Maps,Hash(es) We need more second year reps

Transcription

1 Maps,Hash(es) We need more second year reps

2 Your on the run from the police and a shadowy secret government organization keen to kill you (it happens)

3 Real card/fake card Eve former co-worker has given you a credit card. If it s fake she s trying to kill you

4 Fortunately you know about

5 The Luhn algorithm Step 1 - Starting with the check digit double the value of every other digit (right to left every 2nd digit) Slide 5 is now slide 44

6 The Luhn algorithm Step 2 - If doubling of a number results in a two digits number, add up the digits to get a single digit number. This will results in eight single digit numbers.

7 The Luhn algorithm Step 3 - Now add the un-doubled digits to the odd places

8 The Luhn algorithm Step 4 Add the numbers up if they are a multiple of 10 it s genuine 90 % 10 == 0

9 Error detection from Jupiter to Earth

10 A pixel grid

11 Error detection = = 701 1!= 9 Some kind of error send again

12 Hashing

13 Hashing hash 1 haʃ nouna dish of cooked meat cut into small pieces and cooked again, usually with potatoes. North American a finely chopped mixture: a hash of raw tomatoes, chillies, and coriander. a mixture of jumbled incongruous things; a mess.

14 Hashing is not complex Big numbers go in. They get mashed up. Small number comes out Property Same input gives same output. Very hard to go backwwards ( from small to big )

15 passwords Each letter in your password is a number Combine numbers together Password P a s s w o r d =851

16 Passwords Never store password in database store hash of password as a number. It is possible to generate password which matches and get lucky, Limit attempts to about 5.

17 passwords Generally you make order important to. Like multiply each number by 2 as you add them up. Can get complex ( like xor ) P a s s w o r d w o r d p a s s

18 Java code Sytem.out.println( Hello world.hashcode()) ; //= System.out.println( hello world.hashcode()) ;// System.out.println( World hello.hashcode());//

19 (19 % 7 ) Modulus 7 ( 19 mod 7 ) = 5 Remainder after division If this isn t working for you don t worry. Just treat it as a scrambling thing

20 You want to keep student keys. You have 100 boxes. So you look at the last two digits So ID give 45 so goes in box so into box 13

21 0-9 bag bag coat bag purse bag bag bag bag 90-99

22 Depositing a bag Getting a bag Do you have my bag Look at number of card Look at last to digits Put in that slot. Look at number of card Look at last to digits look in that slot. Look at number of card Look at last to digits look in that slot. VERY QUICK VERY QUICK VERY QUICK

23 What happens to two 34s turn up W W If there is a bag in slot already ( a collision ) Or She adds 17 to the number and tries again. Puts two bags in one hole. Then asks is this your bag.

24 Handle collisions use a linked list

25 Maps In Java Maps use Keys ( index ) To return Values ( the thing being stored ).

26 Mapping converts on thing to another For example convert a location to treasure In a dictionary The key is the word The value is the defintion

27 Another word for a map is dictionary A dictionary maps one word to one definition.

28 Or an associative array (in php)

29 Creating a map // Map<FROM-TYPE,TO-TYPE> nameofmap Map<String,String> dictionary = new HashMap<String,String>(); dictionary.put( thing","an object that one need not, give a specific name to:"); Map<String,Integer> phonebook = new HashMap<String,Integer>(); phonebook.put("richard K", ) ;

30 Looking something up String definition = dictionary.get( Exposition"); if( definition!= null ) { // I found it. } Integer pnum = phonenumber.get("richard K"); if( pnum!= null ) { }

31 Check if something exists if( dictionary.containskey("thing") == true ) { String def = dictionary.get("thing"); }

32 Iterating over all ( with for each ) for( String key : dictionary.keyset()) { String def = dictionary.get( key) ; System.out.println( key + "-> " + def ) ; }

33 Iterating over all ( with for each ) for( String def: dictionary.values()) { System.out.println( key + "-> " + def ) ; }

34 Iterating over all ( with for each ) for (Map.Entry<String, Integer> entry : phonebook.entryset()) { } System.out.println( "name: " + entry.getkey() + "number: " + entry.getvalue());

35 Iterating over all ( with for each ) for (Map.Entry<String, Integer> entry : phonebook.entryset()) { } System.out.println( "name: " + entry.getkey() + "number: " + entry.getvalue());

36 The order of items in a HashMap is not the order you put them in ( get over it )

37 Linked list HashMap It s a linked list and a HashMap Keeps the original adding order Uses a bit more space shaving foam and a dessert topping

38 My Hashmapable class What do you do if you want to make your own class and put it into a Override hashcode(); This must return something which is the same each time but spreads all the values out.

39 Look at the first letter not the last digits Bag,bag, bag,bag, bagf, bag,

40 Make your own class - hashable Class Student { } int age ; String name ; public int hashcode() { int prime = 31; int result = 1; result = prime * result + age; result = prime * result + name.hashcode()); return result; }

41 Map is not part of the collection Interface

42 Who wrote Shakespeare's plays? Philip Marlow The other guy

43 Different words used at different rates

44 public class Freq { public static void main(string[] args) { Map<String, Integer> wordcount = new HashMap<String, Integer>(); String words[] = { To, be, or, not, to, be, etc. } ; for (String a : words) { Integer freq = m.get(a); if( freq == null ) { wordcount.put(a, 1); } else { wordcount.put(a, freq + 1); } } System.out.println(m.size() + " distinct words:"); System.out.println(m); } Notice use of Integer the class version of int. So you can compare }

45 public interface Map <K,V> { //Basic operations void clear(); public Set<K> keyset(); public Collection<V> values(); public Set<Map.Entry<K,V>>entrySet(); } V put(k key, V value); V get(object key); V remove(object key); boolean containskey(object key); boolean containsvalue(object value); int size(); boolean isempty(); //Bulk operations void putall(map<? extends K,? extends V> t);

46 TreeMap Another binary tree. Left Key right

47 TreeMap Another binary tree. Except has value not just key. Down side is you need to define compareto() not hashcode() Left Key Value right

48 public class Freq { public static void main(string[] args) { Map<String, Integer> wordcount = new TreeMap<String, Integer>(); String words[] = { To, be, or, not, to, be, etc. } ; for (String a : words) { Integer freq = m.get(a); if( freq == null ) { wordcount.put(a, 1); } else { wordcount.put(a, freq + 1); } } System.out.println(m.size() + " distinct words:"); System.out.println(m); } Notice use of Integer the class version of int. So you can compare }

49 public class Freq { public static void main(string[] args) { Map<String, Integer> wordcount = new LinkedHashMap <String, Integer>(); String words[] = { To, be, or, not, to, be, etc. } ; for (String a : words) { Integer freq = m.get(a); if( freq == null ) { wordcount.put(a, 1); } else { wordcount.put(a, freq + 1); } } System.out.println(m.size() + " distinct words:"); System.out.println(m); } }

50 Break 10 mins question who is Admiral Grace Hopper? What she contribute to computing?

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54 public class Worker implements Runnable { public static void main (String[] args) { System.out.println("This is currently running on the main thread, " + "the id is: " + Thread.currentThread().getId()); Worker worker = new Worker(); Thread thread = new Thread(worker); thread.start(); } Multi-threading List<Integer> mylist public void run() { System.out.println("This is currently running on a separate thread, " + "the id is: " + Thread.currentThread().getId()); } }

55 List<Object> objlist = Collections.synchronizedList(new ArrayList<Object>()); Creating a list accessible from any class.

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57 Interface Collection Specifies methods for adding an item (or a Collection of items); removing item(s); emptying itself completely; Intersecting with another Collection telling whether it contains an item, or all of another Collection; telling whether it equals another Collection Reporting its size; telling whether it is empty; Iterating over its members Computing a hash value Returning an array of its members

58 Interface Set is a subinterface of Collection It provides all the methods specified by Collection But importantly, a set may not contain a member twice adding an item which is already in the set has no effect A mathematical set abstraction Union, intersection, complement Two common implementations are TreeSet elements are stored in a binary tree HashSet elements are stored in a hashtable.

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60 Interface List is a subinterface of Collection Abstracts the mathematical sequence Has a richer set of methods Can retrieve an item from a particular position in the list Can insert or put an item at a particular position Can find the position of an element Collections & Lists Can contain an element multiple times Defines an ordering of elements Iterator respects this order

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62 Is this The End

63 sequel material Stuff we haven't covered and won t cover but would round out your education and make you a programming guru

64 Graphs or networks- Ways of computing shortest route

65 Priority Queues aka Heaps A queue you can add to the middle of.

66 Bags you don t care about order, you just want to hold stuff

67 Operations on Strings ( e.g. searching )

68 Two or three dimensional arrays Home of AI ( neural networks )

69 Parallel programming Running programs spread over multiple computers. CUDA ( programming GPU ). Still a heavy research area

70 Algorithms Ways of computing stuff which doesn t need clever ways of organizing the data. Eg sorting, searching, Why sometimes it s very important and algorithm is very slow. Next week

71 Remember We ( programmers ) hardly dead in ones we deal with many Everyday working your choosing a data structure Either while programming While planning Choose wisely Choose conceptual simplicity( choose clarity ) Programming time is your life Use pre-done methods over re-writing Choose speed Over slowness

72 Thank you!!! You ve been very cool to teach Think of me when your planning your third year project You need to have an idea of what your going to make before you end second year If you want more Remember to feed back to Student reps Next week back to a responsible adult!