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Indefinite loop variations

Learn how to use the do/while loop in Java, which executes the body statement(s) at least once, regardless of the condition. Includes code examples and suggested reading.

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Indefinite loop variations

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  1. Indefinite loop variations suggested reading: 5.4

  2. The do/while loop • Java has another kind of loop named the do/while loop. • It is almost identical to the while loop, except that its body statement(s) will always execute the first time, regardless of whether the condition is true. • The do/while loop, general syntax: do { <statement(s)>; } while (<condition>); • Example: // roll until we get a number other than 3 Random rand = new Random(); int dice; do { dice = rand.nextInt(); } while (dice == 3);

  3. do/while loop flow chart

  4. "Forever" loop with break • break statement: Immediately exits a loop when reached. • Can be used to write a loop whose test is in the middle. • Such loops are often called "forever" loops because their header's boolean test is often changed to a trivial true. • "forever" loop, general syntax: while (true) { <statement(s)>; if (<condition>) { break; } <statement(s)>; }

  5. More correct code • This code correctly implements a sentinel loop: int sum = 0; while (true) { System.out.print("Enter a number (-1 to quit): "); int inputNumber = console.nextInt(); if (inputNumber == -1) { break; } sum += inputNumber; // inputNumber != -1 here } System.out.println("The total was " + sum);

  6. Random/while question • Write a multiplication tutor program. Example log of execution: This program helps you practice multiplication by asking you random multiplication questions with numbers ranging from 1 to 20 and counting how many you solve correctly. 14 * 8 = 112 Correct! 5 * 12 = 60 Correct! 8 * 3 = 24 Correct! 5 * 5 = 25 Correct! 20 * 14 = 280 Correct! 19 * 14 = 256 Incorrect; the correct answer was 266 You solved 5 correctly. • Use a class constant for the maximum value of 20.

  7. User errors suggested reading: 5.3

  8. Testing for valid user input • A Scanner object has methods that can be used to "look ahead" to test whether the upcoming input token is of a given type: • Each of these methods waits for the user to type input tokens and press Enter, then reports a true or false answer. • The hasNext and hasNextLine methods are not useful until we learn how to read input from files in Chapter 6.

  9. Scanner condition example • The Scanner's hasNext methods are useful for testing whether the user typed the right kind of token for our program to use, before we read it (and potentially cause an exception!). Scanner console = new Scanner(System.in); System.out.print("How old are you? "); if (console.hasNextInt()) { int age = console.nextInt(); // will not throw an exception System.out.println("Retire in " + (65 - age) + " years."); } else if (console.hasNextDouble()) { System.out.println("Please use a whole number for your age!"); } else { System.out.println("You did not type a number."); }

  10. Assertions suggested reading: 5.5

  11. Logical assertions • assertion: A declarative statement that is either true or false. Examples: • Java was created in 1995. • The sky is purple. • 7 is a prime number. • 10 > 20. • x % 2 == 0. (depends on the value of x)

  12. Assertions in code • We can make assertions about our code and ask whether they are true at various points in the code. • Valid answers are ALWAYS, NEVER, or SOMETIMES. System.out.print("Type a nonnegative number: "); double number = console.nextDouble(); // Point A: is number < 0.0 here? while (number < 0.0) { // Point B: is number < 0.0 here? System.out.print("Negative; try again: "); number = console.nextDouble(); // Point C: is number < 0.0 here? } // Point D: is number < 0.0 here?

  13. Assertions in code answers • We can make assertions about our code and ask whether they are true at various points in the code. • Valid answers are ALWAYS, NEVER, or SOMETIMES. System.out.print("Type a nonnegative number: "); double number = console.nextDouble(); // Point A: is number < 0.0 here? (SOMETIMES) while (number < 0.0) { // Point B: is number < 0.0 here? (ALWAYS) System.out.print("Negative; try again: "); number = console.nextDouble(); // Point C: is number < 0.0 here? (SOMETIMES) } // Point D: is number < 0.0 here? (NEVER)

  14. Assertion example 1 public static int mystery(Scanner console) { int prev = 0; int count = 0; int next = console.nextInt(); // Point A while (next != 0) { // Point B if (next == prev) { // Point C count++; } prev = next; next = console.nextInt(); // Point D } // Point E return count; } Which of the following assertions are true at which point(s) in the code? Choose ALWAYS, NEVER, or SOMETIMES.

  15. Assertion example 1 answer public static int mystery(Scanner console) { int prev = 0; int count = 0; int next = console.nextInt(); // Point A while (next != 0) { // Point B if (next == prev) { // Point C count++; } prev = next; next = console.nextInt(); // Point D } // Point E return count; } Which of the following assertions are true at which point(s) in the code? Choose ALWAYS, NEVER, or SOMETIMES.

  16. Assertion example 2 public static void mystery(int x, int y) { int z = 0; // Point A while (x >= y) { // Point B x -= y; // Point C z++; // Point D } // Point E System.out.println(z + " " + x); } Which of the following assertions are true at which point(s) in the code? Choose ALWAYS, NEVER, or SOMETIMES.

  17. Assertion example 2 answer public static void mystery(int x, int y) { int z = 0; // Point A while (x >= y) { // Point B x -= y; // Point C z++; // Point D } // Point E System.out.println(z + " " + x); } Which of the following assertions are true at which point(s) in the code? Choose ALWAYS, NEVER, or SOMETIMES.

  18. Assertion example 3 // pre : y >= 0, post: returns x^y public static int pow(int x, int y) { int prod = 1; // Point A while (y > 0) { // Point B if (y % 2 == 0) { // Point C x *= x; y /= 2; // Point D } else { // Point E prod *= x; y--; // Point F } // Point G } // Point H return prod; } Which of the following assertions are true at which point(s) in the code? Choose ALWAYS, NEVER, or SOMETIMES.

  19. Assertion example 3 answer // pre : y >= 0, post: returns x^y public static int pow(int x, int y) { int prod = 1; // Point A while (y > 0) { // Point B if (y % 2 == 0) { // Point C x *= x; y /= 2; // Point D } else { // Point E prod *= x; y--; // Point F } // Point G } // Point H return prod; } Which of the following assertions are true at which point(s) in the code? Choose ALWAYS, NEVER, or SOMETIMES.

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