Example. Generating random numbers. Write a program which generates 2 random integers and asks the user to answer the math expression.

Transcription

1 Example Generating random numbers Write a program which generates 2 random integers and asks the user to answer the math expression. For example, the program shows =? If the user answers 7, then the program reports Correct. and terminates. Otherwise, the program reports Wrong answer. The correct answer is 7. for this case. You may use Math.random() for a random value between 0.0 and 1.0, excluding themselves. Zheng-Liang Lu Java Programming 102 / 135

2 2 int x = (int) (Math.random() 10); // integers int y = (int) (Math.random() 10); 4 int answer = x + y; 5 6 System.out.println(x + " + " + y + " =?"); 7 8 Scanner input = new Scanner(System.in); 9 int z = input.nextint(); if (z == answer) 12 System.out.println("Correct."); 13 else 14 System.out.println("Wrong. Answer: " + answer); 15 input.close(); Can you extend this program to include + in the math expressions? Zheng-Liang Lu Java Programming 103 / 135

3 Exercise Find Max Write a program which determines the maximum value in 3 random integers whose range from 0 to 99. How many variables do we need? How to compare? How to keep the maximum value? Zheng-Liang Lu Java Programming 104 / 135

4 2 int x = (int) (Math.random() 100); 3 int y = (int) (Math.random() 100); 4 int z = (int) (Math.random() 100); 5 6 int max = x; 7 if (y > max) max = y; 8 if (z > max) max = z; 9 System.out.println("max = " + max); In this case, a scalar variable is not convenient. (Why?) So we need two more elements: arrays and loops. Zheng-Liang Lu Java Programming 105 / 135

5 switch-case Statements A switch-case structure takes actions depending on the target variable. 2 switch (target) { 3 case v1: 4 // statements 5 break; 6 case v2: case vk: 10 // statements 11 break; 12 default: 13 // statements 14 } Zheng-Liang Lu Java Programming 106 / 135

6 A switch-case statement is more convenient than an if statement for multiple discrete conditions. The variable target, always enclosed in parentheses, must yield a value of char, byte, short, int, or String type. The value v 1,..., and v k must have the same data type as the variable target. In each case, a break statement is a must. 1 break is used to break a construct! The default case, which is optional, can be used to perform actions when none of the specified cases matches target. Counterpart to else statements. 1 If not, there will be a fall-through behavior. Zheng-Liang Lu Java Programming 107 / 135

7 Example 2 int trafficlight = (int) (Math.random() 3); 3 switch (trafficlight) { 4 case 0: 5 System.out.println("Red"); 6 break; 7 case 1: 8 System.out.println("Yellow"); 9 break; 10 case 2: 11 System.out.println("Green"); 12 break; 13 } Zheng-Liang Lu Java Programming 108 / 135

8 Conditional Operators A conditional expression evaluates an expression based on the specified condition and returns values accordingly. 2 somevar = booleanexpr? expra : exprb; 3... This is the only ternary operator in Java. If the boolean expression is evaluated true, then return expr A; otherwise, expr B. Zheng-Liang Lu Java Programming 109 / 135

9 For example, 2 if (num1 > num2) 3 max = num1; 4 else 5 max = num2; 6... Alternatively, one can use a conditional expression like this: 2 max = (num1 > num2)? num1 : num2; 3... Zheng-Liang Lu Java Programming 110 / 135

10 1 class Lecture4 { 2 3 "Loops" 4 5 } 6 7 // keywords: 8 while, do, for, break, continue Zheng-Liang Lu Java Programming 111 / 135

11 Loops A loop can be used to make a program execute statements repeatedly without having to code the same statements. For example, a program outputs Hello, Java. for 100 times. 2 System.out.println("Hello, Java."); 3 System.out.println("Hello, Java."); // copy and paste for 100 times 6. 7 System.out.println("Hello, Java."); 8... Zheng-Liang Lu Java Programming 112 / 135

12 2 int cnt = 0; 3 while (cnt < 100) { 4 System.out.println("Hello, Java."); 5 cnt++; 6 } 7... This is a simple example to show the power of loops. In practice, any routine which repeats couples of times 2 can be done by folding them into a loop. 2 I d like to call them patterns. Zheng-Liang Lu Java Programming 113 / 135

13 成也迴圈, 敗也迴圈 Loops provide significant computational power. Loops bring an efficient way of programming. Loops could consume a lot of time. 3 3 We will visit the analysis of algorithms in the end of this lecture. Zheng-Liang Lu Java Programming 114 / 135

14 while Loops A while loop executes statements repeatedly while the condition is true. 2 while (condition) { 3 // loop body 4 } 5... The condition should be a Boolean expression which determines whether or not the execution of the body occurs. If true, the loop body is executed and check the condition again. Otherwise, the entire loop terminates. Zheng-Liang Lu Java Programming 115 / 135

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16 Example Write a program which sums up all integers from 1 to 100. In math, the question can be written as: sum = But this form is not doable in the machine. 4 4 We need to develop computational thinking. Read or Zheng-Liang Lu Java Programming 117 / 135

17 Normally, the machine executes the instructions sequentially. So one needs to decompose the math equation into several steps, like: 2 int sum = 0; 3 sum = sum + 1; 4 sum = sum + 2; sum = sum + 100; 9... It is obvious that many similar statements can be found. Zheng-Liang Lu Java Programming 118 / 135

18 Using a while loop, the program can be rearranged as follows: 2 int sum = 0; 3 int i = 1; 4 while (i <= 100) { 5 sum = sum + i; 6 ++i; 7 } 8... You should guarantee that the loop will terminate as expected. In practice, the number of loop steps (iterations) is unknown until the input data is given. Zheng-Liang Lu Java Programming 119 / 135

19 Malfunctioned Loops It is really easy to make an infinite loop. 2 while (true); 3... The common errors of the loops are: never start never stop not complete exceed the expected number of iterations Zheng-Liang Lu Java Programming 120 / 135

20 Example Write a program which asks the sum of two random integers and lets the user repeatedly enter a new answer until correct. 2 Scanner input = new Scanner(System.in); 3 int x = (int) (Math.random() 10); 4 int y = (int) (Math.random() 10); 5 int ans = x + y; 6 7 System.out.println(x + " + " + y + " =? "); 8 int z = input.nextint(); 9 10 while (z!= ans) { 11 System.out.println("Try again? "); 12 z = input.nextint(); 13 } 14 System.out.println("Correct."); 15 input.close(); Zheng-Liang Lu Java Programming 121 / 135

21 Loop Design Strategy Writing a correct loop is not an easy task for novice programmers. Consider 3 steps when writing a loop: Find the pattern: identify the statements that need to be repeated. Wrap by loops: put these statements in the loop. Set the continuation condition: translate the criterion from the real world problem into computational conditions. 5 5 Not unique. Zheng-Liang Lu Java Programming 122 / 135

22 Sentinel-Controlled Loop Another common technique for controlling a loop is to designate a special value when reading and processing a set of values. This special input value, known as a sentinel value, signifies the end of the loop. For example, the operating systems and the GUI apps. Zheng-Liang Lu Java Programming 123 / 135

23 Example: Cashier Problem Write a program which sums positive integers from the input except for -1 to exit, and displays the sum. 2 Scanner input = new Scanner(System.in); 3 System.out.println("Enter price ( 1 to exit): "); 4 int curritem = input.nextint(); 5 int sum = 0; 6 while (curritem!= 1) { 7 if (curritem > 0) sum += curritem; 8 System.out.println("Enter price ( 1 to exit): "); 9 curritem = input.nextint(); 10 } 11 System.out.println("Sum = " + sum); 12 input.close(); Line 8 and 9 are the recurrence of Line 3 and 4?! Zheng-Liang Lu Java Programming 124 / 135

24 do-while Loops A do-while loop is similar to a while loop except that it does execute the loop body first and then checks the loop continuation condition. 2 do { 3 // loop body 4 } while (condition); // Do not miss the semicolon! 5... Note that there is a semicolon at the end of the do-while loop. The do-while loops are also called posttest loops, in contrast to while loops, which are pretest loops. Zheng-Liang Lu Java Programming 125 / 135

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26 Example (Revisted) Write a program which sums positive integers from consecutive inputs and displays the sum when the input is 1. 2 Scanner input = new Scanner(System.in); 3 int sum = 0; 4 int curritem; 5 do { 6 System.out.println("Enter price ( 1 to exit): "); 7 curritem = input.nextint(); 8 if (curritem >= 0) sum += curritem; 9 } while (curritem!= 1); 10 System.out.println("Sum = " + sum); 11 input.close(); Zheng-Liang Lu Java Programming 127 / 135

27 Exercise Write a program which allows the user to enter positive integers consecutively and outputs the maximum when the input is 1. Zheng-Liang Lu Java Programming 128 / 135

28 2 Scanner input = new Scanner(System.in); 3 int max; 4 5 do { 6 System.out.println("x? ( 1 to exit) "); 7 int x = input.nextint(); 8 if (max < x) { 9 max = x; 10 } 11 } while (x!= 1); input.close(); Zheng-Liang Lu Java Programming 129 / 135

29 for Loops A for loop generally uses a variable to control how many times the loop body is executed and when the loop terminates. 2 for (init action; condition; increment) { 3 // loop body 4 } 5... init-action: declare and initialize a variable condition: set a criterion for loop continuation increment: how the variable changes after each iteration Note that these three terms are separated by semicolons. Zheng-Liang Lu Java Programming 130 / 135

30 Sum from 1 to 100 Example Write a program which sums from 1 to int sum = 0; 3 for (int i = 1; i <= 100; ++i) 4 sum = sum + i; 5... Compared to the while version, 2 int sum = 0; 3 int i = 1; 4 while (i <= 100) { 5 sum = sum + i; 6 ++i; 7 } 8... Zheng-Liang Lu Java Programming 131 / 135

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32 Example: Selection Resided in Loop Display all even numbers Write a program which displays all even numbers smaller than 100. An even number is an integer of the form x = 2k, where k is an integer. Zheng-Liang Lu Java Programming 133 / 135

33 You may use the modular operator (%). 2 for (int i = 1; i <= 100; i++) { 3 if (i % 2 == 0) System.out.println(i); 4 } 5... Also consider this alternative: 2 for (int i = 2; i <= 100; i += 2) { 3 System.out.println(i); 4 } 5... How about odd numbers? Zheng-Liang Lu Java Programming 134 / 135

34 Example: Monte Carlo Simulation 6 Write a program which conducts a Monte Carlo simulation to estimate π. 6 See Zheng-Liang Lu Java Programming 135 / 135