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13.12: Exercises

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    The code for this chapter is in the ch05 directory of ThinkJavaCode. See [Section 0.4] for instructions on how to download the repository. Before you start the exercises, we recommend that you compile and run the examples.

    If you have not already read Appendix A.6, now might be a good time. It describes the DrJava debugger, which is a useful tool for tracing the flow of execution.

    Exercise \(\PageIndex{1}\)

    Logical operators can simplify nested conditional statements. For example, can you rewrite this code using a single if statement?

    if (x > 0) {
        if (x < 10) {
            System.out.println("positive single digit number.");

    Exercise \(\PageIndex{2}\)

    For the following program:

    1. Draw a stack diagram that shows the state of the program the second time zoop is invoked.
    2. What is the complete output?
    public static void zoop(String fred, int bob) {
        if (bob == 5) {
            ping("not ");
        } else {
    public static void main(String[] args) {
        int bizz = 5;
        int buzz = 2;
        zoop("just for", bizz);
        clink(2 * buzz);
    public static void clink(int fork) {
        System.out.print("It's ");
        zoop("breakfast ", fork);
    public static void ping(String strangStrung) {
        System.out.println("any " + strangStrung + "more ");

    Exercise \(\PageIndex{3}\)

    Draw a stack diagram that shows the state of the program in Section 5.8 after main invokes nLines with the parameter n == 4, just before the last invocation of nLines returns.

    Exercise \(\PageIndex{4}\)

    Fermat’s Last Theorem says that there are no integers a, b, and c such that an + bn = cn, except when n ≤ 2.

    Write a method named checkFermat that takes four integers as parameters – a, b, c and n – and checks to see if Fermat’s theorem holds. If n is greater than 2 and an + bn = cn, the program should display “Holy smokes, Fermat was wrong!” Otherwise the program should display “No, that doesn’t work.”


    You may want to use Math.pow.

    Exercise \(\PageIndex{5}\)

    The purpose of this exercise is to take a problem and break it into smaller problems, and to solve the smaller problems by writing simple methods. Consider the first verse of the song “99 Bottles of Beer”:

    99 bottles of beer on the wall,
    99 bottles of beer,
    ya’ take one down, ya’ pass it around,
    98 bottles of beer on the wall.

    Subsequent verses are identical except that the number of bottles gets smaller by one in each verse, until the last verse:

    No bottles of beer on the wall,
    no bottles of beer,
    ya’ can’t take one down, ya’ can’t pass it around,
    ’cause there are no more bottles of beer on the wall!

    And then the song (finally) ends.

    Write a program that displays the entire lyrics of “99 Bottles of Beer”. Your program should include a recursive method that does the hard part, but you might want to write additional methods to separate other parts of the program. As you develop your code, test it with a small number of verses, like 3.

    Exercise \(\PageIndex{6}\)

    This exercise reviews the flow of execution through a program with multiple methods. Read the following code and answer the questions.

    public class Buzz {
        public static void baffle(String blimp) {
            zippo("ping", -5);
        public static void zippo(String quince, int flag) {
            if (flag < 0) {
                System.out.println(quince + " zoop");
            } else {
        public static void main(String[] args) {
            zippo("rattle", 13);
    1. Write the number 1 next to the first line of code in this program that will execute.
    2. Write the number 2 next to the second line of code, and so on until the end of the program. If a line is executed more than once, it might end up with more than one number next to it.
    3. What is the value of the parameter blimp when baffle gets invoked?
    4. What is the output of this program?

    Exercise \(\PageIndex{7}\)

    Now that we have conditional statements, we can get back to the “Guess My Number” game from Exercise 3.12.4.

    You should already have a program that chooses a random number, prompts the user to guess it, and displays the difference between the guess and the chosen number.

    Adding a small amount of code at a time, and testing as you go, modify the program so it tells the user whether the guess is too high or too low, and then prompts the user for another guess.

    The program should continue until the user gets it right. Hint: Use two methods, and make one of them recursive.

    This page titled 13.12: Exercises is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Allen B. Downey (Green Tea Press) .

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