The Art and Science of An Introduction to Computer Science ERIC S. ROBERTS Java Arrays and ArrayLists C H A P T E R 1 1 1 slides partially adapted from.

The Art and Science of

An Introductionto Computer ScienceERIC S. ROBERTS

Java

Arrays and ArrayLists

C H A P T E R 1 1

1

slides partially adapted from UW CSE 142 course slides

Lecture Slides, Part II -- Review

Comp 101 | Özyeğin University 2

A named, ordered collection of variables of identical type

• We do not name every individual element in the collection• Instead, we give a name to the whole collection• We access individual elements in the collection by specifying its order in the collection

– This is called the index

Recap: Arrays

Recap: Arrays

3

int score = 0;int[] scores = new int[10];

0 1 2 3 4 5 6 7 8

0 0 0 0 0 0 0 0 0 09

scores

0score

Element type

Length

index

Acessing Arrays

0 1 2 3 4 5 6 7 8

0 0 0 0 0 0 0 0 0 09

scores

int[] scores = new int[10];

0 1 2 3 4 5 6 7 8

25 0 0 0 0 0 0 0 0 09

scores

scores[0] = 25;

0 1 2 3 4 5 6 7 8

25 17 0 0 0 0 0 0 0 09

scores

scores[1] = 17;

4

Array Length

int[] scores = new int[10];println("Length of the array is " + scores.length);

Length of the array is 10

5

Exercise

CS 101 | Özyeğin University 6

• Guess the output of the following program

int[] numbers = new int[5];

for (int i = 0; i < numbers.length; i++) {numbers[i] = numbers.length - i;

}

for (int i = 0; i < numbers.length; i++) {print(numbers[i] + " ");

}

Exercise

CS 101 | Özyeğin University 7

• Guess the output of the following program

int[] numbers = new int[5];

for (int i = 0; i < numbers.length; i++) {numbers[i] = numbers.length - i;

}

for (int i = 0; i < numbers.length; i++) {print(numbers[i] + " ");

}

5 4 3 2 1

Exercise

CS 101 | Özyeğin University 8

• Write a program that provides a solution for the motivating example:

– Get a set of numbers from the user

– Print out the numbers that are above the average of the whole set of numbers

CS 101 | Özyeğin University 9

int noOfNumbers = readInt("Enter the total amount of numbers: ");

int[] numbers = new int[noOfNumbers];

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

numbers[i] = readInt("Enter a number: ");

}

int total = 0;

for (int i = 0; i < noOfNumbers; i++) {

total = total + numbers[i];

}

double average = (double) total / numbers.length;

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

if(numbers[i] > average) {

println(numbers[i] + " is above average!");

}

}

•An array is a collection of variables

•Each element can be used wherever a simple variable of that type is allowed.

–Assignment, expressions, input/output

•An entire array can’t be treated as a single variable–Can’t assign or compare arrays using =, <, …–Can’t use println to read or write an entire array–But, you can do these things one element at a time

Technicalities

Internal Representation of Arrays• Arrays in Java are implemented as objects, which means that

they are stored in the heap. The value stored in an array variable is simply a reference to the actual array.

int score = 0;int[] scores = new int[10];

0score

0 1 2 3 4 5 6 7 8

0 0 0 0 0 0 0 0 0 09

scores

11

Internal Representation of Arraysint score = 5;int foo = score;foo++;println(score); // prints 5println(foo); // prints 6

5score foo

12

Internal Representation of Arraysint score = 5;int foo = score;foo++;println(score); // prints 5println(foo); // prints 6

5score

5foo

13

Internal Representation of Arraysint score = 5;int foo = score;foo++;println(score); // prints 5println(foo); // prints 6

5score

6foo

14

Internal Representation of Arraysint score = 5;int foo = score;foo++;println(score); // prints 5println(foo); // prints 6

5score

6foo

15

Internal Representation of Arraysint[] scores = new int[10];int[] numbers = scores;numbers[5] = 42;println(scores[5]); // prints 42println(numbers[5]); // prints 42scores[0] = 77;println(scores[0]); // prints 77println(numbers[0]); // prints 77

0 1 2 3 4 5 6 7 8

0 0 0 0 0 0 0 0 0 09

scores

numbers

16

Internal Representation of Arrays

0 1 2 3 4 5 6 7 8

0 0 0 0 0 0 0 0 0 09

scores

numbers

int[] scores = new int[10];int[] numbers = scores;numbers[5] = 42;println(scores[5]); // prints 42println(numbers[5]); // prints 42scores[0] = 77;println(scores[0]); // prints 77println(numbers[0]); // prints 77

17

Internal Representation of Arrays

0 1 2 3 4 5 6 7 8

0 0 0 0 0 42 0 0 0 09

scores

numbers

int[] scores = new int[10];int[] numbers = scores;numbers[5] = 42;println(scores[5]); // prints 42println(numbers[5]); // prints 42scores[0] = 77;println(scores[0]); // prints 77println(numbers[0]); // prints 77

18

Internal Representation of Arrays

0 1 2 3 4 5 6 7 8

0 0 0 0 0 42 0 0 0 09

scores

numbers

int[] scores = new int[10];int[] numbers = scores;numbers[5] = 42;println(scores[5]); // prints 42println(numbers[5]); // prints 42scores[0] = 77;println(scores[0]); // prints 77println(numbers[0]); // prints 77

19

Internal Representation of Arrays

0 1 2 3 4 5 6 7 8

77 0 0 0 0 42 0 0 0 09

scores

numbers

int[] scores = new int[10];int[] numbers = scores;numbers[5] = 42;println(scores[5]); // prints 42println(numbers[5]); // prints 42scores[0] = 77;println(scores[0]); // prints 77println(numbers[0]); // prints 77

20

Internal Representation of Arrays

0 1 2 3 4 5 6 7 8

77 0 0 0 0 42 0 0 0 09

scores

numbers

int[] scores = new int[10];int[] numbers = scores;numbers[5] = 42;println(scores[5]); // prints 42println(numbers[5]); // prints 42scores[0] = 77;println(scores[0]); // prints 77println(numbers[0]); // prints 77

21

•Individual array elements can be used as parameters, just like other simple variables.

•The corresponding element in the array is copied to the argument

•Example:

int result = power(numbers[0], numbers[1]);println(numbers[0] + “ to the power ” + numbers[1] + “ is ” + result) ;

Array elements as arguments

•Array arguments (entire arrays) work differently:

–An array is never copied–The array name is always treated as a reference to the collection of elements

Whole arrays as arguments

Initializing Arrays• Java makes it easy to initialize the elements of an array as

part of a declaration. The syntax is

type[] name = { elements };

where elements is a list of the elements of the array separated by commas. The length of the array is automatically set to be the number of values in the list.

• For example, the following declaration initializes the variable powersOfTen to the values 100, 101, 102, 103, and 104:

int[] powersOfTen = { 1, 10, 100, 1000, 10000 };

This declaration creates an integer array of length 5 and initializes the elements as specified.

24

Exercise

Comp 101 | Özyeğin University 25

• Write a method, which takes an array of integers as argument and returns an array with the same elements in reverse order.

The ReverseArray Program

skip simulation

public void run() { int n = readInt("Enter number of elements: "); int[] intArray = createIndexArray(n); println("Forward: " + arrayToString(intArray)); reverseArray(intArray); println("Reverse: " + arrayToString(intArray));} n

10

intArray

ReverseArray

Enter number of elements: 10Forward: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]Reverse: [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

private int[] createIndexArray(int n) { int[] array = new int[n]; for ( int i = 0; i < n; i++ ) { array[i] = i; } return array;}

10

n arrayi

012345678910

0 1 2 3 4 5 6 7 8

0 0 0 0 0 0 0 0 0 09

09 18 27 36 45 54 63 72 81 90

private String arrayToString(int[] array) { String str = ""; for (int i = 0; i < array.length; i++) { if (i > 0) str += ", "; str += array[i]; } return "[" + str + "]";}

arrayistr

01234567891000, 10, 1, 20, 1, 2, 30, 1, 2, 3, 40, 1, 2, 3, 4, 50, 1, 2, 3, 4, 5, 60, 1, 2, 3, 4, 5, 6, 70, 1, 2, 3, 4, 5, 6, 7, 80, 1, 2, 3, 4, 5, 6, 7, 8, 9

private void reverseArray(int[] array) { for (int i = 0; i < array.length / 2; i++) { swapElements(array, i, array.length - i - 1); }}

arrayi

012345

private void swapElements(int[] array, int p1, int p2) { int temp = array[p1]; array[p1] = array[p2]; array[p2] = temp;}

array

9

p2

0

p1temp

0

public void run() { int n = readInt("Enter number of elements: "); int[] intArray = createIndexArray(n); println("Forward: " + arrayToString(intArray)); reverseArray(intArray); println("Reverse: " + arrayToString(intArray));} n intArray

0 1 2 3 4 5 6 7 8

9 8 7 6 5 4 3 2 1 09

0

10

26

Exercise

Comp 101 | Özyeğin University 27

• Write the DiceRoll program using an array, instead of using a variable for each face value.

Comp 101 | Özyeğin University 28

public class DiceRoll extends ConsoleProgram { private RandomGenerator rgen = RandomGenerator.getInstance(); private static final int NUM_ROLLS = 600000; public void run() { int[] faces = new int[6]; for(int i = 0; i < NUM_ROLLS; i++) { int n = rgen.nextInt(1, 6); faces[n-1]++; } for (int i = 0; i < faces.length; i++) { println("Percentage of " + (i+1) +"'s " + faces[i]*100.0/NUM_ROLLS + " %."); } }}

•Arrays hold multiple values• All values are of the same type

•Notation: [i] selects one array element• [0] is always the first element• Be careful with array bounds!

•Especially useful with large amounts of data• Often processed within loops• Entire array can be passed as an argument

Summary

Extending an array• Arrays are created with fixed length; they cannot be resized.

• Solution:

• Create a new array that is one element longer than the original.

• Copy each element from the original into the new array.

• Set the extra element of the new array.

• Assign the new array back to the original array variable.

Extending an array

a b c d e f

• Create a new array that is one element longer than the original.

originalArray

newArray

Extending an array

a b c d e f

• Copy each element from the original into the new array.

originalArray

newArray

a b c d e f

Extending an array

a b c d e f

• Set the extra element of the new array.

originalArray

newArray

a b c d e f g

Extending an array

a b c d e f

• Assign the new array back to the original array variable.

originalArray

newArray

a b c d e f g