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## Solution for Programming Exercise 4.8

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

### Exercise 4.8:

Write a program that administers a basic addition quiz to the user. There should be ten questions. Each question is a simple addition problem such as 17 + 42, where the numbers in the problem are chosen at random (and are not too big). The program should ask the user all ten questions and get the user's answers. After asking all the questions, the user should print each question again, with the user's answer. If the user got the answer right, the program should say so; if not, the program should give the correct answer. At the end, tell the user their score on the quiz, where each correct answer counts for ten points.

The program should use three subroutines, one to create the quiz, one to administer the quiz, and one to grade the quiz. It can use three arrays, with three global variables of type int[], to refer to the arrays. The first array holds the first number from every question, the second holds the second number from every questions, and the third holds the user's answers.

Discussion

The statement of the problem has already made a lot of decisions for us. We are told to use three global variables of type int[]. The variables must be static, and I have chosen to make them private, so the declarations become:

    private static int[] firstNumbers;  // The first numbers in all ten questions.
    private static int[] secondNumbers; // The second numbers in all ten questions.
    private static int[] userAnswers;   // The user's answers to the ten questions.

Note that these declarations are not in any subroutine. These statements only create variables, not arrays. We still need to create three arrays of length 10. The arrays can be created using the new operator. This initialization could in fact be combined with the declaration of the variables:

    private static int[] firstNumbers = new int[10];
    private static int[] secondNumbers = new int[10];
    private static int[] userAnswers = new int[10];

Another possibility would be to create the three arrays at the beginning of the main() routine. In fact, I chose to create them in the subroutines where they are first used.

The program has three stages—create the problems, administer the quiz, and grade the quiz—and we are told to write a subroutine to do each stage. The main program simply has to call the subroutines:

    public static void main(String[] args) {
        createQuiz();
        administerQuiz();
        gradeQuiz();
    }

(However, I added a couple of output statements at the start.) This is an example of using subroutines to break up a large task into several simpler subtasks, so that programming each subtask becomes a separate problem. The subroutines are short and not too hard to write. Looking at administerQuiz() as an example, note that the user's answers have to be stored in the array, userAnswers, so that they can be used when grading the quiz. I create the array itself before asking any questions, and then store the user's responses into the array as they are read from the user. On the other hand, the arrays that hold the numbers that occur in the questions have already been created and filled before administerQuiz() is called. Here's the 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:  What is %2d + %2d  ? ",
                          questionNum, firstNumbers[i], secondNumbers[i]);
            userAnswers[i] = TextIO.getlnInt();
        }
    }

A couple notes about the output: I wanted to number the questions 1, 2, ..., 10, but the elements of the arrays are numbered 0, 1, ..., 9. This means that the question number that I output has to be one more than the array index. Also, all the numbers that occur in my program are either one or two digits. By allowing exactly two digits for each number in the output, I can get everything to line up neatly. Here's the I/O for a sample run of the program:

    Welcome to the addition quiz!

    Question  1:  What is 40 + 14  ? 54
    Question  2:  What is 18 + 38  ? 46
    Question  3:  What is 21 + 31  ? 52
    Question  4:  What is 18 + 36  ? 54
    Question  5:  What is 43 + 12  ? 55
    Question  6:  What is 21 + 17  ? 38
    Question  7:  What is 33 + 25  ? 58
    Question  8:  What is 39 + 21  ? 70
    Question  9:  What is  1 + 25  ? 26
    Question 10:  What is 39 + 26  ? 65

    Here are the correct answers:
       Question  1:  40 + 14  =  54.  You were CORRECT.
       Question  2:  18 + 38  =  56.  You said 46, which is INCORRECT.
       Question  3:  21 + 31  =  52.  You were CORRECT.
       Question  4:  18 + 36  =  54.  You were CORRECT.
       Question  5:  43 + 12  =  55.  You were CORRECT.
       Question  6:  21 + 17  =  38.  You were CORRECT.
       Question  7:  33 + 25  =  58.  You were CORRECT.
       Question  8:  39 + 21  =  60.  You said 70, which is INCORRECT.
       Question  9:   1 + 25  =  26.  You were CORRECT.
       Question 10:  39 + 26  =  65.  You were CORRECT.

    You got 8 questions correct.
    Your grade on the quiz is 80

I noted in Subsection 4.3.8 that it's considered bad style to use global variables where parameters could be used instead, and that some people are quite hostile to unnecessary global variables. You might ask, how bad was it to use global variables in this case? To some extent this is a matter of opinion. You could argue that the whole program revolves around the three arrays, so it makes sense for them to be global in the program. On the other hand, it is possible to avoid using globals in this case. The three array variables can be declared in main() and the three arrays can be created there and passed as parameters to the subroutines. Here is a main routine that does that:

    public static void main(String[] args) {
        int[] firstNums = new int[10];  // The first numbers in the ten problems
        int[] secondNums = new int[10]; // The second numbers in the ten problems
        int[] answers = new int[10];    // The user's answers.
        System.out.println();
        System.out.println("Welcome to the addition quiz!");
        System.out.println();
        createQuiz(firstNums,secondNums);
        administerQuiz(firstNums,secondNums,answers);
        gradeQuiz(firstNums,secondNums,answers);
    }

A complete second version of the program, with no global variables, is given below. One reason that I asked you to use global variables is that understanding exactly how arrays work as parameters requires an understanding of objects and how they are passed as parameters. Without that understanding, it's hard to get the global-less version correct. You will learn about passing objects as parameters in the next chapter.

The Solution

    Here is the original program, using global variables:

    import textio.TextIO;

    /**
     * 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 AdditionQuiz {

        private static int[] firstNumbers;  // The first numbers in all ten questions.
        private static int[] secondNumbers; // The second numbers in all ten questions.
        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 arrays that hold the numbers for the questions and fills
         * them with random numbers.
         */
        private static void createQuiz() {
            firstNumbers = new int[10];
            secondNumbers = new int[10];
            for ( int i = 0; i < 10; i++ ) {
                firstNumbers[i] = (int)(Math.random() * 50 + 1);  // in the range 1 to 50
                secondNumbers[i] = (int)(Math.random() * 50); // in the range 0 to 49
            }
        }

        /**
         * 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:  What is %2d + %2d  ? ",
                              questionNum, firstNumbers[i], secondNumbers[i]);
                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;
                int correctAnswer = firstNumbers[i] + secondNumbers[i];
                System.out.printf("   Question %2d:  %2d + %2d  =  %2d.  ",
                    questionNum, firstNumbers[i], secondNumbers[i], correctAnswer);
                if ( userAnswers[i] == correctAnswer ) {
                    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 AdditionQuiz

    Here is a version that uses parameters and no global variables:

    import textio.TextIO;

    /**
     * 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 AdditionQuizNoGlobals {

        public static void main(String[] args) {
            int[] firstNums = new int[10];  // The first numbers in the ten problems
            int[] secondNums = new int[10]; // The second numbers in the ten problems
            int[] answers = new int[10];    // The user's answers.
            System.out.println();
            System.out.println("Welcome to the addition quiz!");
            System.out.println();
            createQuiz(firstNums,secondNums);
            administerQuiz(firstNums,secondNums,answers);
            gradeQuiz(firstNums,secondNums,answers);
        }

        /**
         * Creates the arrays that hold the numbers for the questions and fills
         * them with random numbers.  The parameters are arrays that will hold
         * the random numbers for the first and second operands of each addition
         * problem.  The arrays must have already been created when this subroutine
         * is called!
         */
        private static void createQuiz(int[] firstNumbers, int[] secondNumbers) {
            for ( int i = 0; i < 10; i++ ) {
                firstNumbers[i] = (int)(Math.random() * 50 + 1);  // in the range 1 to 50
                secondNumbers[i] = (int)(Math.random() * 50); // in the range 0 to 49
            }
        }

        /**
         * Asks the user each of the ten quiz questions and gets the user's answers.
         * The first two parameters hold the operands for the quiz questions.  The user's
         * answers to the ten problems will be stored in the third array.  All arrays
         * exist before this subroutine is called, and the first two arrays have
         * already been filled with values.
         */
        private static void administerQuiz(int[] firstNumbers, int[] secondNumbers, int[] userAnswers) {
            for (int i = 0; i < 10; i++) {
                int questionNum = i + 1;
                System.out.printf("Question %2d:  What is %2d + %2d  ? ",
                              questionNum, firstNumbers[i], secondNumbers[i]);
                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.  The first two parameters hold the operands for the quiz questions,
         * and the third parameter holds the answers that the user gave to the quiz.
         */
        private static void gradeQuiz(int[] firstNumbers, int[] secondNumbers, int[] userAnswers) {
            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;
                int correctAnswer = firstNumbers[i] + secondNumbers[i];
                System.out.printf("   Question %2d:  %2d + %2d  =  %2d.  ",
                    questionNum, firstNumbers[i], secondNumbers[i], correctAnswer);
                if ( userAnswers[i] == correctAnswer ) {
                    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 AdditionQuizNoGlobals

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