More on collec)ons and sor)ng

Transcription

1 More on collec)ons and sor)ng CSCI 136: Fundamentals of Computer Science II Keith Vertanen Copyright 2013

2 Java Collec/ons API Overview List (last term), e.g. ArrayList Map (last /me), e.g. HashMap Set (today), e.g. HashSet Sor/ng things in a collec/on Collections.sort() Sor/ng your own data types 2

3 Collec/ons API: Java collec/ons API Useful classes for storing stuff List Set Knows about the order of things Knows if something is in the set Map Knows how to get from a key to a value Mul/ple keys can go to same value 3

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5 ArrayList: storing an ordered list Common problem: store dynamic sized list of objects Java ArrayList class More flexible than an array since an ArrayList automa/cally grows as data is added But can only hold reference types Duplicates possible import java.util.arraylist; public class ArrayListExample public static void main(string[] args) ArrayList<String> names = new ArrayList<String>(); names.add("alice"); names.add("bob"); names.add("bob"); names.add("carol"); [alice, bob, bob, carol] System.out.println(names); 5

6 ArrayList example "I want to type ArrayList instead of java.util.arraylist everywhere." "I want an empty ArrayList and I promise to only put String objects in it." import java.util.arraylist; public class ReverseLines public static void main(string[] args) ArrayList<String> lines = new ArrayList<String>(); "Please add this String to my ArrayList." while (!StdIn.isEmpty()) lines.add(stdin.readline()); for (int i = lines.size() - 1; i >= 0; i- - ) System.out.println(lines.get(i)); "How many things are in my list?" "Please return the i th element of the array." 6

7 ArrayList, important methods Method add(object o) size() Descrip)on Add the new element o to the end of the list. Returns the number of items in the list. get(int i) Return the element at the index i. clear() Throws an excep/on if (i < 0 i >= size()) Remove all the elements in the list. remove(int i) Removes the element at index i. Shi[s subsequent elements to the le[. remove(object o) Removes first instance of element o from the list. Shi[s subsequent elements to the le[. Returns true if something removed, false otherwise. 7

8 HashMap: mapping keys to values Common problem: mapping one thing to another Java HashMap, map one reference type to another Mul/ple keys can map to same value (object) Applica)on Purpose Key Value phone book look up phone number name phone number dic/onary look up word word defini/on zip code map city to a zip code city zip code login screen check user knows password username password file system find file on disk filename loca/on on disk web search find all relevant pages search keywords list of pages DNS find IP address given URL URL IP address reverse DNS find URL given an IP address IP address URL key value 8

9 HashMap example Goal: Map a domain name to an IP address put() adds a key/value mapping Replaces the exis/ng value with a new one. import java.util.*; public class HashMapDNS public static void main(string [] args) HashMap<String, String> map = new HashMap<String, String>(); map.put(" " " ); map.put("cs.mtech.edu", " "); map.put(" " " ); map.put("google.com", " " ); map.put(" " "); HashMap is a generic that requires two types. Here both types are the same, but in general they could be different. while (!StdIn.isEmpty()) String str = StdIn.readLine(); System.out.println(str + " - > " + map.get(str)); get() returns the value for a key, or null if key not in the map. 9

10 HashMap, important methods Method Descrip)on get(k) Returns the value associated with k. If key is not in the map, returns null. put(k, v) Creates a map entry from key k to value v. If key already exists, overwrites the exis/ng value with the new value v. containskey(k) Returns true if the map contains the key k, returns false otherwise. clear() Useful for checking if a map contains a key k or not before call get(k). Remove all the key value mappings. remove(k) Remove the mapping for key k. 10

11 HashSet: storing a set of values Common problem: check if something is in a set We only need to know if present in set or not Java HashSet, no duplicates Like a HashMap but with only keys, no values % java HelloHashSet public class HelloHashSet set size = 2 How are you doing? public static void main(string [] args) HashSet<String> set = new HashSet<String>(); set.add("bob"); set.add("carol"); set.add("bob"); System.out.println("set size = " + set.size()); if (set.contains("carol")) System.out.println("How are you doing?"); 11

12 HashSet example 1 Goal: Print all unique words in a file HashSet<String> dict = new HashSet<String>(); try Scanner scan = new Scanner(new File(args[0])); while (scan.hasnext()) dict.add(scan.next().tolowercase()); scan.close(); catch (FileNotFoundException e) e.printstacktrace(); return; for (String s : dict) System.out.println(s); Itera/ng over all the elements of a set. Crea/ng the set, only needs a single reference type. Read in the words from the file. 12

13 HashSet example 2 Goal: Spell check a file, flag words not in dic/onary HashSet<String> dict = new HashSet<String>(); try Scanner scan = new Scanner(new File(args[0])); while (scan.hasnext()) dict.add(scan.next().tolowercase()); scan.close(); Read in the words from the dic/onary. Check for words that aren't in the dic/onary. scan = new Scanner(new File(args[1])); while (scan.hasnext()) String word = scan.next().tolowercase().replaceall("[^a- z']", ""); if (!dict.contains(word)) System.out.println(word); scan.close(); catch (FileNotFoundException e) e.printstacktrace(); 13

14 HashSet, important methods Method add(object o) size() contains(object o) clear() remove(object o) Descrip)on Add the new element o to the set. Returns true if element o was added (not already in the set), returns false if set already contains o. Returns the number of items in the set. Returns true if the set contains element o, false otherwise. Remove all the elements from the set. Removes the element o from the set if present. Returns true if something removed, false otherwise. 14

15 Sor/ng O[en we want things in a certain order e.g. Print all unique words in a file in alphabe/cal order e.g. Display all DictEntry objects in order of probability e.g. Sort all the cards in your hand 15

16 1: Aces high, diamonds > hearts: Sor/ng cards 2: Aces high, hearts > diamonds: We need to define what we mean by "sorted". 3: Aces low, diamonds > hearts: 4: 2 biggest, spade > hearts > clubs > diamonds 16

17 How to sort A large variety of sor/ng algorithms: Bubble sort Inser/on sort Selec/on sort Quick sort Merge sort Stupid sort (aka bogosort, slowstart) Eventually you'll learn about some of these Today: sor/ng using the built- in Java Collec/ons API Works great for List ADTs like ArrayList Java uses an op/mized version of merge sort 17

18 Sor/ng an ArrayList public static void main(string [] args) ArrayList<String> words = new ArrayList<String>(); try Scanner scan = new Scanner(new File(args[0])); while (scan.hasnext()) words.add(scan.next()); scan.close(); catch (FileNotFoundException e) e.printstacktrace(); return; Collections.sort(words); for (String s : words) System.out.println(s); Sort the ArrayList so it is in alphabe/cal order Read in the words from the file. % java WordsSorted wiki10.txt a a a a a ailment an and and and and and appointed 18

19 Sor/ng DictEntry objects Goal: Print DictEntry objects, most probable first Read from file Put in file order into an ArrayList<DictEntry> object Sort the ArrayList Iterate through list prin/ng entries him E-04 across E-04 died E-04 often E-04 problem E-04 that's E-04 19

20 Sor/ng a DictEntry list: failure 20

21 Making objects sortable In order to use Collection.sort(): Reference type in the ArrayList must "implement Comparable" i.e. Class must have a compareto() method 21

22 DictEntry in ascending order public class DictEntry implements Comparable<DictEntry>... public int compareto(dictentry other) if (getprob() > other.getprob()) return 1; else if (getprob() == other.getprob()) return 0; else return - 1; But we wanted the most probable first! % java SortDictEntry entries.txt moore 5.79E- 5 bull 5.79E- 5 craft 5.79E- 5 periodically 5.79E- 5 founder 5.79E- 5 nick 5.79E- 5 sanctions 5.79E- 5 manufactured 5.79E- 5 drill 5.8E

23 DictEntry in descending order public class DictEntry implements Comparable<DictEntry>... public int compareto(dictentry other) if (getprob() > other.getprob()) return - 1; else if (getprob() == other.getprob()) return 0; else return 1; % java SortDictEntry entries.txt and is in to was of for the or

24 Java Collec/ons API Summary Represent lists, maps, and sets: ArrayList HashMap HashSet Also provides ability to sort some collec/ons Built- in reference types work out- of- the- box String, Integer, Double,... You own data types: You must implement the Comparable interface Add a compareto() method that returns - 1, 0, 1 24