StepTracker free response answer 2019

StepTracker free response problem from the 2019 AP Computer Science A Exam

StepTracker is #2 from the from the 2019 AP Computer Science A Free Response problems.

https://apcentral.collegeboard.org/pdf/ap19-frq-computer-science-a.pdf?course=ap-computer-science-a

StepTracker class

public class StepTracker
{
    private final int minStepsActive;
    private int activeDays;
    private int days;
    private int totalSteps;

    public StepTracker(int minStepsActive)
    {
        this.minStepsActive = minStepsActive;
        activeDays = 0;
        days = 0;
        totalSteps = 0;
    }

    public void addDailySteps(int steps)
    {
        if(steps >= minStepsActive)
            activeDays++;

        days++;
        totalSteps += steps;
    }

    public int activeDays()
    {
        return activeDays;
    }

    public double averageSteps()
    {
        if(days == 0)
            return 0;

        return totalSteps / (double) days;
    }
}

It is common for students to unnecessarily store an Arraylist. This makes the code inside the methods unnecessarily complex. See Class writing order for a technique to respond to AP CS FR that request an entire class.

StepTracker class (alternate solution)

I’ve received many questions about whether this problem can/should be done using an ArrayList. There is no benefit to using an ArrayList vs storing only the data you actually need to implement the methods. That said, a correct solution using an ArrayList would receive full credit.

public class StepTracker
{
    private final int minStepsActive;
    private ArrayList<Integer> stepsPerDay;

    public StepTracker(int minStepsActive)
    {
        this.minStepsActive = minStepsActive;
        stepsPerDay = new ArrayList<Integer>();
    }

    public void addDailySteps(int steps)
    {
        stepsPerDay.add(steps);
    }

    public int activeDays()
    {
        int activeDays = 0;

        for(Integer steps : stepsPerDay)
            if(steps >= minStepsActive)
                activeDays++;

        return activeDays;
    }

    public double averageSteps()
    {
        if(stepsPerDay.size() == 0)
            return 0;

        int totalSteps = 0;

        for(Integer steps : stepsPerDay)
            totalSteps += steps;

        return (double) totalSteps / stepsPerDay.size();
    }
}

Again, I don’t recommend this solution. I’ve added it here in response to the many questions I’ve received about it.

2019 AP CS Exam Free Response Solutions

Additional classes & objects resources

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