Chapter 5 Loops PDF

Chapter 5 Loops Liang, Introduction to Java Programming, Tenth Edition, (c) 1 2015 Pearson Education, Inc. All  Motivations Suppose that you need to print a string (e.g., "Welcome to Java!") a hundred times. It would be t...

Chapter 5 Loops Liang, Introduction to Java Programming, Tenth Edition, (c) 1 2015 Pearson Education, Inc. All  Motivations Suppose that you need to print a string (e.g., "Welcome to Java!") a hundred times. It would be tedious to have to write the following statement a hundred times: System.out.println("Welcome to Java!"); So, how do you solve this problem? Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 2  Opening Problem Problem: System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!"); 100 times … … … System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!"); Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 3  Introducing while Loops int count = 0; while (count < 100) { System.out.println("Welcome to Java"); count++; } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 4  Objectives  To write programs for executing statements repeatedly using a while loop (§5.2).  To follow the loop design strategy to develop loops (§§5.2.1–5.2.3).  To control a loop with a sentinel value (§5.2.4).  To obtain large input from a file using input redirection rather than typing from the keyboard (§5.2.5).  To write loops using do-while statements (§5.3).  To write loops using for statements (§5.4).  To discover the similarities and differences of three types of loop statements (§5.5).  To write nested loops (§5.6).  To learn the techniques for minimizing numerical errors (§5.7).  To learn loops from a variety of examples (GCD, FutureTuition, Dec2Hex) (§5.8).  To implement program control with break and continue (§5.9).  To write a program that displays prime numbers (§5.11). Liang, Introduction to Java Programming, Tenth Edition, (c) 5 2015 Pearson Education, Inc. All  while Loop Flow Chart int count = 0; while (loop-continuation-condition) { while (count < 100) { // loop-body; System.out.println("Welcome to Java!"); Statement(s); count++; } } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 6 animation Trace while Loop Initialize count int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 7 animation Trace while Loop, cont. (count < 2) is true int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 8 animation Trace while Loop, cont. Print Welcome to Java int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 9 animation Trace while Loop, cont. Increase count by 1 int count = 0; count is 1 now while (count < 2) { System.out.println("Welcome to Java!"); count++; } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 10 animation Trace while Loop, cont. (count < 2) is still true since count int count = 0; is 1 while (count < 2) { System.out.println("Welcome to Java!"); count++; } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 11 animation Trace while Loop, cont. Print Welcome to Java int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 12 animation Trace while Loop, cont. Increase count by 1 int count = 0; count is 2 now while (count < 2) { System.out.println("Welcome to Java!"); count++; } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 13 animation Trace while Loop, cont. (count < 2) is false since count is 2 int count = 0; now while (count < 2) { System.out.println("Welcome to Java!"); count++; } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 14 animation Trace while Loop The loop exits. Execute the next int count = 0; statement after the loop. while (count < 2) { System.out.println("Welcome to Java!"); count++; } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 15  Problem: Repeat Addition Until Correct Recall that Listing 3.1 AdditionQuiz.java gives a program that prompts the user to enter an answer for a question on addition of two single digits. Using a loop, you can now rewrite the program to let the user enter a new answer until it is correct. IMPORTANT NOTE: If you cannot run the buttons, see www.cs.armstrong.edu/liang/javaslidenote.doc. RepeatAdditionQuiz Run Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 16 import java.util.Scanner; public class RepeatAdditionQuiz { public static void main(String[] args) { int number1 = (int)(Math.random() * 10); int number2 = (int)(Math.random() * 10); // Create a Scanner Scanner input = new Scanner(System.in); System.out.print( "What is " + number1 + " + " + number2 + "? "); int answer = input.nextInt(); while (number1 + number2 != answer) { System.out.print("Wrong answer. Try again. What is " + number1 + " + " + number2 + "? "); answer = input.nextInt(); } System.out.println("You got it!"); } } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 17  Problem: Guessing Numbers Write a program that randomly generates an integer between 0 and 100, inclusive. The program prompts the user to enter a number continuously until the number matches the randomly generated number. For each user input, the program tells the user whether the input is too low or too high, so the user can choose the next input intelligently. Here is a sample run: GuessNumberOneTime Run GuessNumber Run Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 18 import java.util.Scanner; public class GuessNumberOneTime { public static void main(String[] args) { // Generate a random number to be guessed int number = (int)(Math.random() * 101); Scanner input = new Scanner(System.in); System.out.println("Guess a magic number between 0 and 100"); // Prompt the user to guess the number System.out.print("\nEnter your guess: "); int guess = input.nextInt(); if (guess == number) System.out.println("Yes, the number is " + number); else if (guess > number) System.out.println("Your guess is too high"); else System.out.println("Your guess is too low"); } } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 19 Problem: An Advanced Math Learning Tool The Math subtraction learning tool program generates just one question for each run. You can use a loop to generate questions repeatedly. This example gives a program that generates five questions and reports the number of the correct answers after a student answers all five questions. SubtractionQuizLoop Run Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 20 import java.util.Scanner; public class SubtractionQuizLoop { public static void main(String[] args) { final int NUMBER_OF_QUESTIONS = 5; // Number of questions int correctCount = 0; // Count the number of correct answers int count = 0; // Count the number of questions long startTime = System.currentTimeMillis(); String output = ""; // output string is initially empty Scanner input = new Scanner(System.in); while (count < NUMBER_OF_QUESTIONS) { // 1. Generate two random single-digit integers int number1 = (int)(Math.random() * 10); int number2 = (int)(Math.random() * 10); // 2. If number1 < number2, swap number1 with number2 if (number1 < number2) { int temp = number1; number1 = number2; number2 = temp; } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 21 // 3. Prompt the student to answer "What is number1 – number2?" System.out.print( "What is " + number1 + " - " + number2 + "? "); int answer = input.nextInt(); // 4. Grade the answer and display the result if (number1 - number2 == answer) { System.out.println("You are correct!\n"); correctCount++; } else System.out.println("Your answer is wrong.\n" + number1 + " - " + number2 + " should be " + (number1 - number2) + "\n"); // Increase the count count++; output += "\n" + number1 + "-" + number2 + "=" + answer + ((number1 - number2 == answer) ? " correct" : " wrong"); } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 22 long endTime = System.currentTimeMillis(); long testTime = endTime - startTime; System.out.println("Correct count is " + correctCount + "\nTest time is " + testTime / 1000 + " seconds\n" + output); } } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 23 Ending a Loop with a Sentinel Value Often the number of times a loop is executed is not predetermined. You may use an input value to signify the end of the loop. Such a value is known as a sentinel value. Write a program that reads and calculates the sum of an unspecified number of integers. The input 0 signifies the end of the input. SentinelValue Run Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 24 import java.util.Scanner; public class SentinelValue { public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Read an initial data System.out.print( "Enter an integer (the input ends if it is 0): "); int data = input.nextInt(); // Keep reading data until the input is 0 int sum = 0; while (data != 0) { sum += data; // Read the next data System.out.print( "Enter an integer (the input ends if it is 0): "); data = input.nextInt(); } System.out.println("The sum is " + sum); } } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 25  Caution Don’t use floating-point values for equality checking in a loop control. Since floating-point values are approximations for some values, using them could result in imprecise counter values and inaccurate results. Consider the following code for computing 1 + 0.9 + 0.8 +... + 0.1: double item = 1; double sum = 0; while (item != 0) { // No guarantee item will be 0 sum += item; item -= 0.1; } System.out.println(sum); Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 26  do-while Loop do { // Loop body; Statement(s); } while (loop-continuation-condition); Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 27  for Loops for (initial-action; loop- int i; continuation-condition; action- for (i = 0; i < 100; i++) { after-each-iteration) { System.out.println( // loop body; Statement(s); "Welcome to Java!"); } } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 28 animation Trace for Loop Declare i int i; for (i = 0; i < 2; i++) { System.out.println( "Welcome to Java!"); } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 29 animation Trace for Loop, cont. Execute initializer int i; i is now 0 for (i = 0; i < 2; i++) { System.out.println( "Welcome to Java!"); } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 30 animation Trace for Loop, cont. (i < 2) is true int i; since i is 0 for (i = 0; i < 2; i++) { System.out.println( "Welcome to Java!"); } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 31 animation Trace for Loop, cont. Print Welcome to Java int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 32 animation Trace for Loop, cont. Execute adjustment statement int i; i now is 1 for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 33 animation Trace for Loop, cont. (i < 2) is still true int i; since i is 1 for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 34 animation Trace for Loop, cont. Print Welcome to Java int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 35 animation Trace for Loop, cont. Execute adjustment statement int i; i now is 2 for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 36 animation Trace for Loop, cont. (i < 2) is false int i; since i is 2 for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 37 animation Trace for Loop, cont. Exit the loop. Execute the next int i; statement after the loop for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 38  Note The initial-action in a for loop can be a list of zero or more comma-separated expressions. The action-after-each- iteration in a for loop can be a list of zero or more comma- separated statements. Therefore, the following two for loops are correct. They are rarely used in practice, however. for (int i = 1; i < 100; System.out.println(i++)); for (int i = 0, j = 0; (i + j < 10); i++, j++) { // Do something } Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 39  Note If the loop-continuation-condition in a for loop is omitted, it is implicitly true. Thus the statement given below in (a), which is an infinite loop, is correct. Nevertheless, it is better to use the equivalent loop in (b) to avoid confusion: for ( ; ; ) { Equivalent while (true) { // Do something // Do something } } (a) (b) Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 40  Caution Adding a semicolon at the end of the for clause before the loop body is a common mistake, as shown below: Logic Error for (int i=0; i