University of Washington, AP/CS A

Lab 5: while loops

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp, Stuart Reges and Whitaker Brand

Today's lab

Goals for this lab:

• use while loops for indefinite repetition
• Where you see this icon, you can click it to check the problem in Practice-It!

while Loops

for loops are fantastic for when we know how many times we want to repeat something. But sometimes we won't know how many times we'll want to repeat something in advance! while loops repeat indefinitely while a given condition is true.

while (test) {
    statement(s);
}

Example:

int num = 1;
while (num < 5) {
    System.out.print(num + " ");     // output: 1 2 3 4
    num++;
}

Exercise : while loop basics

Consider the following loop.

int x = 1;
System.out.print(x);
while (x < 100) {
    x = x + x;
    System.out.print(", " + x);
}

Exercise : while loop mystery

Fill in the boxes at right with the output produced by each method call.

public static void mystery(int x) {
    int y = 1;
    int z = 0;
    while (2 * y <= x) {
        y = y * 2;
        z++;
    }
    System.out.println(y + " " + z);
}

1 0

4 2

16 4

32 5

64 6

Exercise : while loop mystery

Fill in the boxes at right with the output produced by each method call.

public static void mystery2(int x, int y) {
    int z = 0;
    while (x % y != 0) {
        x = x / y;
        z++;
        System.out.print(x + ", ");
    }

    System.out.println(z);
}

12, 1

0

1034, 103, 10, 3

31, 15, 7, 3, 1, 0, 6

Exercise : while loop mystery

Fill in the boxes at right with the output produced by each method call.

public static void mystery3(int x) {
    int y = 0;
    while (x % 2 == 0) {
        y++;
        x = x / 2;
    }
    System.out.println(x + " " + y);
}

19 0

21 1

3 4

5 3

1 6

Exercise : while loop mystery

Fill in the boxes at right with the output produced by each method call. (Review how to do this type of problem in lab 5!)

public static void mystery4(int n) {
    int x = 1;
    int y = 2;
    while (y < n) {
        if (n % y == 0) {
            n = n / y;
            x++;
        } else {
            y++;
        }
    }
    System.out.println(x + " " + n);
}

1 2

1 5

4 3

3 7

Exercise : assertions

Write always/never/sometimes true at each point. If unsure how to, check out the assertions tutorial!

public static void mystery(int x, int y) {
    int z = 0;

    // Point A
    while (x != y) {
        // Point B
        z++;
        if (x > y) {
            // Point C
            x = x / 10;
        } else {
            // Point D
            y = y / 10;
        }
    }

    // Point E
    System.out.println(x + " " + y + " " + z);
}

You can also solve this problem in Practice-It by clicking on the check-mark above.

Exercise : allDigitsOdd

Write a method named allDigitsOdd that returns whether every digit of a positive integer is odd. Your method should return true if the number consists entirely of odd digits and false if any of its digits are even. 0, 2, 4, 6, and 8 are even digits, and 1, 3, 5, 7, 9 are odd digits.

For example, allDigitsOdd(135319) returns true but allDigitsOdd(9145293) returns false.

Hint: You can pull apart a number into its digits using / 10 and % 10.

Exercise : firstDigit

• Write a method named firstDigit that returns the first digit of an integer. For example, firstDigit(3572) should return 3. It should work for negative numbers as well. For example, firstDigit(-947) should return 9.
• Try solving this problem in Practice-It! using the link above.
• (Hint: This problem requires a loop.)

Exercise : digitsInARow

Write a static method called digitsInARow that takes an integer n as a parameter and that returns the highest number of digits that appear in a row in the base-10 representation of n. For many numbers the answer will be 1 because they don't have adjacent digits that match. But for a number like 3555585, the answer is 4 because there are four occurrences of the digit 5 that appear in a row. Below are sample calls on the method.

Method                  Value          Method                  Value
 Call                  Returned         Call                  Returned
-------------------------------        -------------------------------
digitsInARow(0)           1            digitsInARow(8823)        2
digitsInARow(18)          1            digitsInARow(777)         3
digitsInARow(394)         1            digitsInARow(82888)       3
digitsInARow(99)          2            digitsInARow(7111171)     4
digitsInARow(8229)        2            digitsInARow(233333888)   5

You are not allowed to use a string to solve this problem. You may assume that the value passed to the method is greater than or equal to 0.

Hint: You can pull apart a number into its digits using / 10 and % 10.

Solve this problem in Practice-It by clicking on the check-mark above.

Exercise : digitSum

Write a method named digitSum that accepts an integer as a parameter and returns the sum of the digits of that number. For example, the call digitSum(29107) returns 2+9+1+0+7 or 19. For negative numbers, return the same value that would result if the number were positive. For example, digitSum(-456) returns 4+5+6 or 15. The call digitSum(0) returns 0.

(Hint: This is a cumulative algorithm. To extract a digit from a number, use / 10 and % 10 operations.)

Exercise : flip

Write a method flip that takes a Random object as a parameter and that prints information about a coin-flipping simulation.

Your method should use the Random object to produce a sequence of simulated coin flips, printing whether each flip comes up "heads" or "tails". Each outcome should be equally likely. Your method should stop flipping when you see three heads in a row.

Solve this problem in Practice-It by clicking on the check-mark above.

Exercise : makeGuesses

Write a method named makeGuesses that will output random numbers between 1 and 50 inclusive until it outputs one of at least 48. Output each guess and the total number of guesses made. Below is a sample execution:

guess = 43
guess = 47
guess = 45
guess = 27
guess = 49
total guesses = 5

If you're not sure how make/use a Random, review them here.

Exercise : printAverage

• Write a method named printAverage that accepts a Scanner for the console as a parameter and repeatedly prompts the user for numbers. Once any number less than zero is typed, the average of all non-negative numbers typed is displayed. Display the average as a double, and do not round it. For example, a call to your method might look like this:

  Scanner console = new Scanner(System.in);
  printAverage(console);
  

  The following is one example log of execution for your method:

  Type a number: 10
  Type a number: 15
  Type a number: -1
  Average was 12.5
  

  If the first number typed is negative, do not print an average.
• Try solving this problem in Practice-It! using the link above.
• (Hint: To find the average, you will need to know both the sum and a count of the number of numbers to average.)

Exercise : ProcessName

Copy/paste the following code into jGRASP, or download it. Then go to the next slide.

import java.util.*;  // for Scanner

public class ProcessName {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("Type your name: ");
        
        // read the entire input as a single line
        String input = console.nextLine();
        
        // your code goes here
        
        System.out.println("Your name is: " + name);
    }
}

continued on the next slide ...

Exercise - code to add

Add code to the program so that it reads the user's first and last name (reading the entire line of input as a single string), then prints the last name followed by a comma and the first initial. Assume that the user types a single first name, a space, and then a single last name.

Type your name: Jessica Miller
Your name is: Miller, J.

Exercise : ProcessName Solution

import java.util.*;  // for Scanner

public class ProcessName {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("Type your name: ");
        
        // read the entire input as a single line
        String input = console.nextLine();
        
        int spaceIndex = input.indexOf(" ");
        String lastName = input.substring(spaceIndex + 1);
        char firstInitial = input.charAt(0);
        
        String name = lastName + ", " + firstInitial + ".";
        
        System.out.println("Your name is: " + name);
    }
}

Exercise : ProcessName2

Modify your previous ProcessName program so that it re-prompts until the user types a name that is at least 5 letters total in length and has at least one space in it. Example:

Type your name: Joe
Error, must be at least 5 chars with a space.
Type your name: O K!
Error, must be at least 5 chars with a space.
Type your name: what
Error, must be at least 5 chars with a space.
Type your name: Tyler Durden
Your name is: Durden, T.