COSC 236 Section 101 Computer Science 1 -- Prof. Michael A. Soderstrand

Transcription

1 COSC 236 Section 101 Computer Science 1 -- Prof. Michael A. Soderstrand

2 COSC 236 Web Site You will always find the course material at: or or From this site you can click on the COSC-236 tab to download the PowerPoint lectures, the Quiz solutions and the Laboratory assignments. 2

3 3

4 Review of Quiz 17 PART 1: You were given the program and asked to modify it to read the file Quiz17.java You were to run the file and confirm the output as shown. You were encouraged to copy and paste the program from the Quiz 17 assignment sheet. 4

5 Review of Quiz 17 PART 1: Only one minor change required: Change "Test.txt" to "Quiz17.java" import java.io.*; // for File import java.util.*; // Java utilities public class FileRead { public static void main(string[] args) throws FileNotFoundException { File f = new File("Test.txt"); Scanner input = new Scanner(f); System.out.println("exists returns " + f.exists()); System.out.println("canRead returns " + f.canread()); System.out.println("length returns " + f.length()); System.out.println("getAbsolutePath returns " + f.getabsolutepath()); int count = 0; while (input.hasnext()) { String word = input.next(); count++; } System.out.println("total words = " + count); } } 5

6 Review of Quiz 17 PART 1: Only one minor change required: Change "Test.txt" to "Quiz17.java" import java.io.*; // for File import java.util.*; // Java utilities public class FileRead { public static void main(string[] args) throws FileNotFoundException { File f = new File("Test.txt"); File( Quiz17.java); Scanner input = new Scanner(f); System.out.println("exists returns " + f.exists()); System.out.println("canRead returns " + f.canread()); System.out.println("length returns " + f.length()); System.out.println("getAbsolutePath returns " + f.getabsolutepath()); int count = 0; while (input.hasnext()) { String word = input.next(); count++; } System.out.println("total words = " + count); } } 6

7 Review of Quiz 17 PART 2: Modify the program to print out any comment lines and exclude them from the count In class you were also told to add a comment at the beginning of the program: // Report some basic information about a file. 7

8 Review of Quiz 17 // Report some basic information about a file. import java.io.*; // for File import java.util.*; // Java utilities ADD Comment Change file name public class Quiz17 { public static void main(string[] args) throws FileNotFoundException { File f = new File("Quiz17.java"); Scanner input = new Scanner(f); System.out.println("exists returns " + f.exists()); System.out.println("canRead returns " + f.canread()); System.out.println("length returns " + f.length()); System.out.println("getAbsolutePath returns " + f.getabsolutepath()); 8

9 Review of Quiz 17 Set up counter for words } int count1 = 0; int count2 = 0; while (input.hasnext()) { String word = input.next(); if (word.equals("//")) { count2++; String temp = input.nextline(); Set up counter for comments Check token for comment System.out.println("Comment " + count2 + " = //" + " " + temp);} else {count1++;} } System.out.println("total words excluding comments = " + count1 + "\nthere are " + count2 + " comments."); } 9

10 } Review of Quiz 17 Increment comment counter int count1 = 0; int count2 = 0; while (input.hasnext()) { String word = input.next(); if (word.equals("//")) { count2++; String temp = input.nextline(); System.out.println("Comment " + count2 + " = //" + " " + temp);} The rest of the line is comment Print out comment number and comment If not a comment, increment word counter else {count1++;} } System.out.println("total words excluding comments = " + count1 + "\nthere are " + count2 + " comments."); } No change in the rest of the program 10

11 Chapter 7 Arrays (pp ) 7.1 Array Basics (Today s Lecture, pp ) Constructing and Traversing an Array Accessing an Array A Complete Array Program Random Access Arrays and Methods The For-Each Loop Initializing Arrays The Arrays Class 11

12 Chapter 7 Arrays (pp ) 7.2 Array-Traversal Algorithms (Lecture 19) Printing an Array Searching and Replacing Testing for Equality Reversing an Array String Traversal Algorithms 12

13 Chapter 7 Arrays (pp ) 7.3 Reference Semantics (Lecture 20) Multiple Objects 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array Arrays of Objects Command-Line Arguments Nested Loop Algorithms 13

14 Chapter 7 Arrays (pp ) 7.5 Multidimensional Arrays (Lecture 21) Rectangular Two-Dimensional Arrays Jagged Arrays 7.6 Case Study: Benford s Law (Lecture 21) Tallying Values Completing the Program 14

15 Introduction to Chapter 7 and Arrays The sequential nature of files severely limits the number of interesting things that you can do easily with them. The algorithms we have examined so far have all been sequential algorithms Sequential algorithms MUST be performed by examining each data item once, in sequence. An entirely different class of algorithms can be performed when you can access the data items multiple times and in an arbitrary order 15

16 Introduction to Chapter 7 and Arrays This chapter examines a new object called an array that provides this more flexible kind of access. The concept of arrays is not complex, but it can take a while for a novice to learn all of the different ways that an array can be used. The chapter begins with a general discussion of arrays The discussion then moves into a discussion of common array manipulations Finally advanced array techniques are covered. The chapter also includes a discussion of special rules known as reference semantics that apply only to objects like arrays strings 16

17 Constructing and Traversing an Array (pp ) Accessing an Array (pp ) A Complete Array Program (pp ) Random Access (pp ) Arrays and Methods (pp ) The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 17

18 Constructing and Traversing an Array (pp ) An array is a flexible structure that holds multiple values of the same type A String is a special case of an array that holds multiple values of type char Like Strings, each element of the array has an index Like Strings, the index is zero biased (that is the index starts at zero) 18

19 Constructing and Traversing an Array (pp ) Suppose you want to write a program that keeps track of the high temperature each day You could store each temperature in a separate variable: double temperature1; double temperature2; double temperature3; That works for three temperatures, but what if you have 100 temperatures to store! The solution to the problem is an array: double[] temperature = new double[100]; 19

20 Constructing and Traversing an Array (pp ) double[] temperature = new double[100]; The array object created sets up 100 temperatures with index running from 0 to 99 The array is auto-initialized (i.e.: automatically initialized) to all zeros: However, you can easily change any value you wish: temperature[0] = 74.3; temperature[1] = 68.4; temperature[2] = 70.3; NOTE: You do not have to change the values in order you can change any value you want! 20

21 Constructing and Traversing an Array (pp ) What the array is auto-initialized to depends on the type of the array: int[] is initialized to 0 double[] is initialized to 0.0 char[] is initialized to null boolean[] is initialized to false object[] is initialized to null 21

22 Constructing and Traversing an Array (pp ) Instead of setting array values in the program, we often read them in from the console or a file: temperature[0] = input.nextdouble(); temperature[1] = input.nextdouble(); temperature[2] = input.nextdouble(); For a large number of temperatures, this is more easily done in a loop: for (int i = 0; i < temperature.length; i++) { temperature[i] = input.nextdouble();} NOTICE: temperature.length is an array method that provides us the length of the array temperature 22

23 Constructing and Traversing an Array (pp ) A loop that fills up an array completely is referred to as array transversal for (int i = 0; i < temperature.length; i++) { temperature[i] = input.nextdouble();} The array transversal pattern will be used often 23

24 Constructing and Traversing an Array (pp ) Accessing an Array (pp ) A Complete Array Program (pp ) Random Access (pp ) Arrays and Methods (pp ) The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 24

25 Accessing an Array (pp ) A Complete Array Program (pp ) Random Access (pp ) Arrays and Methods (pp ) The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 25

26 Accessing an Array (pp ) Consider the following: int[] list = new int[5]; for (int i = 0; i < list.length; i++) { list[i] = 2 * i + 1; } First we create the array: Then we put the first five odd integers into the array: 26

27 Accessing an Array (pp ) In this example we have data in ascending order (lowest to highest) int[] list = new int[5]; for (int i = 0; i < list.length; i++) { list[i] = 2 * i + 1; } Whenever we have data sorted into ascending (or descending) order, we can easily find the low, high and median values: low = list[0]; median = list[2]; high = list[4]; 27

28 Accessing an Array (pp ) Whenever we have data sorted into ascending order with length ODD, we can easily find the low, high and median values: low = list[0]; This works for any ODD array (list.length is odd) median = list[list.length/2]; high = list[list.length-1]; 28

29 Accessing an Array (pp ) If the data is even (list.length is even) we need to modify the calculation of the median: This works for any EVEN array (list.length is even) median = (list[list.length/2] + list[list.length/2 1])/2; low = list[0]; high = list[list.length-1]; 29

30 Constructing and Traversing an Array (pp ) Accessing an Array (pp ) A Complete Array Program (pp ) Random Access (pp ) Arrays and Methods (pp ) The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 30

31 A Complete Array Program (pp ) Random Access (pp ) Arrays and Methods (pp ) The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 31

32 A Complete Array Program (pp ) Consider this weather charting program that DOES NOT use arrays: import java.util.*; // Reads temperatures from the user, computes average temperature. public class Weather1 { public static void main(string[] args) { Scanner console = new Scanner(System.in); System.out.print("How many days' temperatures? "); int days = console.nextint(); double sum = 0; for (int i = 0; i < days; i++) { // read each day's temperature System.out.print("Day " + (i + 1) + "'s high temp: "); double temp = console.nextdouble(); sum += temp;} double average = sum/days; System.out.printf("Average temp = %.1f\n", average); // report results } } 32

33 How can we add to this a calculation of how many days were above average? 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. 33

34 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. 34

35 Arrays are the perfect solution 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 35

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

37 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 double 0.0 boolean String or other object false Default value null (means, "no object") 37

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

39 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 39

40 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]); index value // okay // okay // exception // exception 40

41 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. 41

42 Weather answer (p. 451) // Reads temperatures from the user, computes average and # days above average. import java.util.*; 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(); double[] temps = new double[days]; // array to store days' temperatures double sum = 0; for (int i = 0; i < days; i++) { // read/store each day's temperature System.out.print("Day " + (i + 1) + "'s high temp: "); temps[i] = console.nextdouble(); sum += temps[i];} double average = sum / days; int count = 0; // see if each day is above average for (int i = 0; i < days; i++) { if (temps[i] > average) { count++;} } // report results System.out.printf("Average temp = %.1f\n", average); System.out.println(count + " days above average"); } } 42

43 Weather answer (p. 451) // Reads temperatures from the user, computes average and # days above average. import java.util.*; 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(); double[] temps = new double[days]; // array to store days' temperatures Standard boilerplate for arrays 43

44 Weather answer (p. 451) } double sum = 0; for (int i = 0; i < days; i++) { // read/store each day's temperature System.out.print("Day " + (i + 1) + "'s high temp: "); temps[i] = console.nextdouble(); sum += temps[i];} double average = sum / days; int count = 0; for (int i = 0; i < days; i++) { } if (temps[i] > average) { count++;} // report results System.out.printf("Average temp = %.1f\n", average); System.out.println(count + " days above average"); } // see if each day is above average 44

45 Constructing and Traversing an Array (pp ) Accessing an Array (pp ) A Complete Array Program (pp ) Random Access (pp ) Arrays and Methods (pp ) The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 45

46 Random Access (pp ) Arrays and Methods (pp ) The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 46

47 Accessing array elements (pp ) 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 index numbers value

48 Constructing and Traversing an Array (pp ) Accessing an Array (pp ) A Complete Array Program (pp ) Random Access (pp ) Arrays and Methods (pp ) The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 48

49 Arrays and Methods (pp ) The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 49

50 Arrays and Methods (pp ) 50

51 Arrays and Methods (pp ) Arrays.copyOf(array, newsize) Arrays.copyOfRange(array, startix, stopix) Arrays.equals(array1, array2) Arrays.fill(array, value) Arrays.sort(array) Arrays.toString(array) 51

52 Limitations of arrays You cannot resize an existing array: int[] a = new int[4]; a.length = 10; // error You cannot compare arrays with == or equals: int[] a1 = {42, -7, 1, 15}; int[] a2 = {42, -7, 1, 15}; if (a1 == a2) {... } if (a1.equals(a2)) {... } An array does not know how to print itself: int[] a1 = {42, -7, 1, 15}; System.out.println(a1); // false! // false! // [I@98f8c4] 52

53 Class Arrays in package java.util has useful static methods for manipulating arrays: Method name binarysearch(array, value) copyof(array, length) equals(array1, array2) fill(array, value) sort(array) tostring(array) Description returns the index of the given value in a sorted array (or < 0 if not found) returns a new copy of an array returns true if the two arrays contain same elements in the same order sets every element to the given value arranges the elements into sorted order returns a string representing the array, such as "[10, 30, -25, 17]" Syntax: Arrays.methodName(parameters) 53

54 Constructing and Traversing an Array (pp ) Accessing an Array (pp ) A Complete Array Program (pp ) Random Access (pp ) Arrays and Methods (pp ) The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 54

55 The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 55

56 Arrays and for loops 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

57 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? 57

58 Constructing and Traversing an Array (pp ) Accessing an Array (pp ) A Complete Array Program (pp ) Random Access (pp ) Arrays and Methods (pp ) The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 58

59 Initializing Arrays (p. 459) The Arrays Class (p. 460) 59

60 Quick array initialization 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 60

61 "Array mystery" problem traversal: An examination of each element of an array. 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

62 Constructing and Traversing an Array (pp ) Accessing an Array (pp ) A Complete Array Program (pp ) Random Access (pp ) Arrays and Methods (pp ) The For-Each Loop (pp ) Initializing Arrays (p. 459) The Arrays Class (p. 460) 62

63 The Arrays Class (p. 460) 63

64 The Arrays class (p. 460) Class Arrays in package java.util has useful static methods for manipulating arrays: Syntax: Method name binarysearch(array, value) copyof(array, length) equals(array1, array2) fill(array, value) sort(array) tostring(array) Arrays.methodName(parameters) Description returns the index of the given value in a sorted array (or < 0 if not found) returns a new copy of an array returns true if the two arrays contain same elements in the same order sets every element to the given value arranges the elements into sorted order returns a string representing the array, such as "[10, 30, -25, 17]" 64

65 Arrays.toString (p. 460) Arrays.toString accepts an array as a parameter and returns a String representation of its elements. int[] e = {0, 2, 4, 6, 8}; e[1] = e[3] + e[4]; System.out.println("e is " + Arrays.toString(e)); Output: e is [0, 14, 4, 6, 8] Must import java.util.*; 65

66 Weather question 2 Modify the weather program to print the following output: 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. Temperatures: [45, 44, 39, 48, 37, 46, 53] Two coldest days: 37, 39 Two hottest days: 53, 48 66

67 Weather answer 2 // Reads temperatures from the user, computes average and # days above average. import java.util.*; public class Weather2 { public static void main(string[] args) {... int[] temps = new int[days]; // array to store days' temperatures... (same as Weather program) // report results System.out.printf("Average temp = %.1f\n", average); System.out.println(count + " days above average"); } } System.out.println("Temperatures: " + Arrays.toString(temps)); Arrays.sort(temps); System.out.println("Two coldest days: " + temps[0] + ", " + temps[1]); System.out.println("Two hottest days: " + temps[temps.length - 1] + ", " + temps[temps.length - 2]); 67

68 Assignments for this week 1. Laboratory for Chapter 6 due TODAY (Monday 11/03) IMPORTANT: When you me your laboratory Word Document, be sure it is all in one file 2. Laboratory for Chapter 7 due Monday 11/17 IMPORTANT: When you me your laboratory Word Document, be sure it is all in one file 3. Read pp (Chapter 7) for Wednesday 4. Be sure to complete Quiz 18 before leaving class tonight This is another program to write You must demonstrate the program to me before you leave lab 68