University of Washington, CSE 142 (190)

Lab 5: while loops, Strings, and fencepost loops

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 Strings to represent and manipulate text data
• more practice downloading textbook program examples and using the jGRASP debugger
• use while loops for indefinite repetition
• write methods that return values based on various tests and loops
• exposure to fencepost and sentinel loop patterns
• Where you see this icon, you can click it to check the problem in Practice-It!

Recall: String Methods

Exercise : String Expressions

Write the results of each expression with Strings in "quotes" and characters in single quotes ('a')

//       index 0123456789012345
String str1 = "Frodo Baggins";
String str2 = "Gandalf the GRAY";

13

'a'

'G'

2

"GANDALF THE GRAY"

-1

" Baggins"

"dalf the GR"

"Goondoolf the GRAY"

"Gandalf the GRAY"

"strange1"

Exercise : Getting textbook code files

We are once again going to download a program from the textbook that we will use for a debugging exercise. To do so, follow these steps:

1. Go to the class web page.
2. Click on the "textbook" tab.
3. Find the section labeled "Code Files" and click on the "code files" link.
4. This will bring you to a directory listing that includes an entry for each chapter. Click on the link for "ch4".
5. You want to download and save the file Hailstone.java. Right-click on the file name and choose the option to save the link in whatever folder you have been using for lab work.
6. Compile and run Hailstone.java in jGRASP.

Exercise : jGRASP Debugger

We are going to practice setting stops and continuing execution. We will explore Hailstone.java. This program computes a sequence known as a hailstone sequence. This is related to an unsolved problem in mathematics known as the Collatz Conjecture.

To complete these problems, you will want to set breakpoints and to use the "Resume" button that resumes execution. The "Resume" button is the one that uses a standard "play" icon (the sixth one from the left below):

continued on the next slide...

Exercise - jGRASP Debugger

The first call on the method asks it to compute the sequence starting with the value 7. Set a stop that will allow you to determine the first five values that the variable value takes on not counting the initial value of 7.

22

11

34

17

52

continued on the next slide...

Exercise - jGRASP Debugger

You should still be in the middle of the first execution of the method. Clear your old stop by clicking on it a second time and set a stop for the second method call in main. Then resume execution. You should end up back in main just before the second call is made.

Now we want to determine the sequence of values that the variable min takes on in this second call. It is initialized to the first value of 7 and is reset when a new minimum is encountered. Set a stop that will allow you to determine the first four values that the variable min takes on not counting the initial value of 7.

5

4

2

1

Exercise : while loop basics

Consider the following loop.

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

How many times does the code in the body while loop above execute? 7

What output is produced? 1, 2, 4, 8, 16, 32, 64, 128

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 : 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 = 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 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 : 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 : printLetters

• Write a method called printLetters that takes a String as its parameter and that prints the letters of the String, separated by commas. For example, the call of printLetters("Rabbit") should print:

  R, a, b, b, i, t
  

• Try solving this problem in Practice-It! using the link above.
• (Hint: This is a fencepost problem. Remember that each String object has a length method that tells you how many characters are in the String and a charAt method that gets you individual characters of the String.)

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 : lastDigit

• Write a method named lastDigit that returns the last digit of an integer. For example, lastDigit(3572) should return 2. It should work for negative numbers as well. For example, lastDigit(-947) should return 7.
• Try solving this problem in Practice-It! using the link above.
• (Hint: This is a short method.)

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 : season

• Write a method named season that takes two integers as parameters representing a month and day and that returns a String indicating the season for that month and day. Assume that months are specified as an integer between 1 and 12 (1 for January, 2 for February, and so on) and that the day of the month is a number between 1 and 31.
• If the date falls between 12/16 and 3/15, you should return "Winter". If the date falls between 3/16 and 6/15, you should return "Spring". If the date falls between 6/16 and 9/15, you should return "Summer". And if the date falls between 9/16 and 12/15, you should return "Fall".
• Try solving this problem in Practice-It! using the link above.

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 : 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;
}

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 : 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 : 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.

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!