[ Exercises | Chapter Index | Main Index ]

## Solution for Programming Exercise 5.6

This page contains a sample solution to one of the exercises from Introduction to Programming Using Java.

### Exercise 5.6:

Exercise 4.7 asked you to write a program that administers a 10-question addition quiz. Rewrite that program so that it uses the following class to represent addition questions:

    public class AdditionQuestion {

        private int a, b;  // The numbers in the problem.

        public AdditionQuestion() { // constructor
            a = (int)(Math.random() * 50 + 1);
            b = (int)(Math.random() * 50);
        }

        public String getQuestion() {
            return "What is " + a + " + " + b + " ?";
        }

        public int getCorrectAnswer() {
            return a + b;
        }

    }

Discussion

The original program used two array of type int[] to represent the addition questions. One array held the first number from each of the ten problems, while the other array held the second number. This representation is an example of parallel arrays: The data for a single question is split between two arrays. (The arrays are called "parallel" because to see all the data for a problem next to each other, you would have to lay the arrays down next to each other, like parallel lines, so the i-th element in one array lies next to the i-th element in the other array.)

The new version of the program can use an array of ten objects of type AdditionQuestion to represent the quiz. The data that we need for any one question can now be found in the same place -- the object that represents that question. This makes for a neater solution than using parallel arrays.

Some of the functionality that we need to create, administer, and grade the quiz has been incorporated into the AdditionQuestion class. For example, the constructor in that class already selects random numbers to be used in the problem, so creating a problem is just a matter of calling the constructor, so there is no need to use random numbers when making a problem in the createQuiz() method. Also, to get the question and correct answer for a problem, we can just call questions[i].getQuestion() and questions[i].getCorrectAnswer(). In fact, we can't get the question any other way, since the two instance variables in an ArithmeticQuestion are private. One downside of this is that I couln't format the output from gradeQuis() as neatly as in the first version.

All-in-all, t is fairly straightforward to translate the old version of the program into the new version. You can read my solution below.

By the way, I considered adding the user's answer to the AdditionQuestion class, along with the correct answer, but the user's answer doesn't seem to me to be logically part of a question. So I think it makes more sense to keep it out of the object that represents the question.

Remember that this program depends on the AdditionQuiz class that is given in the statement of the problem.

The Solution

    /**
     * This program administers a ten-question addition quiz to the user.  The numbers
     * for the problem are chosen at random.  The numbers and the answers are one or
     * two digits.  After asking the user the ten questions, the computer grades the
     * quiz, telling the user the correct answer for any problem they got wrong.
     */
    public class AdditionQuizWithObjects {

        private static AdditionQuestion[] questions;  // The questions for the quiz

        private static int[] userAnswers;   // The user's answers to the ten questions.

        public static void main(String[] args) {
            System.out.println();
            System.out.println("Welcome to the addition quiz!");
            System.out.println();
            createQuiz();
            administerQuiz();
            gradeQuiz();
        }

        /**
         * Creates the array of objects that holds the quiz questions
         */
        private static void createQuiz() {
            questions = new AdditionQuestion[10];
            for ( int i = 0; i < 10; i++ ) {
                questions[i] = new AdditionQuestion();
            }
        }

        /**
         * Asks the user each of the ten quiz questions and gets the user's answers.
         * The answers are stored in an array, which is created in this subroutine.
         */
        private static void administerQuiz() {
            userAnswers = new int[10];
            for (int i = 0; i < 10; i++) {
                int questionNum = i + 1;
                System.out.printf("Question %2d:  %s ",
                                      questionNum, questions[i].getQuestion());
                userAnswers[i] = TextIO.getlnInt();
            }
        }

        /**
         * Shows all the questions, with their correct answers, and computes a grade
         * for the quiz.  For each question, the user is told whether they got
         * it right.
         */
        private static void gradeQuiz() {
            System.out.println();
            System.out.println("Here are the correct answers:");
            int numberCorrect = 0;
            int grade;
            for (int i = 0; i < 10; i++) {
                int questionNum = i + 1;
                System.out.printf("   Question %2d:  %s  Correct answer is %d  ",
                    questionNum, questions[i].getQuestion(), questions[i].getCorrectAnswer());
                if ( userAnswers[i] == questions[i].getCorrectAnswer() ) {
                    System.out.println("You were CORRECT.");
                    numberCorrect++;
                }
                else {
                    System.out.println("You said " + userAnswers[i] + ", which is INCORRECT.");
                }
            }
            grade = numberCorrect * 10;
            System.out.println();
            System.out.println("You got " + numberCorrect + " questions correct.");
            System.out.println("Your grade on the quiz is " + grade);
            System.out.println();
        }

    } // end class AdditionQuizWithObjects

[ Exercises | Chapter Index | Main Index ]