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 Grades for COS-236 as of

4 Grades for COS-236 as of

5 5

6 Review of Quiz 19 User enters the number of test scores and then is prompted to enter each test score Program calculates the mean, median, high and low score The Weather Program of Lecture 18, slide 42 does most of what is needed. We need only to replace the day s above average loop with the calculation of low, high and median 6

7 Review of Quiz 19 The Weather Program: // 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 Arrays.sort(temps); count = 0; // see if each day is above average for (int i = 0; i < days; i++) { Replace double median = 0; if (temps.length if (temps[i] > % average) 2 == 1) {median { = temps[temps.length/2]; else { count++; this median = (temps[temps.length/2] + temps[temps.length/2-1])/2; // report results System.out.printf("Average High Score: temp = %6.2f\n %.1f\n", Mean average); Score: %6.2f\n", temps[temps.length-1], average); System.out.println(count System.out.printf("Median + Score: " days %6.2f\n above average"); Low Score: %6.2f\n", median, temps[0]); 7

8 Review of Quiz 19 The Weather Program: // 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 Arrays.sort(temps); count = 0; // see if each day is above average for double (intmedian i = 0; = i 0; < days; i++) { if (temps.length if (temps[i] > % average) 2 == 1) {median { = temps[temps.length/2]; else { count++; median = (temps[temps.length/2] + temps[temps.length/2-1])/2; // report results System.out.printf("Average High Score: temp = %6.2f\n %.1f\n", Mean average); Score: %6.2f\n", temps[temps.length-1], average); System.out.println(count System.out.printf("Median + Score: " days %6.2f\n above average"); Low Score: %6.2f\n", median, temps[0]); 8

9 Review of Quiz 19 OUTPUT: We need only to modify the prompts to get the desired output 9

10 Review of Quiz 19 // Reads test scores and calculates high, low, mean and median import java.util.*; Complete Program: public class Quiz19 { public static void main(string[] args) { Scanner console = new Scanner(System.in); System.out.print("How many test scores do you wish to enter? "); int number = console.nextint(); double[] scores = new double[number]; // array to store scores double sum = 0; for (int i = 0; i < number; i++) { // read/store each score System.out.print("Please enter test " + (i + 1) + "'s score: "); scores[i] = console.nextdouble(); sum += scores[i]; double mean = sum / number; Arrays.sort(scores); double high = scores[number-1]; double low = scores[0]; double median = 0; if(number%2 == 0) { median = (scores[number/2] + scores[number/2-1])/2; else { median = scores[number/2]; // report results System.out.printf("Test Statistics:\n"); System.out.printf(" High Score: %6.2f\n", high); System.out.printf(" Mean Score: %6.2f\n", mean); System.out.printf(" Median Score: %6.2f\n", median); System.out.printf(" Low Score: %6.2f\n", low); 10

11 Review of Quiz 19 // Reads test scores and calculates high, low, mean and median import java.util.*; Complete Program: public class Quiz19 { public static void main(string[] args) { Scanner console = new Scanner(System.in); System.out.print("How many test scores do you wish to enter? "); int number = console.nextint(); double[] scores = new double[number]; // array to store scores double sum = 0; for (int i = 0; i < number; i++) { // read/store each score System.out.print("Please enter test " + (i + 1) + "'s score: "); scores[i] = console.nextdouble(); sum += scores[i]; double mean = sum / number; Arrays.sort(scores); double high = scores[number-1]; double low = scores[0]; double median = 0; if(number%2 == 0) { median = (scores[number/2] + scores[number/2-1])/2; else { median = scores[number/2]; // report results System.out.printf("Test Statistics:\n"); System.out.printf(" High Score: %6.2f\n", high); System.out.printf(" Mean Score: %6.2f\n", mean); System.out.printf(" Median Score: %6.2f\n", median); System.out.printf(" Low Score: %6.2f\n", low); 11

12 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) Multiple Objects (pp ) 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Arrays of Objects (pp ) Command-Line Arguments (pp ) Nested Loop Algorithms (pp ) 12

13 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) In Java, arrays are objects. We have been using objects since Chapter 3 We haven t yet discussed in detail how they are stored However, we have illustrated the storage differently: Primitive Object 13

14 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) Storing Primitives vs Objects. When we store a primitive (eg: x = 8;) the variable stores the actual data: When we store an object (eg: int[] list = new int[5];) the variable list stores a reference to where the data is actually stored: 14

15 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) Storing Primitives vs Objects. The difference between value (primitives) and reference (objects) makes a slight difference in the efficiency of storage But the real significant difference is in how data is stored One example is what happens when we call a method with a primitive vs and object 15

16 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) Storing Primitives vs Objects in Methods: Let s look at a simple method that takes two parameters: The primitive x The object list[] The method then returns a primitive (int) 16

17 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) import java.util.*; public class Lecture20S16 { public static void main(string[] args) { int x = 0; Storing Primitives vs Objects in Methods: Let s look at a simple method that takes two int[] list = new int[1]; parameters: int y = valref(x, list); System.out.printf("x = %d, list[0] = %d, y = %d\n", x, list[0], y); public static int valref(int x, int[] list) { The primitive x x The = 8; object list[] list[0] = 8; The return method x; then returns a primitive (int) 17

18 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) import java.util.*; public class Lecture20S16 { public static void main(string[] args) { int x = 0; Storing Primitives vs Objects in Methods: Let s look at a simple method that takes two int[] list = new int[1]; parameters: int y = valref(x, list); System.out.printf("x = %d, list[0] = %d, y = %d\n", x, list[0], y); public static int valref(int x, int[] list) { The primitive x x The = 8; object list[] list[0] = 8; In the main program: Set integer x to zero The return method x; then returns a primitive (int) 18

19 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) import java.util.*; public class Lecture20S16 { public static void main(string[] args) { int x = 0; Storing Primitives vs Objects in Methods: Let s look at a simple method that takes two int[] list = new int[1]; parameters: int y = valref(x, list); System.out.printf("x = %d, list[0] = %d, y = %d\n", x, list[0], y); public static int valref(int x, int[] list) { In the main program: Set integer x to zero The primitive x x The = 8; object list[] list[0] = 8; Set integer y to result of calling method valref The return method x; then returns a primitive (int) Print x, list[0], and y Set object list[] to zero 19

20 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) import java.util.*; public class Lecture20S16 { public static void main(string[] args) { int x = 0; Storing Primitives vs Objects in Methods: Let s look at a simple method that takes two int[] list = new int[1]; parameters: int y = valref(x, list); System.out.printf("x = %d, list[0] = %d, y = %d\n", x, list[0], y); public static int valref(int x, int[] list) { The primitive x x The = 8; object list[] list[0] = 8; In the method valref: Set x to 8 Set list[0] to 8 Return x = 8 The return method x; then returns a primitive (int) 20

21 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) import java.util.*; public class Lecture20S16 { public static void main(string[] args) { int x = 0; Storing Primitives vs Objects in Methods: Let s look at a simple method that takes two int[] list = new int[1]; parameters: int y = valref(x, list); System.out.printf("x = %d, list[0] = %d, y = %d\n", x, list[0], y); public static int valref(int x, int[] list) { The primitive x x The = 8; object list[] list[0] = 8; In the method valref: Set x to 8 Set list[0] to 8 Return x = 8 The return method x; then returns a primitive (int) 21

22 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) import java.util.*; public class Lecture20S16 { public static void main(string[] args) { int x = 0; Storing Primitives vs Objects in Methods: Let s look at a simple method that takes two int[] list = new int[1]; parameters: int y = valref(x, list); System.out.printf("x = %d, list[0] = %d, y = %d\n", x, list[0], y); public static int valref(int x, int[] list) { The primitive x x The = 8; object list[] list[0] = 8; Since x is passed to the method by value, changing x to 8 in the method has no effect on x in the main program The return method x; then returns a primitive (int) 22

23 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) import java.util.*; public class Lecture20S16 { public static void main(string[] args) { int x = 0; Storing Primitives vs Objects in Methods: Let s look at a simple method that takes two int[] list = new int[1]; parameters: int y = valref(x, list); System.out.printf("x = %d, list[0] = %d, y = %d\n", x, list[0], y); public static int valref(int x, int[] list) { The primitive x x The = 8; object list[] list[0] = 8; However, list[0] is passed by reference, so changing list[0] in the method also changes list[0] in the main program The return method x; then returns a primitive (int) 23

24 7.3 Reference Semantics (Lecture 20) Reference vs Value (pp ) import java.util.*; public class Lecture20S16 { public static void main(string[] args) { int x = 0; Storing Primitives vs Objects in Methods: Let s look at a simple method that takes two int[] list = new int[1]; parameters: int y = valref(x, list); System.out.printf("x = %d, list[0] = %d, y = %d\n", x, list[0], y); public static int valref(int x, int[] list) { The primitive x x The = 8; object list[] list[0] = 8; Since we return the value x=8 from the method to the main program, y will contain the value of x in the method (x=0) The return method x; then returns a primitive (int) 24

25 Reference semantics (pp ) Let s explore more deeply value vs reference 25 25

26 A swap method? (pp ) Does the following swap method work? Why or why not? public static void main(string[] args) { int a = 7; int b = 35; // swap a with b? swap(a, b); System.out.println(a + " " + b); public static void swap(int a, int b) { int temp = a; a = b; b = temp; 26

27 Value semantics (pp ) value semantics: Behavior where values are copied when assigned, passed as parameters, or returned. All primitive types in Java use value semantics. When one variable is assigned to another, its value is copied. Modifying the value of one variable does not affect others. int x = 5; int y = x; // x = 5, y = 5 y = 17; // x = 5, y = 17 x = 8; // x = 8, y = 17 27

28 Reference semantics (multiple objects pp ) reference semantics: Behavior where variables actually store the address of an object in memory. When one variable is assigned to another, the object is not copied; both variables refer to the same object. Modifying the value of one variable will affect others. int[] a1 = {4, 15, 8; int[] a2 = a1; // refer to same array as a1 a2[0] = 7; System.out.println(Arrays.toString(a1)); // [7, 15, 8] a1 index value a2 28

29 References and objects (pp ) Arrays and objects use reference semantics. Why? efficiency. Copying large objects slows down a program. sharing. It's useful to share an object's data among methods. DrawingPanel panel1 = new DrawingPanel(80, 50); DrawingPanel panel2 = panel1; // same window panel2.setbackground(color.cyan); panel1 panel2 29

30 Objects as parameters (pp ) When an object is passed as a parameter, the object is not copied. The parameter refers to the same object. If the parameter is modified, it will affect the original object. public static void main(string[] args) { DrawingPanel window = new DrawingPanel(80, 50); window.setbackground(color.yellow); example(window); window public static void example(drawingpanel panel) { panel.setbackground(color.cyan);... panel 30

31 Arrays pass by reference (pp ) Arrays are passed as parameters by reference. Changes made in the method are also seen by the caller. public static void main(string[] args) { int[] iq = {126, 167, 95; increase(iq); System.out.println(Arrays.toString(iq)); public static void increase(int[] a) { for (int i = 0; i < a.length; i++) { a[i] = a[i] * 2; Output: [252, 334, 190] a iq index value

32 Array reverse question 2 (pp ) Turn your array reversal code into a reverse method. Accept the array of integers to reverse as a parameter. int[] numbers = {11, 42, -5, 27, 0, 89; reverse(numbers); Solution: public static void reverse(int[] numbers) { for (int i = 0; i < numbers.length / 2; i++) { int temp = numbers[i]; numbers[i] = numbers[numbers.length i]; numbers[numbers.length i] = temp; 32

33 Array parameter questions (pp ) Write a method swap that accepts an array of integers and two indexes and swaps the elements at those indexes. int[] a1 = {12, 34, 56; swap(a1, 1, 2); System.out.println(Arrays.toString(a1)); // [12, 56, 34] Write a method swapall that accepts two arrays of integers as parameters and swaps their entire contents. Assume that the two arrays are the same length. int[] a1 = {12, 34, 56; int[] a2 = {20, 50, 80; swapall(a1, a2); System.out.println(Arrays.toString(a1)); // [20, 50, 80] System.out.println(Arrays.toString(a2)); // [12, 34, 56] 33

34 Array parameter answers (pp ) // Swaps the values at the given two indexes. public static void swap(int[] a, int i, int j) { int temp = a[i]; a[i] = a[j]; a[j] = temp; // Swaps the entire contents of a1 with those of a2. public static void swapall(int[] a1, int[] a2) { for (int i = 0; i < a1.length; i++) { int temp = a1[i]; a1[i] = a2[i]; a2[i] = temp; 34

35 Array return question (pp ) Write a method merge that accepts two arrays of integers and returns a new array containing all elements of the first array followed by all elements of the second. int[] a1 = {12, 34, 56; int[] a2 = {7, 8, 9, 10; int[] a3 = merge(a1, a2); System.out.println(Arrays.toString(a3)); // [12, 34, 56, 7, 8, 9, 10] Write a method merge3 that merges 3 arrays similarly. int[] a1 = {12, 34, 56; int[] a2 = {7, 8, 9, 10; int[] a3 = {444, 222, -1; int[] a4 = merge3(a1, a2, a3); System.out.println(Arrays.toString(a4)); // [12, 34, 56, 7, 8, 9, 10, 444, 222, -1] 35

36 Array return answer 1 (pp ) // Returns a new array containing all elements of a1 // followed by all elements of a2. public static int[] merge(int[] a1, int[] a2) { int[] result = new int[a1.length + a2.length]; for (int i = 0; i < a1.length; i++) { result[i] = a1[i]; for (int i = 0; i < a2.length; i++) { result[a1.length + i] = a2[i]; return result; 36

37 Array return answer 2 (pp ) // Returns a new array containing all elements of a1,a2,a3. public static int[] merge3(int[] a1, int[] a2, int[] a3) { int[] a4 = new int[a1.length + a2.length + a3.length]; for (int i = 0; i < a1.length; i++) { a4[i] = a1[i]; for (int i = 0; i < a2.length; i++) { a4[a1.length + i] = a2[i]; for (int i = 0; i < a3.length; i++) { a4[a1.length + a2.length + i] = a3[i]; return a4; // Shorter version that calls merge. public static int[] merge3(int[] a1, int[] a2, int[] a3) { return merge(merge(a1, a2), a3); 37

38 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Arrays of Objects (pp ) Command-Line Arguments (pp ) Nested Loop Algorithms (pp ) 38

39 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Rotating an array one place to the left: 39

40 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Rotating an array one place to the left: We need to save the first value in a temporary location Then we need to shift each element of the array to the left 40

41 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Rotating an array one place to the left: We need to save the first value in a temporary location Then we need to shift each element of the array to the left Finally, we need to take the temporarily stored value and replace the right-most place with the element originally in the left-most place 41

42 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Rotating an array one place to the left: public static void rotateleft(int[] list) { int first = list[0]; for (int i = 0; i < list.length 1; i++) { list[i] = list[i + 1]; list[list.length 1] = first; 42

43 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Rotating an array one place to the right: Start at the end of the array Save the last (right most) value in temporary storage int last = list[list.length 1]; Copy the shift from the top to the bottom of the array for (int i = list.length 1; i >= 1; i ) { list[i] = list[i 1]; Replace the list[0] with last list[0] = last; 43

44 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Rotating an array one place to the right: Save last last 44

45 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Rotating an array one place to the right: Save last Shift 3 to 4 last 45

46 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Rotating an array one place to the right: Save last Shift 3 to 4 Shift 2 to 3 last 46

47 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Rotating an array one place to the right: Save last Shift 3 to 4 Shift 2 to 3 Shift 1 to 2 last 47

48 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Rotating an array one place to the right: Save last Shift 3 to 4 Shift 2 to 3 Shift 1 to 2 Shift 0 to 1 last 48

49 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Rotating an array one place to the right: Save last last Shift 3 to 4 Shift 2 to 3 Shift 1 to 2 Shift 0 to 1 Shift last to 0 49

50 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Rotating an array one place to the right: public static void rotateright(int[] list) { int last = list[list.length 1]; for (int i = list.length 1; i >= 1; i ) { list[i] = list[i 1]; list[0] = last; 50

51 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Arrays of Objects (pp ) Command-Line Arguments (pp ) Nested Loop Algorithms (pp ) 51

52 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Arrays of Objects (pp ) 52

53 7.4 Advanced Array Techniques (Lecture 20) Arrays of Objects (pp ) All of the arrays we have looked at so far have stored primitive values like simple int values However, you can have arrays of any Java type. Arrays of objects behave slightly differently, though, because objects are stored as references rather than as data values. Constructing an array of objects is usually a two-step process: First construct the array Then construct the individual objects 53

54 7.4 Advanced Array Techniques (Lecture 20) Arrays of Objects (pp ) As an example, Java has a Point class as part of its java.awt package. Each Point object is used for storing the (x, y) coordinates of a point in two-dimensional space. We will discuss this class in more detail in the next chapter, but for now we will just construct a few objects from it. 54

55 7.4 Advanced Array Techniques (Lecture 20) Arrays of Objects (pp ) Suppose that you want to construct an array of Point objects: Point[] points = new Point[3]; This statement declares a variable called points that refers to an array of length 3 that stores references to Point objects. Using the new keyword to construct the array doesn t construct any actual Point objects. Instead it constructs an array of length 3, each element of which can store a reference to a Point. When Java constructs the array, it auto-initializes these array elements to the zero-equivalent for the type. 55

56 7.4 Advanced Array Techniques (Lecture 20) Arrays of Objects (pp ) First construct the array initialized to null Point[] points = new Point[3]; Then construct the individual points: points[0] = new Point(3, 7); points[1] = new Point(4, 5); points[2] = new Point(6, 2); 56

57 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Arrays of Objects (pp ) Command-Line Arguments (pp ) Nested Loop Algorithms (pp ) 57

58 7.4 Advanced Array Techniques (Lecture 20) Command-Line Arguments (pp ) Nested Loop Algorithms (pp ) 58

59 7.4 Advanced Array Techniques (Lecture 20) Command-Line Arguments (pp ) As you ve seen since Chapter 1, whenever you define a main method: You are required to include as its parameter String[] args This is an array of String objects 59

60 7.4 Advanced Array Techniques (Lecture 20) Command-Line Arguments (pp ) Java itself initializes this array if the user provides what are known as command-line arguments when invoking Java. The user could execute a Java class called DoSomething from a command prompt or terminal by using a command like: java DoSomething The user has the option to type extra arguments, as in the following: java DoSomething temperature.dat temperature.out 60

61 7.4 Advanced Array Techniques (Lecture 20) Command-Line Arguments (pp ) In this case the user has specified two extra arguments that are file names that the program should use: The names of an input and output file The String[] args parameter to main is initialized to an array of length 2 that stores these two strings: 61

62 7.4 Advanced Array Techniques (Lecture 20) Shifting Values in an Array (pp ) Arrays of Objects (pp ) Command-Line Arguments (pp ) Nested Loop Algorithms (pp ) 62

63 7.4 Advanced Array Techniques (Lecture 20) Nested Loop Algorithms (pp ) Consider the problem of printing inversions in an array of numbers An inversion is a pair of numbers in which the first number is greater than the second A sorted (ascending) array has no inversions A reversed (descending) array has many inversions 63

64 7.4 Advanced Array Techniques (Lecture 20) Nested Loop Algorithms (pp ) Inversions in a reversed (descending) array [4, 3, 2, 1]: 64

65 7.4 Advanced Array Techniques (Lecture 20) Nested Loop Algorithms (pp ) Inversions in a reversed (descending) array [4, 3, 2, 1]: public class Lecture20S62 { public static void main(string[] args) { int[] data = {4, 3, 2, 1; for (int i = 0; i < data.length; i++) { for (int j = 0; j < data.length; j++) { if (data[i] > data[j]) { This works, but is not optimized System.out.println("(" + data[i] + ", " + data[j] + ")"); 65

66 7.4 Advanced Array Techniques (Lecture 20) Nested Loop Algorithms (pp ) Inversions in a reversed (descending) array [4, 3, 2, 1]: public class Lecture20S62 { public static void main(string[] args) { int[] data = {4, 3, 2, 1; for (int i = 0; i < data.length; i++) { for (int j = 0; j < data.length; j++) { if (data[i] > data[j]) { We do not need to check the last location in the outer loop The inner loop does not have to check 0 through i System.out.println("(" + data[i] + ", " + data[j] + ")"); 66

67 7.4 Advanced Array Techniques (Lecture 20) Nested Loop Algorithms (pp ) Inversions in a reversed (descending) array [4, 3, 2, 1]: public class Lecture20S62 { public static void main(string[] args) { int[] data = {4, 3, 2, 1; for (int i = 0; i < data.length-1; i++) { for (int j = i + 1; j < data.length; j++) { if (data[i] > data[j]) { System.out.println("(" + data[i] + ", " + data[j] + ")"); Optimized algorithm 67

68 Assignments for this week 1. Laboratory for Chapter 7 due Monday 11/17 IMPORTANT: When you me your laboratory Word Document, be sure it is all in one file 2. Read pp (Chapter 7) for Wednesday 11/12 3. Be sure to complete Quiz 20 before leaving class tonight This is another program to write You must demonstrate the program to me before you leave lab 68