Monday, April 16, 2012

Test 3

Here are the two tests, with some sample solutions.
  1. (25%) What does the following program print out when run?
    public class Q1 {
    public static boolean swing (int strength) throws Exception{
        if (strength < 5)
            throw new Exception("Too weak");
        if (strength > 10)
            return false;
        return true;
    }
    public static void main(String[] args)  {
        int[] strengths = {5,6,-1,12};
        for (int s : strengths) {
            try {
                if (swing(s))
                    System.out.println("Homerun");
                else
                    System.out.println("Strike!");
            } catch (RuntimeException e){
                System.out.println("You are OUT!");
            } catch (Exception e) {
                System.out.println(e.getMessage());
            }
        }
        System.out.println("Done.");
    }
}

  2. (25%)Given a text file called sales.txt which contains data that looks like 
    aba 55.99
akdkd8dj 12.95
iwjwhe 4
Iji8keJl 44.99
ijdddawe 10.00
...and so on for many more rows...
    where each row represents one item that is for sale in a store. The first column is a String that uniquely identifies the item and the second column is a double that represents the price of the item. Write a program which


    1. Implements a simple Item class that can represent one item.
    2. In the main, your program reads the complete contents of the file sales.txt into an ArrayList<Item> variable called items.
    Answer:
    public class Item {
    private String sku;

    private double price;

    public Item(String sku, double price){
        this.sku = sku;
        this.price = price;
    }

    public String toString(){
        return sku + " " + price;
    }
    public static void main(String[] args) {
        try {
            Scanner in = new Scanner(new FileInputStream("sales.txt"));
            ArrayList<Item> items = new ArrayList<Item>();
            while (in.hasNext()){
                String sku = in.next();
                double price = in.nextDouble();
                items.add(new Item(sku,price));
            }
            System.out.println(items);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
   }
}
  3. (25%) Read over the code below and implement the empty method.
    import java.util.ArrayList;

public class Q3 {

    private ArrayList<Q3> data;

    private Integer number;

    public Q3(Integer number){
        data = new ArrayList<Q3>();
        this.number = number;
    }

    public void add(Q3 q){
        data.add(q);
    }

    public void add(Integer number){
        data.add(new Q3(number));
    }
    
    /**
     * Calculates and returns the sum of all even numbers in
     * this object or in any of the ones in 'data', and so on
     * recursively.
     * @return The sum.
     */
    public int sumEven(){
        //TODO: implement this method
        int result = 0;
        if (number % 2 == 0)
            result = number;
        for (Q3 q: data){
            result += q.sumEven();
        }
        return result;
    }
    

    public static void main(String[] args) {
        Q3 q = new Q3(0);
        Q3 first = new Q3(1);
        q.add(first);
        Q3 second = new Q3(2);
        q.add(second);
        q.add(3);
        q.add(4);
        first.add(8);
        second.add(5); 
        System.out.println(q.sumEven());
    }
}
    For example, when the main above is run it should print out
    14
  4. (25%) Implement a method String mostFrequent(String s) which returns the 3-character substring that appears most frequently in s, or one of the most frequent if there is a tie. For example
    mostFrequent("AABABA") returns "ABA" because "ABA" appears twice
mostFrequent("AAAAAB") returns "AAA" because it appears 3 times
mostFrequent("ATATATA") returns "ATA" because it appears 3 times
    Answer:
    public static String mostFrequent(String seq){
        HashMap<String, Integer> count = new HashMap<String, Integer>();
        int max = 0;
        String maxKey = null;
        //add them all
        for (int i = 0; i <= seq.length() - 3; i++) {
            String key = seq.substring(i,i+3);
            if (count.containsKey(key))
                count.put(key, count.get(key) + 1);
            else
                count.put(key, 1);
            if (count.get(key) > max){
                max = count.get(key);
                maxKey = key;
            }
        }
        return maxKey;
    }



And, the other one:


  1. (25%) What does the following program print out when run?

    public class Q1 {
    public static boolean pitch (String type) throws Exception {
        if (type.startsWith("f"))
            return true;
        if (type.startsWith("c"))
            return false;
        if (type.startsWith("x"))
            throw new Exception("Oops");
        return true;
    } 
    public static void main(String[] args)  {
        String[] types = {"fast", "foo", "curve", "xoom"};
        for (String t : types) {
            try {
                if (pitch(t))
                    System.out.println("Hit");
                else
                    System.out.println("Miss");
            } catch (RuntimeException e){
                System.out.println("You hit the umpire");
            }
            catch (Exception e) {
                System.out.println(e.getMessage());
            }
        }
        System.out.println("Done.");
    }
}

  2. (25%) Given a text file called points.txt which contains data that looks like 
    1.44 5.666
12.999 0
9.163 3.1415
6 7
5 6.999
...and so on for many more rows...
    where each row represents the x and y coordinates of a point in 2-dimensional space. Both columns are doubles. Write a program which


    1. Implements a simple Point class that can represent one point in space.
    2. In the main, your program reads the complete contents of the file points.txt into an ArrayList<Point> variable called points.
    Answer:
    public class Point {

    private double x;
    private double y;
    public Point(double x, double y){
        this.x = x;
        this.y = y;
    }
    public String toString(){
        return "(" + x + "," + y + ")";
    }
    public static void main(String[] args) {
        try {
            Scanner in = new Scanner(new FileInputStream("points.txt"));
            ArrayList<Point> points = new ArrayList<Point>();
            while (in.hasNext()){
                double x= in.nextDouble();
                double y = in.nextDouble();
                points.add(new Point(x,y));
            }
            System.out.println(points);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}
  3. (25%) Read over the code below and implement the empty method.
    import java.util.ArrayList;

public class Q3 {

    private ArrayList<Q3> data;

    private Integer number;

    public Q3(Integer number){
        data = new ArrayList<Q3>();
        this.number = number;
    }

    public void add(Q3 q){
        data.add(q);
    }

    public void add(Integer number){
        data.add(new Q3(number));
    }

    
    /**
     * Find if x is either equal to number, or equal to one of
     * the numbers in 'data', and so on recursively.
     * @param x the number we are looking for
     * @return true if x is there, false otherwise.
     */
    public boolean find(Integer x){
        //TODO: implement this method
        if (number == x)
            return true;
        for (Q3 q : data) {
            if (q.find(x))
                return true;
        }
        return false;
    }

    public static void main(String[] args) {
        Q3 q = new Q3(0);
        Q3 first = new Q3(1);
        q.add(first);
        Q3 second = new Q3(2);
        q.add(second);
        q.add(3);
        q.add(4);
        first.add(8);
        second.add(5); 
        System.out.println(q.find(3));
        System.out.println(q.find(5));
        System.out.println(q.find(10));
    }
}
    For example, when the main above is run it should print
    out
    true
true
false
  4. (25%) Implement a method String mostPoints(String[] name, int[] points) which returns the name that has the most points, assuming that for all indeces i we have that points[i] has the number of points for name[i]. For example
       String[] name1 = {"a", "b", "c", "a", "b", "a"};
   int[] points1 =  { 1,   2,   7,   4 ,  3,   1};
   mostPoints(name1, points1); //returns "c" because it has 7
   // points and "a" has 6, and "b" has 5.

   String[] name2 = {"a", "b", "c", "a", "b", "a"};
   int[] points2 =  { 5,   2,   7,   4 ,  3,   1};
   mostPoints(name2, points2); //returns "a"

   String[] name3 = {"a", "a", "a", "a", "b", "c"};
   int[] points3 =  { 1,   1,   1,   1 ,  3,   2};
   mostPoints(name3, points3); //returns "a"
    Answer:
    public static String mostPoints(String[] name, int[] points){
        HashMap<String, Integer> count = new HashMap<String, Integer>();
        int max = 0;
        String maxKey = null;

        for (int i = 0; i < points.length; i++) {
            if (count.containsKey(name[i]))
                count.put(name[i], points[i] + count.get(name[i]));
            else
                count.put(name[i], points[i]);
            if (count.get(name[i]) > max){
                max = count.get(name[i]);
                maxKey = name[i];
            }
        }
        return maxKey;
    }
Posted by jmvidal at 4/16/2012 05:00:00 PM
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