Compsci 201, Maps+Hashing

Transcription

1 Compsci 201, Maps+Hashing Owen Astrachan Jeff Forbes September 13, /15/17 Compsci 201, Fall 2017, Maps/Hashing 1

2 Plan for the Day Extend Fast ClassScores solution to Not just values in [0..100], not just integers Motivate the java.util.map interface, hashing How does hashing work: in general, in Java Mapping objects to numbers Coping with collisions Implementing the Map interface: OO concept 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 2

3 Logistics Interlude 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 3

4 On Being (Close to) Done 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 4

5 F is for Folder aka Directory where things are stored in Git FIFO How a Queue works, and not a Stack Floating Point What is a double in Java Function Abstraction a method in Java 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 5

6 Re-orientation from Wednesday Using ClassScores to equate counter[32] with frequency[ policy ] Counting number of occurrences of 32 Counting number of occurrences of policy In Java we can t use strings to index an array, but we can use strings to index a Map Keys in a map are immutable Values don t have to be integers, anything! 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 6

7 Thinking about Efficiency ClassScores code: Review code that created int[] of 101 counters Index into array was mapped to # occurrences counters[30] is number of times 30 appears Access counters[99] fast! Same as counters[1024] Same as counters[index] Any value of index! Constant time indexing in arrays 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 7

8 Counting words Look for search trends in Google? Look for authorship based on statistics? We d like a fast solution, just as counters[32] is # of occurrences of 32, scount[ policy ] could be # of occurrences of policy Languages like Python allow this syntax! Languages like Java require alternate syntax Map aka Hash aka Dictionary 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 8

9 A Rose by Any Other Name 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 9

10 Compare two approaches the fox cried fat tears Parallel arrays: word[k] occurs count[k] times What do you do when reading a word? Look up in words, increment parallel location Map index to values in two arrays (ArrayLists) 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 10

11 Using parallel arrays MethodA in Read string, find index in ArrayList words If found, increment value at index in counter Else new word? Add at end of words/counter the fox cried fat tears /15/17 Compsci 201, Fall 2017, Maps/Hashing 11

12 Compare Similar Approach MethodB Similar to parallel arrays, but using one structure Store (String,Integer) or (word, # occurrences) Fast lookup by String, update # occurrences Store (key,value): lookup by key, obtain value How to make this fast? Stay tuned for details! Bottleneck in MethodA? indexof method 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 12

13 Map conceptually (key,value) Search engine: (K,V) is (query, list of web pages) Key is word or phrase, Value: list of pages Maps query to list of web pages/urls Color Name/RGB triple: (K,V) is (name, (r,g,b)) Duke Blue maps to (0,0,156) Dartmouth Green maps to (0,105,62) Stanford Cardinal maps to (140,21,21) 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 13

14 We use java.util.map HashMap<Key,Value> or TreeMap<K,V> Nested/inner class Map.Entry<K,V> Method return purpose Map.size() int # keys Map.keySet() Set<K> Set of keys Map.values() Collection<V> All values Map.containsKey(K) boolean Is key in Map? Map.put(K,V) V (ignored) Insert (K,V) Map.entrySet() Set<Map.Entry> Get (K,V) pairs Map.clear() void Remove all keys 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 14

15 Examining Map Code Why initialize value to 0 when stored in map? What does map.get(key) return? What does map.put(key,value) do? HashMap<String,Integer> map = new HashMap<>(); Scanner s = new Scanner(new File(f)); while (s.hasnext()){ String w = s.next(); if (! map.containskey(w)) { map.put(w, 0); } map.put(w, map.get(w) + 1); } // map complete here 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 15

16 Thinking about Maps and Tradeoffs Looking at more than one approach 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 16

17 HashSet<> and HashMap<> Underneath same concept: hash table HashSet actually implemented with HashMap Keys are unique, all mapped to same value, that value is ignored Read API: il/hashmap.html 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 17

18 Abstract View of Hashing Every object knows where it belongs in a hash table, just as 39 is index into counters[39] Ask object like tiger where it belongs Ask any object of any class where it belongs Call hashcode, get an int value 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 18

19 What does hashcode return? Ideally: an integer that ranges over all possible values for a locker that stores objects Hash table is an array, hashcode like an index IRL? Math.abs(hashCode())% ht.length What if every object returns 15? Or more than one does? Collisions will happen! Conceptually what s in a locker? Room for stuff 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 19

20 Ideal world? Real world! 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 20

21 The hashcode contract Every object has.hashcode() method Inherited from Object, but typically overridden What does inherit mean? Override mean? Must respect.equals(): If a.equals(b)? a.hashcode() == b.hashcode() Converse not true! There will be collisions 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 21

22 What s a HashTable? Indexable structure, like row of lockers ArrayList rather than array so it can grow Constant time to find a locker given the number: locker 2 same as locker ArrayList< > of what? What s in the locker Able to store several elements: ArrayList! ArrayList<ArrayList<String>> Coping with collisions in a simple way 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 22

23 Alternate Ideas? One object per locker, if full? Try next one Linear probing, has it s own issues (# lockers?) Use better structures per locker Low-level linked lists (up until Java 8) Low-level search trees (Java 8) Deal with worst-case, generate good average case performance 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 23

24 Maria Klawe President of Harvey Mudd Dean of Engineering at Princeton, ACM Fellow, College Dropout (and re-enroller) I personally believe that the most important thing we have to do today is use technology to address societal problems, especially in developing regions Coding is today's language of creativity. All our children deserve a chance to become creators instead consumers of computer science. 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 24

25 Hashing and Maps Use maps to solve problems Create maps, understand API HashMap<Key,Value> or TreeMap<Key,Value> Tradeoff of speed v order à orderable! Understand hashing and performance Create your own HashMap! Complicated by <Key,Value>, study via sets 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 25

26 DIY Set with ArrayList First we look at ArraySet.java and SetDriver.java Add an element? Search, then add if not found More elements added? Slower.add is Desktop Laptop 4103 words of from data/melville.txt in words of from data/hawthorne.txt in words of from data/kjv10.txt in words of from data/melville.txt in words of from data/hawthorne.txt in words of from data/kjv10.txt in /15/17 Compsci 201, Fall 2017, Maps/Hashing 26

27 ArraySet deconstructed This class has a generic parameter <Type> Could be <Foo>, use like ArrayList<String> Implementations of.contains,.remove use loop Could use ArrayList equivalent methods! Examine.add: 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 27

28 DIY Set, SimpleHashSet Look at SimpleHashSet and compare ArraySet What is an element of ArrayList myelements? ArraySet 4103 words of from data/melville.txt in words of from data/hawthorne.txt in words of from data/kjv10.txt in SimpleHashSet 4103 words of from data/melville.txt in words of from data/hawthorne.txt in words of from data/kjv10.txt in /15/17 Compsci 201, Fall 2017, Maps/Hashing 28

29 SimpleHashSet Deconstructed Constructor adds 10,000 ArrayLists Alternatives to constructor? What if 10,000 changes to 1,000 or 100 or 10 Helper method.gethash Used:.contains(),.remove(),.add() 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 29

30 Interplay of.hashcode/.equals Use StringWrapper class to help understand Static variables that count calls of methods When is.hashcode called? When is.equals called? What is interplay between these? Change size of hash table and see what happens! What calls change, why do they change? 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 30

31 Thinking about Hashing What does StringWrapper help us understand? 9/15/17 Compsci 201, Fall 2017, Maps/Hashing 31