CS 106A, Lecture 7 Parameters and Return

Transcription

1 CS 106A, Lecture 7 Parameters and Return suggested reading: Java Ch This document is copyright (C) Stanford Computer Science and Marty Stepp, licensed under Creative Commons Attribution 2.5 License. All rights reserved. Based on slides created by Keith Schwarz, Mehran Sahami, Eric Roberts, Stuart Reges, and others.

2 Plan For Today Announcements Recap: For Loops Recap: Scope Parameters Return 2

3 Plan For Today Announcements Recap: For Loops Recap: Scope Parameters Return 3

4 For Loops in Java Repeats the loop if this condition passes for (int i = 0; i < 3; i++) { println("i love CS 106A!"); 4

5 Nested loops nested loop: A loop placed inside another loop. for (int i = 0; i < 5; i++) { for (int j = 0; j < 10; j++) { print("*"); println(); // to end the line Output: ********** ********** ********** ********** ********** The outer loop repeats 5 times; the inner one 10 times. 5

6 Nested loop question 2 How would we produce the following output? Answer: for (int i = 0; i < 5; i++) { for (int j = 0; j < 5 i - 1; j++) { print("."); for (int j = 0; j <= i; j++) { print(i + 1); println(); 6

7 Methods in Java We can define new methods in Java just like in Karel: private void name() { statement; statement;... For example: private void printgreeting() { println("hello world!"); println("i hope you have a great day."); 7

8 Methods in Java public void run() { int x = 2; printx(); private void printx() { // ERROR! "Undefined variable x" println("x has the value " + x); 8

9 Plan For Today Announcements Recap: For Loops Recap: Scope Parameters Return 9

10 By Chris Piech 10

11 Variable Scope The scope of a variable refers to the section of code where a variable can be accessed. Scope starts where the variable is declared. Scope ends at the termination of the code block in which the variable was declared. A code block is a chunk of code between { brackets 11

12 Variable Scope You cannot have two variables with the same name in the same scope. for (int i = 1; i <= 100 * line; i++) { int i = 2; // ERROR print("/"); 12

13 Variable Scope You cannot have two variables with the same name in the same scope. for (int i = 1; i <= 100 * line; i++) { int i = 2; // ERROR while (...) { int i = 5; // ERROR 13

14 Variable Scope You can have two variables with the same name in separate scopes. public void run() { for (int i = 0; i < 5; i++) { int w = 2; // i ok here // w ok here for (int i = 0; i < 2; i++) { int w = 3; // i ok here // w ok here 14

15 Variable Scope You can have two variables with the same name in separate scopes. public void run() { int num = 5; cow(); println(num); // prints 5 private void cow() { int num = 10; println(num); // prints 10 15

16 Variable Scope You can have two variables with the same name in different scopes. public void run() { int num = 5; cow(); println(num); run 5 num private void cow() { int num = 10; println(num); 16

17 Variable Scope You can have two variables with the same name in different scopes. public void run() { int num = 5; cow(); println(num); run cow 5 num private void cow() { int num = 10; println(num); 17

18 Variable Scope You can have two variables with the same name in different scopes. public void run() { int num = 5; cow(); println(num); run cow 5 num num 10 private void cow() { int num = 10; println(num); 18

19 Variable Scope You can have two variables with the same name in different scopes. public void run() { int num = 5; cow(); println(num); run cow 5 num num 10 private void cow() { int num = 10; println(num); 19

20 Variable Scope You can have two variables with the same name in different scopes. public void run() { int num = 5; cow(); println(num); run 5 num private void cow() { int num = 10; println(num); 20

21 Plan For Today Announcements Recap: For Loops Recap: Scope Parameters Return 21

22 Parameters Parameters let you provide a method some information when you are calling it. 22

23 Methods = Toasters parameter 23

24 Example: readint readint( Your guess? "); 24

25 Example: readint We call readint We give readint some information in parenthesis (the text to print to the user) readint( Your guess? "); 25

26 Example: printgreeting printgreeting(5); (Prints a greeting a certain number of times) 26

27 Wouldn t it be nice if We call printgreeting We give printgreeting some information (the number of greetings to print) printgreeting(5); 27

28 Methods with Parameters Tells Java this method needs one int in order to execute. private void printgreeting(int times) { // use times to print the greeting 28

29 Methods with Parameters printgreeting(5); private void printgreeting(int times) { // use times to print the greeting 29

30 Methods with Parameters printgreeting(5); private void printgreeting(int times) { for (int i = 0; i < times; i++) { println("hello world!"); 30

31 Methods with Parameters public void run() { int repeats = 5; printgreeting(repeats); private void printgreeting(int times) { for (int i = 0; i < times; i++) { println("hello world!"); 31

32 Methods with Parameters public void run() { int repeats = 5; printgreeting(repeats); private void printgreeting(int times) { for (int i = 0; i < times; i++) { println("hello world!"); run 5 repeats printgreeting? times 32

33 Methods with Parameters public void run() { int repeats = 5; printgreeting(repeats); private void printgreeting(int times) { for (int i = 0; i < times; i++) { println("hello world!"); run 5 repeats printgreeting 5 times 33

34 Methods with Parameters public void run() { int times = 5; printgreeting(times); private void printgreeting(int times) { for (int i = 0; i < times; i++) { println("hello world!"); run 5 times printgreeting? times 34

35 Methods with Parameters public void run() { int times = 5; printgreeting(times); private void printgreeting(int times) { for (int i = 0; i < times; i++) { println("hello world!"); run 5 times printgreeting 5 times 35

36 Parameters are Copies // NOTE: This program is buggy!! public void run() { int x = 3; run addfive(x); // prints "x = 3"! println("x = " + x); 3 x private void addfive(int x) { x += 5; 36

37 Parameters are Copies // NOTE: This program is buggy!! public void run() { int x = 3; run addfive addfive(x); // prints "x = 3"! println("x = " + x); private void addfive(int x) { x += 5; 3 x 3 x 37

38 Parameters are Copies // NOTE: This program is buggy!! public void run() { int x = 3; run addfive addfive(x); // prints "x = 3"! println("x = " + x); private void addfive(int x) { x += 5; 3 x 8 x 38

39 Parameters are Copies // NOTE: This program is buggy!! public void run() { int x = 3; run addfive(x); // prints "x = 3"! println("x = " + x); 3 x private void addfive(int x) { x += 5; 39

40 Parameters parameter: A value passed to a method by its caller. Write a method drawbox to draw a box of any size. When declaring the method, we will state that it requires the caller to tell it the width and height of the box. When calling the method, we will specify the width and height to use. run 10, 4 drawbox ********** * * * * ********** 7, 5 drawbox ******* * * * * * * ******* 40

41 Declaring a parameter Stating that a method requires a parameter in order to run private void name(type name) { statements; Example: private void password(int code) { println("the password is: " + code); When password is called, the caller must specify the integer code to print. 41

42 Multiple parameters A method can accept multiple parameters separated by commas:, When calling it, you must pass values for each parameter. Declaration: private void name(type name,..., type name) { statements; Call: name(value, value,..., value); 42

43 Passing a parameter Calling a method and specifying values for its parameters methodname(expression); Example: public void run() { password(42); password(12345); Output: The password is 42 The password is Illegal to call without passing an int for that parameter. password(); // Error password(3.7); // Error 43

44 How params are passed When the method is called: The value is stored into the parameter variable. The method's code executes using that value. public void run() { chant(7); 7 private void chant(int times) { for (int i = 0; i < times; i++) { println("java is great!"); 44

45 Drawing boxes Lets write a program that uses methods and parameters to print the following boxes: ********** * * * * ********** ******* * * * * * * * * ******* The code to draw each box will be very similar. Would variables help? Would constants help? 45

46 drawbox drawbox(10, 4); 46

47 drawbox We call drawbox We give drawbox some information (the size of the box we want) drawbox(10, 4); 47

48 drawbox private void drawbox(int width, int height) { // use width and height variables // to draw a box 48

49 drawbox ********** * * * * ********** private void drawbox(int width, int height) { line(width); for (int line = 0; line < height - 2; line++) { boxside(width); line(width); 49

50 drawbox ********** * * * * ********** private void drawbox(int width, int height) { line(width); for (int line = 0; line < height - 2; line++) { boxside(width); line(width); 50

51 drawbox ********** * * * * ********** private void drawbox(int width, int height) { line(width); for (int line = 0; line < height - 2; line++) { boxside(width); line(width); 51

52 drawbox ********** * * * * ********** private void drawbox(int width, int height) { line(width); for (int line = 0; line < height - 2; line++) { boxside(width); line(width); 52

53 drawbox ********** * * * * ********** private void drawbox(int width, int height) { line(width); for (int line = 0; line < height - 2; line++) { boxside(width); line(width); 53

54 line ********** private void line(int count) { for (int i = 0; i < count; i++) { print("*"); println(); 54

55 boxside * * private void boxside(int width) { print("*"); for (int i = 0; i < width - 2; i++) { print(" "); println("*"); 55

56 boxside public void run() { drawbox(10, 4); drawbox(7, 6); ********** * * * * ********** ******* * * * * * * * * ******* 56

57 Plan For Today Announcements Recap: For Loops Recap: Scope Parameters Return 57

58 Return Return values let you give back some information when a method is finished. 58

59 Methods = Toasters parameter 59

60 Methods = Toasters parameter 60

61 Methods = Toasters 61

62 Methods = Toasters 62

63 Methods = Toasters return 63

64 Example: readint int x = readint( Your guess? "); 64

65 Example: readint We call readint We give readint some information (the text to print to the user) int x = readint( Your guess? "); 65

66 Example: readint When finished, readint gives us information back (the user s number) and we put it in x. int x = readint( Your guess? "); 66

67 Example: readint When we set a variable equal to a method, this tells Java to save the return value of the method in that variable. int x = readint( Your guess? "); 67

68 Example: meterstocm double cm = meterstocm(5); (Returns the given number of m as cm) 68

69 Example: meterstocm We call meterstocm We give meterstocm some information (the number of meters) double cm = meterstocm(5); 69

70 Example: meterstocm When meterstocm finishes, it returns the number of cm, and we put that in this variable. double cm = meterstocm(5); 70

71 Methods and Return Tells Java this method needs one double in order to execute. private double meterstocm(double meters) {... 71

72 Methods and Return Tells Java that, when this method finishes, it will return a double. private double meterstocm(double meters) {... 72

73 Methods and Return Tells Java that, when this method finishes, it will return a double. (Void meant returns nothing) private double meterstocm(double meters) { return 100 * meters; 73

74 Methods and Return public void run() { double meters = readdouble("#meters? ); double cm = meterstocm(meters); println(cm + " centimeters."); private double meterstocm(double meters) { return 100 * meters; 74

75 Methods and Return public void run() { double meters = readdouble("#meters? ); double cm = meterstocm(meters); println(cm + " centimeters."); private double meterstocm(double meters) { return 100 * meters; run? meters 75

76 Methods and Return public void run() { double meters = readdouble("#meters? ); double cm = meterstocm(meters); println(cm + " centimeters."); private double meterstocm(double meters) { return 100 * meters; run 5 meters 76

77 Methods and Return public void run() { double meters = readdouble("#meters? ); double cm = meterstocm(meters); println(cm + " centimeters."); private double meterstocm(double meters) { return 100 * meters; run 5 meters meterstocm? meters 77

78 Methods and Return public void run() { double meters = readdouble("#meters? ); double cm = meterstocm(meters); println(cm + " centimeters."); private double meterstocm(double meters) { return 100 * meters; run 5 meters meterstocm 5 meters 78

79 Methods and Return public void run() { double meters = readdouble("#meters? ); double cm = meterstocm(meters); println(cm + " centimeters."); private double meterstocm(double meters) { return 100 * meters; 500 run 5 meters meterstocm 5 meters 79

80 Methods and Return public void run() { double meters = readdouble("#meters? ); double cm = meterstocm(meters); println(cm + " centimeters."); private double meterstocm(double meters) { return 100 * meters; 500 run 500 cm 5 meters meterstocm 5 meters 80

81 Methods and Return public void run() { double meters = readdouble("#meters? ); double cm = meterstocm(meters); println(cm + " centimeters."); private double meterstocm(double meters) { return 100 * meters; run 500 cm 5 meters 81

82 Methods and Return public void run() { double meters = readdouble("#meters? ); println(meterstocm(meters) + "cm."); private double meterstocm(double meters) { return 100 * meters; If a method returns something, you can use it directly in an expression! 82

83 Return return: To send out a value as the result of a method. Parameters send information in from the caller to the method. Return values send information out from a method to its caller. A call to the method can be used as part of an expression. -42 Math.abs(-42) run Math.round(2.71) Q: Why return? Why not just println the result value? 83

84 Methods visibility type nameofmethod(parameters) { statements visibility: usually private or public type: type returned by method (e.g., int, double, etc.) Can be void to indicate that nothing is returned parameters: information passed into method 84

85 Returning Booleans private boolean iseven(int number) { 85

86 Returning Booleans private boolean iseven(int number) { if (number % 2 == 0) { return true; else { return false; 86

87 Returning Booleans private boolean iseven(int number) { if (number % 2 == 0) { return true; else { return false; public void run() { int num = readint("? "); if (iseven(num)) { println("even!"); else { println("odd!"); 87

88 Returning Booleans private boolean iseven(int number) { return number % 2 == 0; 88

89 Returning Booleans public void run() { for(int i = 1; i <= 100; i++) { if(isdivisibleby(i, 7)) { println(i); private void isdivisibleby(int a, int b) { return a % b == 0; 89

90 Return Return ends a method s execution. private int multiplybytwo(int num) { return num * 2; println("hello world?"); // not executed! 90

91 Return Return ends a method s execution. private int max(int num1, int num2) { if(num1 >= num2) { return num1; return num2; // here only if num1 < num2 public void run() { println(max(2,3)); 91

92 Revisiting a Bug // NOTE: This program is buggy!! public void run() { int x = 3; addfive(x); // prints "x = 3"! println("x = " + x); private void addfive(int x) { x += 5; 92

93 Fixed! // NOTE: This program is feeling just fine public void run() { int x = 3; x = addfive(x); // prints "x = 5"! println("x = " + x); private int addfive(int x) { x += 5; return x; 93

94 Recap Announcements Recap: For Loops Recap: Scope Parameters Return Next time: Strings (new variable type!) 94