CSE 12 Basic Data Structures. Dr. Cynthia Bailey Lee

Transcription

1 CSE 12 Basic Data Structures Dr. Cynthia Bailey Lee

2 2 Today s Topics 1. Map interface revisited Quick const tips (common troubles with PQ) Templates (similar to Java generics) 2. TreeMap 3. HashMap

3 Review: the Map Interface

4 Map Interface Want to hold (key, value) associations Want to be able to look up values quickly using the key Siri, what is Maria s phone number? key = Maria, value = We want key and value to be customizable Unlike your PQ assignment where all keys were string You ve been using customizable classes like this: Map<int,string> Now we will learn how to make one!

5 Map Interface: hash-map.h template <typename Key, typename Value> // CUSTOM! class HashMap { public: HashMap(int sizehint); ~HashMap(); bool isempty() const { return size() == 0; } int size() const { return count; } bool containskey(const Key& key) const; void put(const Key& key, const Value& value); Value get(const Key& key) const; Value& operator[](const Key& key); bool MYSTERY(const Key& key1, const Key& key2);

6 Will this compile? bool HashMap::MYSTERY(const Key& key1, const Key& key2){ return key1 == key2; } A. YES! B. No, there is a problem with const C. No, there is a problem with Key D. No, there is some other problem E. Other/none/more

7 Will this compile? bool HashMap::MYSTERY(const Key& key1, const Key& key2){ return key1 == key2; } Syntax detail: We can t actually just say HashMap::MYSTERY We need to note that this is a template class: template <typename Key, typename Value> bool HashMap<Key,Value>::MYSTERY (const Key& key1, const Key& key2){ return key1 == key2; }

8 Map Interface: hash-map.h private: struct node { Key key; Value value; node *next; }; node **buckets; int numbuckets; int count; int hash(const Key& key) const; node *ensurenodeexists(const Key& key); const node *findnode(const Key& key) const; }; #include "hash-map-impl.h // WHY??

9 bool containskey(const Key& key) const; const on the key means that the containskey method promises not to change key user entrusts us with key reference only because of this contract const at the end means containskey method promises not to change the data of the HashMap class Why do we care about this promise? We ARE the HashMap class! Why would we promise not to change our own private data?

10 const class PQVec { public: void myconstmethod() const; private: }; Vector<int> myvec; void PQVec::myconstmethod() const { } vector: int size() const; myvec.add(5); //will not compile myvec.size(); //will compile because //size() is const

11 Hash Tables (HashMaps) Implementing the Map interface with Hash Tables

12 Imagine you want to look up your neighbors names, based on their house number (all on your same street) House numbers: through (roughly 1000 houses there are varying gaps in house numbers between houses) Names: one last name per house

13 Options Some kind of tree (next week!) Linked list Array

14 Hash Table is just a modified, more flexible array Keys don t have to be integers 0-(size-1) (Ideally) avoids big gaps like our gap from 0 to in the house numbers and between numbers Hash function is what makes this possible!

15 Closed Addressing Where does key= Annie value=10 go if hashkey( Annie ) = 3? Where does key= Solange value=12 go if hashkey( Solange ) = 5? Array index Value

16 Map Interface: hash-map.h private: struct node { Key key; Value value; node *next; }; node **buckets; int numbuckets; int count; int hash(const Key& key) const; node *ensurenodeexists(const Key& key); const node *findnode(const Key& key) const; }; #include "hash-map-impl.h // WHY??

17 Closed Addressing Assume hashkey( Julian ) = 3. How many of the following cause a hash collision? A. Try to insert key= Annie B. Try to insert key= Julian C. Try to insert value=10 D. Other/none/more Array index Value

18 Closed Addressing Where does key= Annie value=55 go if hashkey( Annie ) = 3? A. 55 overwrites 10 at 3 B. A link list node is added at 3 C. Other/none/ more than one Array index Value

19 Hash key collisions Hash function takes key and maps it to an integer Sometimes will map two DIFFERENT keys to the same integer This is a collision We can NOT overwrite the value the way we would if it really were the same key Need a way of storing multiple values in a given place in the hash table

20 TreeMap An implementation of the Map (or Dictionary ) interface that has guaranteed log(n) worst case.

21 Implementing Map interface with a Binary Search Tree (BST) Usually we think of a hash table as the goto implementation of the Map interface But Binary Search Trees are another option C++ s Standard Template Library (STL) uses a Red-Black tree for their Map

22 Binary Search Tree