COP 3337
  Section U05
  Spring 2017



                         PRACTICE MIDTERM EXAM ANSWERS
                         =============================




  Question 1. (15 points)

    // return the percentage of cars in the array
    public static double CarPercentage(Vehicle[] arr)
    {

        // check for empty array
        if (arr == null || arr.length == 0)
            throw new IllegalArgumentException("Empty array");

        int cars = 0; // the number of cars;

        // count the number of cars
        for (Vehicle v : arr)
            if (v instanceof Car)
                cars++;

        // compute the percentage
        return (double) cars / arr.length * 100.00;

    }


  Question 2. (15 points)

  mysteryF(3,5) = 10

  Question 3. (30 points)

    // the isNavlakha method recognizes the Navlakha
    // strings
    public static boolean isNavlakha(String in)
    {
        if (in == null)
            return false;
        // the base cases
        if (in.equals("abab") || in.equals("bab"))
            return true;
        int len = in.length();
        // the base cases are the only Navlakha strings
        // shorter than 7 and every Navlakha string ends in b
        if (len < 7 || in.charAt(len-1) != 'b')
           return false;
        // treat case 3
        if (in.charAt(0)=='a' && len % 2 == 0)
        {
            // get the 2 S's
            int mid = len/2;
            String s1 = in.substring(1,mid);
            String s2 = in.substring(mid,len-1);
            if (s1.equals(s2) && isNavlakha(s1))
                return true;
        }    
        
        // check rule 4
        if (in.charAt(0) == 'b')
            for (int i = 4; i < len -3; i++)
            {
                // extract u and v                
                String u = in.substring(1,i);
                String v = in.substring(i);
                if (isNavlakha(u) && isNavlakha(v))
                    return true;
            }    
        // all 4 cases failed
        return false;
    }    

  Question 4. (20 points)

  1. F  2. T  3. F  4. T  5. T  6. T  7. F  8. F  9. F  10. F  11. T  

  12. F  13. F  14. T  15. F  16. F  17. T  18. F  19. F  20. T