Arrays. Weather Problem Array Declaration Accessing Elements Arrays and for Loops Array length field Quick Array Initialization Array Traversals

Transcription

1 Arrays Weather Problem Array Declaration Accessing Elements Arrays and for Loops Array length field Quick Array Initialization Array Traversals

2 Can we solve this problem? Consider the following program (input underlined): How many days' temperatures? 7 Day 1's high temp: 45 Day 2's high temp: 44 Day 3's high temp: 39 Day 4's high temp: 48 Day 5's high temp: 37 Day 6's high temp: 46 Day 7's high temp: 53 Average temp = days were above average. 2

3 Why the problem is hard We need each input value twice: to compute the average (a cumulative sum) to count how many were above average We could read each value into a variable... but we: don't know how many days are needed until the program runs don't know how many variables to declare We need a way to declare many variables in one step. Luckily, Java has a built-in structure, the array, that makes this problem very easy to solve. 3

4 Arrays array: object that stores many values of the same type. element: One value in an array. index: A 0-based integer to access an element from an array. index value element 0 element 4 element 9 4

5 Array declaration type[] name = new type[length]; Example: int[] numbers = new int[10]; index value

6 Array declaration, cont. The length can be any integer expression. int x = 2 * 3 + 1; int[] data = new int[x % 5 + 2]; Each element initially gets a "zero-equivalent" value. Type int 0 Default value double 0.0 boolean String or other object false null (means, "no object") 6

7 Accessing elements name[index] name[index] = value; // access // modify Example: numbers[0] = 27; numbers[3] = -6; System.out.println(numbers[0]); if (numbers[3] < 0) { System.out.println("Element 3 is negative."); index value

8 Arrays of other types double[] results = new double[5]; results[2] = 3.4; results[4] = -0.5; index value boolean[] tests = new boolean[6]; tests[3] = true; index value false false false true false false 10

9 Out-of-bounds Legal indexes: between 0 and the array's length - 1. Reading or writing any index outside this range will throw an ArrayIndexOutOfBoundsException. Example: int[] data = new int[10]; System.out.println(data[0]); System.out.println(data[9]); System.out.println(data[-1]); System.out.println(data[10]); // okay // okay // exception // exception index value

10 Ways of Accessing Arrays Sequential Access: accessing and/or manipulating elements of array in a sequential manner from first element to last element for loops Random Access: accessing and/or manipulating elements of array in any order whatsoever with quick access to each element element by index 10

11 Accessing array elements randomly example int[] numbers = new int[8]; numbers[1] = 3; numbers[4] = 99; numbers[6] = 2; int x = numbers[1]; numbers[x] = 42; numbers[numbers[6]] = 11; // use numbers[6] as index x index numbers value 11

12 Accessing array elements randomly example solution int[] numbers = new int[8]; numbers[1] = 3; numbers[4] = 99; numbers[6] = 2; int x = numbers[1]; numbers[x] = 42; numbers[numbers[6]] = 11; // use numbers[6] as index x 3 numbers index value

13 Accessing array elements randomly another problem int[] data = new int[8]; data[7] = 3; data[4] = 1; int x = data[2]; data[2] = 31; data[x] = 4; data[data[0]] = 6; x index data value 13

14 Arrays and for loops (Sequential Access) It is common to use for loops to access array elements. for (int i = 0; i < 8; i++) { System.out.print(numbers[i] + " "); System.out.println(); // output: Sometimes we assign each element a value in a loop. for (int i = 0; i < 8; i++) { numbers[i] = 2 * i; index value

15 The length field An array's length field stores its number of elements. name.length for (int i = 0; i < numbers.length; i++) { System.out.print(numbers[i] + " "); // output: It does not use parentheses like a String's.length(). What expressions refer to: The last element of any array? The middle element? The first element? 15

16 The length field (cont) Last element: numbers[numbers.length 1]; Middle element: numbers[numbers.length / 2]; Middle element: numbers[(numbers.length - 1) / 2]; First element: numbers[0]; 16

17 Initializing Arrays User Input Because arrays are indexed, using a loop to initialize arrays is really efficient Prompt the user for the size of the array and then create an integer array of that size and fill with values that are prompted from the user. Scanner console = new Scanner(System.in); System.out.print("How many numbers? "); int size = console.nextint(); int[] array = new int[size]; for (int i = 0; i < array.length; i++) { System.out.print("Integer: "); while(!console.hasnextint()){ console.next(); System.out.println("Not an integer"); System.out.print("Integer: "); array[i] = console.nextint(); 17

18 Initializing Arrays Random Create and fill a 100 element integer array with random values between 0 and 99, inclusive. public static final int ARRAY_SIZE = 100; public static final int VALUE_RANGE = 100; Random r = new Random(); int[] a = new int[array_size]; for (int i = 0; i < a.length; i++) { a[i] = r.nextint(value_range); 20

19 Quick array initialization You can also create an array and give it initial values for each element using a shortcut: type[] name = {value, value, value; Example: int[] numbers = {12, 49, -2, 26, 5, 17, -6; index value Useful when you know what the array's elements will be The compiler figures out the size by counting the values 19

20 "Array mystery" problem What element values are stored in the following array? int[] a = {1, 7, 5, 6, 4, 14, 11; for (int i = 0; i < a.length - 1; i++) { if (a[i] > a[i + 1]) { a[i + 1] = a[i + 1] * 2; index value 20

21 "Array mystery" solution What element values are stored in the following array? int[] a = {1, 7, 5, 6, 4, 14, 11; for (int i = 0; i < a.length - 1; i++) { if (a[i] > a[i + 1]) { a[i + 1] = a[i + 1] * 2; index value

22 Array traversals traversal: An examination of each element of an array. for (int i = 0; i < array.length; i++) { do something with array[i]; Examples: printing the elements searching for a specific value rearranging the elements computing the sum, product, etc. 22

23 Back to the Weather question Use an array to solve the weather problem: How many days' temperatures? 7 Day 1's high temp: 45 Day 2's high temp: 44 Day 3's high temp: 39 Day 4's high temp: 48 Day 5's high temp: 37 Day 6's high temp: 46 Day 7's high temp: 53 Average temp = days were above average. 23

24 Weather answer // Reads temperatures from the user, computes average and # days import java.util.*; above average. public class Weather { public static void main(string[] args) { Scanner console = new Scanner(System.in); System.out.print("How many days' temperatures? "); int days = console.nextint(); int[] temps = new int[days]; int sum = 0; // array to store days' temperatures for (int i = 0; i < days; i++) { System.out.print("Day " + (i + 1) + "'s high temp: temps[i] = console.nextint(); sum += temps[i]; double average = (double) sum / days; // read/store each day's temperature "); int count = 0; for (int i = 0; i < days; i++) { if (temps[i] > average) { count++; // see if each day is above average // report results System.out.printf("Average System.out.println(count + temp = " days %.1f\n", average); above average"); 26

25 Lab Exercise Go to the moodle page and work on the ArrayCalculations.java assignment: Write a class called ArrayCalculations that does the following: Declares an integer array named data with the elements 7, -1, 13, 24, and 6. Use only one statement to initialize the array. Calculates and prints the maximum value in the array. Calculates and prints the minimum value in the array. Calculates and prints the sum of all the elements in the array. Calculates and prints the average (a double) of all the elements in the array. System.out.println("\nThe maximum value in the array is: " + maximum); System.out.println("The minimum value in the array is: " + minimum); System.out.println("The sum of the values in the array is: " + sum); System.out.println("The average of the values in the array is: " + average); 27