CSC 212 –Data Structures

Lecture 31:

Last Word On Dictionaries

Problem of the Day

What is the smallest positive integer that cannot be defined in less than twenty-five syllables?

What is the smallest positive integer that cannot be defined in less than twenty-five syllables?

The highlighted phrase has fewer than 25 syllables, so this cannot exist!

Comparing Data Items

Ordering entries means comparing keys Comparison depends on key’s type

Use <, >, == for numeric dataUse compareTo() for StringsWhat about Car, IMClient, or Prof instances?

Want abstract approach to this problemNo OrderedDictionary class for each key type

Comparator ADT

Implements a total order relationObjects less than, equal to, or greater thanFollows rule that if a > b & b > c, then a > c

Comparator’s methods are abstract Implementation is specific to key typeBut still independent of the key class

Can reweight keys by changing how comparator orders them

Comparator Interface

public interface Comparator<E> { public int compare(E x, E y);}public class StrComp implements Comparator<String> { public int compare(String x, String y){ return(x.compareTo(y));

}}

Returns integer < 0 when x < y 0 when x == y

> 0 when x > y

Ordered Dictionaries

Ordered dictionary constructor includes Comparator parameter Now works with any type of key!

Can rewrite binary search more abstractly as:if (c.compare(key, table[m].getKey()) > 0) { l = m + 1;} else if (c.compare(key, table[m].getKey()) < 0) { h = m - 1;} else { return m;}

Ordered Dictionary

Key feature of ordered dictionary: they maintain entries in orderCan be a performance win (faster searching)But can also be an important feature

Think of how first use a dictionary:Q: “Mom, how do I spell _______?”A: “Look it up.”

Cannot do this with Dictionary ADT

Ordered Dictionary Interface

public interface OrderedDictionary<K,V> extends Dictionary<K,V> { public Entry<K,V> first(); // Entry with smallest key public Entry<K,V> last(); // Entry with largest key public Iterator<Entry<K,V>> successors(K k); public Iterator<Entry<K,V>> predecessors(K k);

}

Includes all methods in Dictionary Includes first() & last() methodssuccessors() iterates over larger keyspredecessors() iterates through smaller keysMay not include entries with the key k

Writing an Ordered Dictionary

public class ODict<K,V> implements... {private IndexList<Entry<K,V>> table;private Comparator<K> comp;

public Entry<K,V> first() throws EmptyDictionaryException { try { return table.get(0); } catch (NoSuchElementException e) { throw EmptyDictionaryException(“Dummkopf.”); }

}

Writing an Ordered Dictionary

public Iterator<Entry<K,V>> successors(K k) {IteratorClass<Entry<K,V>> retVal = new ...; int rank = binSearch(k);while (rank < table.size()) { if (comp.compare(k, table.get(rank).getKey)==0) { rank++; } else { break; }}for (; rank < table.size(); rank++) { retVal.addLast(table.get(rank));}return retVal;

}

Why Should We Care?

We often care about ordering dataPricesSchedulesQPAsNumber of watts consumedProcessor speed

Your Turn

Get back into groups and do activity

Before Next Lecture…

Keep up with your reading!Cannot stress this enough

Finish Week #12 Assignment Start Programming Assignment #4 Prepare for Midterm #2 next Monday

Will also be open book, open noteCovers from last midterm through this week