cs230 Lecture Notes
Week 11 Friday

Homework 9 solutions
--------------------

Guardian patterns

A good one (from main in Sort.java)

       if (args.length == 0 || args[0] == null || args[0].length() == 0) {
            c = 's';
        } else {
            c = args[0].charAt(0);          
        }

A not so good one (from merge in Sort.java)

            if (i == a1.length || j != a2.length && a2[j] > a1[i]) {
                result[k] = a2[j];  j++;
            } else {
                result[k] = a1[i];  i++;
            }

Homework 10
-----------

Key, Value, Key-Value pair

Keys are unique -- values are not necessarily.

Example: count the frequency of the words in a file

    public void processWord (String word) {
        Integer i = (Integer) ht.get (word);
        if (i == null) {
            ht.put (word, new Integer (1));
        } else {
            Integer j = new Integer (i.intValue() + 1);
            ht.put (word, j);
        }
    }


How do we traverse the entries in a table?

The keys method returns an Enumeration!

    (just like an Iterator -- left over from previous)

        Enumeration enum = ht.keys ();
        while (enum.hasMoreElements ()) {
            String key = (String) enum.nextElement ();


How do we find the top 20?

class Pair implements Comparable {
    String s, Integer w;
}


Table implementation
--------------------

class KeyValuePair {
    Object key, value;

    public KeyValuePair (Object key, Object value) {
        this.key = key;
        this.value = value;
    }

    public String toString () {
        return "{ " + key + ", " + value + " }";
    }
}

Vector implementation of a Table

public class Table {
    Vector v;

    public void put (Object key, Object value) {
        KeyValuePair pair = new KeyValuePair (key, value);
        v.add (pair);
    }

    public Object get (Object key) {
        Iterator iterator = v.iterator ();
        while (iterator.hasNext ()) {
            KeyValuePair pair = (KeyValuePair) iterator.next ();
            if (key.equals (pair.key)) {
                return pair.value;
            }
        }
        return null;
    }
}

Actually, put has to update the thing if it already exists.

Quiz Monday!