double amount) { sales += amount; }} // end class SalesClerk

52

Arrays of Objects

Beware: To create an array of objects, you must instantiate the

array with the new operator, and you must also instantiate each object that's stored in the array with individual new operators.

For example, the SalesClerks constructor instantiates an array of SalesClerk objects like this:

clerks = new SalesClerk[maxClerks];

You might think that since the above instantiation specifies maxClerks number of SalesClerk objects, the JVM instantiates all the SalesClerk objects.

On the contrary, the JVM only instantiates an array object and each array element holds null.

To fill up the clerks array with SalesClerk objects, you need to instantiate the SalesClerk objects individually.

53

For-Each Loop

A for-each loop is an alternative to the traditional for loop.

It’s handy when you want to iterate through all the elements in an array and you don’t know or care where particular elements are located.

Syntax:

for (<element-type> <element-reference> : <array-reference>) { ... }

When you read this, say to yourself:

“For each <element-reference> in <array-reference>,

...”

54

For-Each Loop

Here's a for-each loop example that prints the numbers in a primes array:

int[] primes = {2, 3, 5, 7, 11, 13};

for (int p : primes)

{

System.out.println(p);

}

Say, “For each p in primes, print p.” Benefits of the for-each loop (over the standard for loop):

It reduces code clutter. It avoids the effort of index initialization (e.g., i=0), index

comparison (e.g., i<10), and index updating (e.g., i++). It simplifies element access within the loop body by providing

a simple name for the current element (e.g., p, not primes[i]).

55

For-Each Loop

Although the for-each loop's lack of an index variable leads to less cluttered code, the lack of an index variable can be a drawback if there's a need for an index value within the loop. For example, given the primes array in the earlier slide, which type of loop (standard or for-each) should you use to print the following?primes[0] = 2primes[1] = 3...primes[5] = 13

As a practice exercise, provide a standard for loop that prints the above, and also provide a for-each loop that prints the above.

56