University of Washington, CSE 142 (190)

Lab 5: while loops, fencepost, Random numbers, boolean logic, midterm practice

Except where otherwise noted, the contents of this document are Copyright 2010 Stuart Reges and Marty Stepp.

lab document created by Whitaker Brand and Marty Stepp

Today's lab

Goals for today:

• use while loops for indefinite repetition
• exposure to fencepost and sentinel loop patterns
• use Random objects to produce random numbers
• use boolean expressions and variables to represent logical true/false expressions
• examine logical assertions that can be made about a running program
• practice problems similar to what will be on the midterm exam
• Where you see this icon, you can click it to check the problem in Practice-It!

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) {
    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 : 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.
• Try solving this problem in Practice-It! using the link above.
• (Hint: To extract a digit from a number, use / 10 and % 10 operations.)

Exercise : digitSum - solution

public static int digitSum(int n) {
    n = Math.abs(n);            // handle negatives
    int sum = 0;
    while (n > 0) {
        int lastDigit = n % 10;
        sum = sum + lastDigit;  // add last digit to sum
        n = n / 10;             // remove last digit from n
    }
    return sum;
}

Recall: Random Methods

To use these methods, you need a variable of type Random in scope:

Random randy = new Random();
int aRandomNumber = randy.nextInt(10);  // 0-9

Exercise : Random Expressions

• Fill in the boxes to produce expressions that will generate random numbers in the provided ranges. Assume that the following Random variable has been declared:

  Random rand = new Random();
  

Exercise : Boolean Expressions

Write the result of each expression as either true or false, given the following variables. Recall the logical operators: && (and), || (or), ! (not).

int x = 12;
int y = 7;
int z = 28;
String s = "mid term";

true

false

true

true

false

true

true

false

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

Try solving this problem in Practice-It! from the link above.

Exercise : assertions

Identify whether each assertion is always/never/sometimes true at each point.

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);
}

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 : hopscotch

Write a method named hopscotch that accepts an integer parameter for a number of "hops" and prints a hopscotch board of that many hops.

For example, the call hopscotch(3); would produce the following output:

   1
2     3
   4
5     6
   7
8     9
   10

Try solving this problem in Practice-It: click on the check-mark above!

Exercise : hasMidpoint

Write a method hasMidpoint that accepts three integers as parameters, and returns true if one of the numbers is the midpoint of the other two and returns false otherwise.

For example, the call hasMidpoint(3, 7, 5) would return true because one of the parameters (5) is the midpoint of the other two (3 and 7).

Try to solve this problem in Practice-It: click on the check-mark above!

Exercise : longestName

Write a method named longestName that reads names typed by the user and prints the longest name (the name that contains the most characters) in the format shown below. Your method should accept a console Scanner and an integer n as parameters and should then prompt for n names.

A sample execution of the call longestName(console, 4) might look like the following:

name #1? roy
name #2? DANE
name #3? sTeFaNiE
name #4? Erik
Stefanie's name is longest

Try to solve this problem in Practice-It: click on the check-mark above!

Exercise : Syntax errors

• The following Java program has 11 errors. Can you find them all?

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

  public class StringOops { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("Type your name: "); String name = console.nextString(); process(name); } public static void process(string "name") { if (name == Whitaker) { System.out.println("You must be really awesome."); } replace("a", "e"); toUppercase(name); name.substring(0, 3); System.out.println(name + " has " + name.length + " letters"); } }

• Copy and paste the code into jGrasp and see if you can fix the errors.

Exercise - answer

1. line 5: nextString should be next
2. line 9: string should be String
3. line 9: name should not be in quotes
4. line 10: Whitaker should be in quotes
5. line 10: cannot compare strings with ==; must use .equals
6. line 13: cannot call replace without specifying a string object (name)
7. line 14: toUppercase should be toUpperCase
8. line 14: name. should come before toUpperCase, not passed as a parameter to it
9. line 14: must say name = to store the result of toUpperCase
10. line 15: must say name = to store the result of substring
11. line 16: must use parentheses () when calling length

Exercise - Corrected version

public class StringOops {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("Type your name: ");
        String name = console.next();
        process(name);
    }

    public static void process(String "name") {
        if (name.equals("Whitaker")) {
            System.out.println("You must be really awesome.");
        }
        name = name.replace("a", "e");
        name = name.toUpperCase();
        name = name.substring(0, 3);
        System.out.println(name + " has " + name.length() + " letters");
    }
}

Exercise : ProcessName

• Copy/paste and save this program in jGRASP, 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.println("Type your name: ");
          
          // your code goes here
          
          System.out.println("Your name is: " + name);
      }
  }
  

Exercise - code to add

• Add code to the program so that it reads the user's first and last name (read an entire line as a single string), then prints the last name followed by a comma and the first initial. (Assume that the user types a valid name.) Example:

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

• (Notice that the program reads an entire line of user input, not just one word.)

Exercise - answer

• The following is a completed version of the program:

  import java.util.*;  // for Scanner
  
  public class ProcessName {
      public static void main(String[] args) {
          Scanner console = new Scanner(System.in);
          System.out.println("Type your name: ");
          
          // your code goes here
          String name = console.nextLine();
          int space = name.indexOf(" ");
          String first = name.substring(0, space);
          String last = name.substring(space + 1, name.length());
          String firstInitial = first.substring(0, 1);
          name = last + ", " + firstInitial + ".";
          
          System.out.println("Your name is: " + name);
      }
  }
  

Exercise : ProcessName2

• Modify your 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.

Exercise - answer

import java.util.*;  // for Scanner

public class ProcessName2 {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.println("Type your name: ");
        String name = console.nextLine();
        
        while (name.length() < 5 || name.indexOf(" ") < 0) {
            System.out.println("Error, must be at least 5 chars with a space.");
            System.out.println("Type your name: ");
            name = console.nextLine();
        }
        
        int space = name.indexOf(" ");
        String first = name.substring(0, space);
        String last = name.substring(space + 1, name.length());
        String firstInitial = first.substring(0, 1);
        name = last + ", " + firstInitial + ".";
        System.out.println("Your name is: " + name);
    }
}

Exercise : repl

• Write a method named repl that accepts a String and a number of repetitions as parameters and returns the String concatenated that many times. For example, the call repl("hello", 3) returns "hellohellohello". If the number of repetitions is 0 or less, an empty string is returned.
• Try solving this problem in Practice-It! using the link above.
• (Hint: This is best solved with a cumulative algorithm. Start with an empty string and build it up piece by piece.)

Exercise : ZuneBug

The following code from Microsoft's Zune music player calculates today's date from the years/days passed since 1980. But all Zune players locked up on Jan 1 2009. Why? Download ZuneBug.java and modify it to fix the bug.

int days = getTotalDaysSince1980();   // pretend this method exists
int year = 1980;
while (days > 365) {                  // subtract out years
    if (isLeapYear(year)) {           // pretend this method exists
        if (days > 366) {
            days = days - 366;
            year++;
        }
    } else {
        days = days - 365;
        year++;
    }
}

Exercise : ZuneBug - answer

The bug occurs when the current year is a leap year and there are exactly 366 days left (i.e., if today is Jan 1 on a year after a leap year). The code gets stuck in an infinite loop with days == 366 because the while test is true but the if (days > 366) is false. Here is a fixed version:

int days = getTotalDaysSince1980();   // pretend this method exists
int year = 1980;
while (days > 365 || (isLeapYear(year) && days > 366)) {
    if (isLeapYear(year)) {
        days = days - 366;
    } else {
        days = days - 365;
    }
    year++;
}

Exercise : printLetters

Consider the following flawed method printLetters, which accepts a String as its parameter and attempts to print the letters of the String, separated by dashes. For example, the call of printLetters("Rabbit") should print R-a-b-b-i-t .

public static void printLetters(String text) {
    for (int i = 0; i < text.length(); i++) {
        System.out.print(text.substring(i, i + 1) + "-");
    }
    System.out.println();   // to end the line of output
}

What is wrong with the code? Paste the code in Practice-It! and fix it.

Exercise : printLetters - answer

public static void printLetters(String text) {
    if (text.length() > 0) {
        System.out.print(text.substring(0, 1));   // fencepost
        for (int i = 1; i < text.length(); i++) {
            System.out.print("-" + text.substring(i, i + 1));
        }
        System.out.println();   // to end the line of output
    }
}

Exercise : printFactors

Write a method named printFactors that accepts an integer as its parameter and uses a fencepost loop to print the factors of that number, separated by the word "and". For example, the call printFactors(24) should print as the following output:

1 and 2 and 3 and 4 and 6 and 8 and 12 and 24

You may assume that the parameter value passed is greater than 0.

• Try solving this problem in Practice-It! using the link above.
• (Hint: This is an example of a fencepost problem.)

Exercise : swapPairs

• Write a method named swapPairs that accepts a String as a parameter and returns that String with each pair of adjacent letters reversed. If the String has an odd number of letters, the last letter is unchanged. For example, the call swapPairs("forget") should return "ofgrte" and the call swapPairs("hello there") should return "ehll ohtree".
• Try solving this problem in Practice-It! using the link above.

Exercise : "Boolean Zen"

This attempted solution to Self-Check 5.15 (isVowel) has several problems:

// Returns whether the given string represents a vowel:
// a, e, i, o, or u, case insensitively.
public static boolean isVowel(String s) {
    if (s == "a") {
        return true;
    } else if (s == "e") {
        return true;
    } else if (s == "i") {
        return true;
    } else if (s == "o") {
        return true;
    } else if (s == "u") {
        return true;
    } else {
        return false;
    }
}

Open Practice-It from the link above, copy/paste this code into it, then see the next slide.

Exercise - things to fix

Fix the following aspects of the code:

• It has a bug related to how strings are compared.
• It isn't case sensitive; it fails for uppercase vowels.
• It has too many unnecessary if/else statements.
• (advanced) It does not use "Boolean Zen" as described in textbook section 5.3.

Exercise - answer

public static boolean isVowel(String s) {
    s = s.toLowerCase();
    if (s.equals("a") || s.equals("e") || s.equals("i")
            || s.equals("o") || s.equals("u")) {
        return true;
    } else {
        return false;
    }
}

The above can be improved. "Boolean Zen" version:

public static boolean isVowel(String s) {
    s = s.toLowerCase();
	return s.equals("a") || s.equals("e") || s.equals("i")
	        || s.equals("o") || s.equals("u");
}

If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from Chapter 5 and try to solve them!