switch-case Statements

Transcription

1 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 AP Computer Science A: Java Programming 106 / 143

2 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 AP Computer Science A: Java Programming 107 / 143

3 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 AP Computer Science A: Java Programming 108 / 143

4 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 AP Computer Science A: Java Programming 109 / 143

5 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 AP Computer Science A: Java Programming 110 / 143

6 1 class Lecture4 { 2 3 "Loops" 4 5 } 6 7 // keywords: 8 while, do, for, break, continue Zheng-Liang Lu AP Computer Science A: Java Programming 111 / 143

7 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 AP Computer Science A: Java Programming 112 / 143

8 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 AP Computer Science A: Java Programming 113 / 143

9 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 AP Computer Science A: Java Programming 114 / 143

10 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 AP Computer Science A: Java Programming 115 / 143

11 Zheng-Liang Lu AP Computer Science A: Java Programming 116 / 143

12 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 AP Computer Science A: Java Programming 117 / 143

13 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 AP Computer Science A: Java Programming 118 / 143

14 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 AP Computer Science A: Java Programming 119 / 143

15 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 AP Computer Science A: Java Programming 120 / 143

16 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 AP Computer Science A: Java Programming 121 / 143

17 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 AP Computer Science A: Java Programming 122 / 143

18 Sentinel-Controlled Loops 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 AP Computer Science A: Java Programming 123 / 143

19 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 AP Computer Science A: Java Programming 124 / 143

20 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 AP Computer Science A: Java Programming 125 / 143

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22 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 AP Computer Science A: Java Programming 127 / 143

23 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 AP Computer Science A: Java Programming 128 / 143

24 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 AP Computer Science A: Java Programming 129 / 143

25 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 AP Computer Science A: Java Programming 130 / 143

26 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 AP Computer Science A: Java Programming 131 / 143

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28 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 AP Computer Science A: Java Programming 133 / 143

29 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 AP Computer Science A: Java Programming 134 / 143

30 Example: Monte Carlo Simulation 6 Write a program which conducts a Monte Carlo simulation to estimate π. 6 See Zheng-Liang Lu AP Computer Science A: Java Programming 135 / 143

31 Jump Statements The keyword break and continue are often used in repetition structures to provide additional controls. break: the loop is terminated right after a break statement is executed. continue: the loop skips this iteration right after a continue statement is executed. In practice, jump statements in loops should be conditioned. Zheng-Liang Lu AP Computer Science A: Java Programming 136 / 143

32 Example: Primality Write a program which determines if the input integer is a prime number. Let x > 1 be any natural number. Then x is said to be a prime number if x has no positive divisors other than 1 and itself. It is then straightforward to check if it is prime by dividing x by all natural numbers smaller than x. For speedup, you can divide x by only numbers smaller than x. (Why?) Zheng-Liang Lu AP Computer Science A: Java Programming 137 / 143

33 2 Scanner input = new Scanner(System.in); 3 System.out.println("Enter x > 2?"); 4 int x = input.nextint(); 5 boolean isprime = true; 6 input.close(); 7 8 for (int y = 2; y < Math.sqrt(x); y++) { 9 if (x % y == 0) { 10 isprime = false; 11 break; 12 } 13 } if (isprime) { 16 System.out.println("Prime"); 17 } else { 18 System.out.println("Composite"); 19 } Zheng-Liang Lu AP Computer Science A: Java Programming 138 / 143

34 Exercise (Revisited) Redo the cashier problem by using an infinite loop with a break statement. 2 while (true) { 3 System.out.println("Enter price ( 1 to exit):"); 4 curritem = input.nextint(); 5 if (curritem >= 0) { 6 amount += curritem; 7 } else if (curritem == 1) { 8 System.out.println("Total = " + amount); 9 break; 10 } 11 } Zheng-Liang Lu AP Computer Science A: Java Programming 139 / 143

35 Another Example Write a program which determines the holding years for an investment doubling its value. Let curr be the current amount, goal be the goal of this investment, and r be the annual interest rate. Then this investment should take at least n years so that the balance of the investment can double its value. Recall that the compounding formula is given by curr = curr (1 + r/100). Zheng-Liang Lu AP Computer Science A: Java Programming 140 / 143

36 2 int r = 18; // 18% 3 int curr = 100; 4 int goal = 200; 5 6 int years = 0; 7 while (curr <= goal) { 8 curr = (1 + r / 100.0); 9 years++; 10 } System.out.println("Years = " + years); 13 System.out.println("Balance = " + curr); Zheng-Liang Lu AP Computer Science A: Java Programming 141 / 143

37 2 int years = 0; // should be declared here; scope issue 3 for (; curr <= goal; years++) { 4 curr = (1 + r / 100); 5 } int years = 1; // check this initial value 3 for (; true; years++) { 4 curr = (1 + r / 100); 5 if (curr >= goal) break; 6 } 7... A for loop can be an infinite loop by setting true or simply leaving empty in the condition statement. An infinite for loop with an if-break statement is equivalent to a normal while loop. Zheng-Liang Lu AP Computer Science A: Java Programming 142 / 143

38 Equivalence: while and for Loops (Concluded) In general, a for loop may be used if the number of repetitions is known in advance. If not, a while loop is preferred. Zheng-Liang Lu AP Computer Science A: Java Programming 143 / 143