COMP 103 Maps and Queues

RECAP  Iterators (for-each loop)  Bag, Sets, and Stacks - a class, not interface TODAY  Maps and Queues 2 RECAP-TODAY QUICK TIP: Comment your assignment code for extra points & remember to fill in your details (name etc) at the top of the file.

Maps 3 Key Value Not this kind of MAP Something that MAPs a key to a value NameAge Bob25 Rita34 Sam15 Adam 6 : : :

Maps  Collection of data, but not of single values:  Map = a set of pairs of keys and values  Constrained access: get values via keys.  No duplicate keys  Lots of implementations, most common is HashMap. “Sharon” ⇒ 442 “Marcus” ⇒ 427 “Rashina” ⇒ 426 “John” ⇒ 337 “Pondy” ⇒ 429 get (“Rashina”) put (“Marcus”, 443) put (“Sharon”, 336) 4

Maps  When declaring and constructing, must specify two types:  Type of the key, and type of the value private Map phoneBook; : phoneBook = new HashMap ();  Central operations:  get(key) → returns value associated with key (or null)  put(key, value) → sets the value associated with key (and returns the old value, if any)  remove(key) → removes the key and associated value (and returns the old value, if any)  containsKey(key) → boolean  size() 5

Iterating through a Map  How do you iterate through a Map ? (eg, to print it out)  A Map isn’t just a collection of single items! ⇒ could iterate through the collection of keys ⇒ could iterate through the collection of values ⇒ could iterate through the collection of key-value pairs  Java Map allows all three!  keySet()→ Set of all keys for (String name : phonebook.keySet()){….  values()→ Collection of all values for (Integer num : phonebook.values()){….  entrySet()→ Set of all Map.Entry’s for (Map.Entry entry : phonebook.entrySet()) { … entry.getKey() … … entry.getValue()… 6 Why Collection of values not Set of values like the other two ?

Example of using Map  Find the highest frequency word in a file ⇒ must count frequency of every word. ie, need to associate a count (int) with each word (String) ⇒ use a Map of “word–count” pairs:  Two Steps:  construct the counts of each word: countWords(file) → map  find the highest countfindMaxCount(map) → word UI.println( findMaxCount( countWords(file) ) ); 7

Example of using Map (similar to Assign#2) /** Construct histogram of counts of all words in a file */ public Map countWords(Scanner sc){ // construct new map // for each word in file // if word is in the map, increment its count // else, put it in map with a count of 1 // return map } /** Find word in histogram with highest count */ public String findMaxCount(Map counts){ // for each word in map // if has higher count than current max, record it // return current max word } 8 Design the step-by-step solution (algorithm) before you code!

Example of using Map /** Construct histogram of counts of all words in a file */ public Map countWords(Scanner scan){ Map counts = new HashMap (); while (scan.hasNext()){ String word = scan.next(); if ( counts.containsKey(word) ) counts.put(word, counts.get(word)+1); else counts.put(word, 1); } return counts; } /** Find word in histogram with highest count */ public String findMaxCount(Map counts){ // for each word in map // if has higher count than current max, record it // return current max word } 9

Iterating through Map: entrySet public String findMaxCount(Map counts){ String maxWord = null; int maxCount = -1; for (Map.Entry entry : counts.entrySet() ){ if (entry.getValue() > maxCount){ maxCount = entry.getValue(); maxWord = entry.getKey(); } return maxWord; } “public” ⇒ 1 “Map” ⇒ 2 “String” ⇒ 5 “counts” ⇒ 2 “findMaxCount” ⇒ 1 Map.Entry - getKey() - getValue() 10

Queues  Queues are like Stacks, but First In First Out principle (FIFO)  Collection of values with an order  Constrained access: Only remove from the front  Two varieties: Ordinary queues: only add at the back Priority queues: add with a given priority 11

Queues  Used for  Operating Systems, Network Applications, multi-user systems Handling requests/events/jobs that must be done in order (often called a “buffer” in this context)  Simulation programs Representing queues in the real world (traffic, customers, deliveries, ….) Managing the events that must happen in the future  Search Algorithms Computer Games Artificial Intelligence  Java provides  a Queue interface  several classes (ie. implementations) : LinkedList, PriorityQueue, …. 12

Queue Operations  offer(value) ⇒ boolean  add a value to the queue  (sometimes called “enqueue”, like “push” )  poll() ⇒ value  remove and return value at front/head of queue or null if the queue is empty  (sometimes called “dequeue”, like “pop”)  peek() ⇒ value  return value at head of queue, or null if queue is empty (doesn’t remove from queue)  remove() and element()  like poll() and peek(), but throw exception if queue is empty.  Why use Queue (or Stack), instead of List ?? 13

Summary  Map  key-value pairs  3 ways to iterate: over set of keys, over collection of values, over set of key-value pairs  Key-value pairs represented using Map.Entry  Queue  First in, first out  Always remove from one end  An interface, concrete class implementations: linkedlist, priority queue  Ordinary queue: add end other end  Priority queue: add anywhere with priority 14