COMP T2 Lecture 5 School of Engineering and Computer Science, Victoria University of Wellington Thomas Kuehne Maps, Stacks  Thomas Kuehne, Marcus Frean,

RECAP So far we’ve looked at these types of Collection  Bag, List, Set, Queue and Map TODAY  Stack (the last of the Collection types we will look at)  Collections of collections of... RECAP-TODAY 2

Maps  When declaring and constructing, we must specify two types:  Type of the key, and type of the value private Map phoneBook; : phoneBook = new HashMap ();  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() 3

Iterating through a Map  How do you iterate through a Map ? (e.g,. 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 Collection library’s Map allows all of the above!  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()… 4 Why Collection of values, not Set of values like the other two ? Type for key  value pair

Example of using Map  Find the highest frequency word in a file ⇒ must count frequency of every word i.e., 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 count:maxCountWord(map) → word UI.println( maxCountWord( countWords(file) ) ); 5

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 maxCountWord(Map counts) { // for each word in map // if word has higher count than current maximum count, record it // return last recorded maximum count word } 6 “pseudocode” Design the step- by-step solution (algorithm) before you code “pseudocode” 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; } 7

Example of using Map (faster) // 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(); Integer frequency = counts.get(word); counts.put(word, frequency == null ? 1 : frequency + 1); } return counts; } 8

Iterating through Map: entrySet public String maxCountWord(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 “Bob” ⇒ 2 “Alice” ⇒ 1 “eavesdrops” ⇒ 5 “private” ⇒ 1 Map.Entry - getKey() - getValue() 9 Map.Entry - getKey() - getValue()

Stacks  Based on the “first in, last out” principle (cf. Queues…)  A special kind of List:  Constrained access: add, get, and remove only from one end  We should have a Stack interface, and different implementations of it (ArrayStack, LinkedStack, etc.), but...  In Java’s Collections library, it is a class that implements List (actually, it extends Vector)  So you make one like this: Stack myNums = new Stack (); 10

Stacks  Stacks have extra operations: push(value), pop(), peek()  push(value): Put value on top of stack  pop():Removes and returns top of stack  peek():Returns top of stack, without removing  plus the other List operations… ( oops! ) 11

Stacks example  Reversing the items from a file:  read and push onto a stack  pop them off the stack public void reverseNums(Scanner sc){ Stack myNums = new Stack (); while (sc.hasNextInt()) myNums.push(sc.nextInt()) while (! myNums.isEmpty()) UI.print(myNums.pop() + “\n”); } 12

Applications of Stacks  Programs that deal with programs  e.g., program execution,  e.g., expression execution, (6 + 4) * ((10 * √ 64) – ( √ 81/ 3))  working on subtasks, returning to previous task.  Processing files of structured (nested) data.  e.g., reading files of with structured markup (HTML, XML,…)  Undo in editors 13

Stack for evaluating expressions (6 + 4) * ((10 * √ 64) – ( √ 81/ 3)) ⇒ ( (6, 4)+, ((10, (64) √ ) *, ((81) √, 3)/ )- )* ⇒ √ * 81 √ 3 / - * Algorithm: if operand:push on stack if operator:pop arguments from stack compute value push on stack if done:pop answer from stack Postfix order (also known as “Reverse Polish notation”) 14

Summary of these recent collection types  Queue  First in, first out (FIFO)  Always remove from one end  Ordinary queue: add at the opposed end  Priority queue: add anywhere, according to priority  Stack  First in, last out (FILO)  [Java: No Stack interface, just a class, named “Stack”, with lax control]  Map  Key  Value pairs  Three ways to iterate:over set of Keys, over collection of Values, over set of Key  Value pairs  Key  Value pairs represented using Map.Entry 15

Collections of collections? Yes! A collection is an object. Often really useful, for instance,  a List of Sets  a Map, from Strings into Lists (of...) 16