Week 8(Maps)

1. Read http://java.sun.com/j2se/1.4.2/docs/api/java/util/AbstractMap.html.

2. Try week5/WordSort.java on the file week5/rrr.

3. Replace the TreeMap with HashMap in week5/WordSort.java and see the difference.

4. A Map <String,Integer> can be thought of as a function which maps Strings to Integers.

   For example, if we run WordSort on rrr we get:

   ape : 3
   cat : 1
   dog : 1
   paper : 2
   rabbit : 3
   schlongle : 1
   

   This function can be inverted to a function which maps Integers to Sets of Strings as follows:

   1 : [cat, schlongle, dog]
   2 : [paper]
   3 : [rabbit, ape]
   

   Write a method whose heading is:

   public static TreeMap <Integer,HashSet <String>> invert (TreeMap<String,Integer> t)
   

   which produces a TreeMap which inverts the TreeMap in the way described. week5/WordSort1.java

5. Write a generic class which generalises the notion of inverting a function. Solution

6. Rewrite week5/WordSort1.java to use this generic invert. Solution

7. (Easy Assignment) Rewrite Solution so we map each word to its first Character. i.e We now have

   TreeMap <String,Character> map = new TreeMap <String,Character>(  );
   

   Watch video about easy assignment for week 5

   Solution

8. (Hard Assignment) Given the generic class

   public class pair <I,S> {
    
   I i;
   S s;  
    
   	public pair (I i1, S s1)
   	{
   		i=i1; s=s1;
   	
   	}
   
   	public String toString()
   	{
   		return "("+i+","+s+")";
   	}
   }
   

   Complete the following generic class:

   public class mapToSetPair <A,B> {
      public HashSet <pair<A,B>> make (TreeMap <A,HashSet <B>> t)
     {
     	 	
     }
    
    
   }
   

   Which from a function from A to Set(B) produces a HashSet of pairs in A X B. e.g. applied to rrr gives the set of pairs:

   (a,apple)
   (r,rock)
   (c,cat)
   (r,rabbit)
   (a,ape)
   (s,schlongle)
   (d,dog)
   (p,paper)
   

   Solution

   Adapt WordSort4.java to use it. Solution

   Week 8 Resources