Iterator and For-Each Loops COMP110 - Lecture 18

Transcription

1 Iterator and For-Each Loops COMP110 - Lecture 18

2 Announcements PS3 - Paternity Test is out! Game of Thrones: Spoilers vs. No Spoilers Due Sunday at 11:59pm 400 Points - Get started early! Whiteboard Hours

3 Announcements Women of COMP110 Taking 401 in the Spring? Considering taking 401? Women on the COMP110 Team are hosting a casual meet & greet / info session on what comes after 110! Tomorrow, Wednesday the 2nd, at 7:30 in SN14

4 Abstract Data Types An ADT describes an organization of data. It specifies characteristics and capabilities from the user s perspective. An ADT does not specify an implementation of its characteristics and capabilities. In Java, when we want to declare capabilities (methods) without defining them, what do we use? Interfaces! Java s Standard Library ADT s are specified as interfaces. Java s Standard Library also provides many classes implementing each individual ADT interface.

5 Commonly Used Collection ADTs Lists Set Map 0 1 Kim Kardashian Dodge Stratus UNC Roy Dean Dome Keys Jeffrey Meggie Ahmed 2 3 Sky UNC Basketball Helen Zihe Values

6 ADT Interfaces vs. ADT Implementations We will always type our variables, parameters, and return values using an ADT interface. When you construct a new ADT object, you will use an implementing class. Interface Implementation Construction List<E> ArrayList<E> List<String> alist = new ArrayList<String>(); Set<E> HashSet<E> Set<String> aset = new HashSet<String>(); Map<K,V> HashMap<K,V> Map<String,String> amap = new HashMap<String,String>();

7 How do we process each item in a Set? A Set has no guaranteed order However, we still need to be able to work through each item 1-by-1 UNC Dean Dome Roy Basketball We need to learn a new, useful concept called iterators.

8 To use: import java.util.iterator; Iterator<T> Iterator<T> is an interface Its purpose is to simplify iterating over a collection. Read: work through a collection item-by-item. You can do three things with an Iterator: 1. Ask it do you have another item? 2. Ask it give me the next item. 3. Optionally tell it remove the last item you gave me.

9 To use: import java.util.iterator; Iterator<T> 1. Ask it do you have another item? boolean hasnext() 2. Ask it give me the next item. T next() 3. Optionally tell it to remove the last item you gave me. void remove()

10 Example usage: Hey set, give me an iterator! Iterator<String> items = set.iterator(); while (items.hasnext()) { String item = items.next(); Hey iterator, do you have another item? System.out.println(item); Hey iterator, ok, I ll take the next item.

11 Follow-Along: Iterate Through a Set public static void iteratethroughaset() { System.out.println("*** iteratethroughaset ***"); Set<String> aset = new HashSet<String>(); aset.add("unc"); aset.add("dean Dome"); aset.add("basketball"); aset.add("roy"); System.out.println("The Set: " + aset); Iterator<String> items = aset.iterator(); while (items.hasnext()) { String item = items.next(); System.out.println(item);

12 Are Lists and Maps iterable like Sets? 1. Yes! Every Collection in Java can give you an Iterator 2. How? 1. List and Map implement the Iterable interface. Each collection in Java (Set, List, Map, etc) implements the Iterable interface. The Iterable interface guarantees the following method: Iterator<T> iterator(); 3. In 401, you ll learn how to write your own Iterables.

13 Hands-On: Iterate through a List 1. Uncomment the static method call to the iteratethroughalist method. 2. After the TODO comment, declare a variable of type Iterator<String> named items. 3. Initialize items to be the return value of calling alist s iterator method. 4. Using a while loop and the items iterator, print each item. 5. Check in on PollEv.com/comp110

14 public static void iteratethroughalist() { System.out.println("*** iteratethroughaset ***"); List<String> alist = new ArrayList<String>(); alist.add("unc"); alist.add("basketball"); alist.add("basketball"); alist.add("basketball"); alist.add("dean Dome"); alist.add("roy"); System.out.println("The List: " + alist); Iterator<String> items = alist.iterator(); while (items.hasnext()) { System.out.println(items.next());

15 How do we iterate through a Map? Keys There are a few ways, actually! In COMP110, we ll focus on iterating through a Map s keys and using the key to get values. Jeffrey Meggie Ahmed Helen Zihe Values Why? Because a Map s keys are a Set and we know how to iterate through a Set!

16 Follow-Along: Iterate Through a Map public static void iteratethroughamap() { System.out.println("*** iteratethroughamap ***"); Map<String, String> amap = new HashMap<String, String>(); amap.put("kris", "Kris Jordan"); amap.put("meggie", "Meggie Cruser"); System.out.println("The Map: " + amap); // TODO: Iterate through amap using an iterator Iterator<String> keys = amap.keyset().iterator(); while (keys.hasnext()) { String key = keys.next(); System.out.println(aMap.get(key));

17 Introducing: The For-Each Loop Using Iterators to work through collections is common. So common, Java provides a really nice, special loop for doing so more succinctly: for ( T item : Iterable<T> ) { // for-each body Concrete example: List<String> names = new ArrayList<String>(); // Elided: names are added to the list. for ( String name : names ) { // for-each body

18 How to Read a For-Each Loop Set<String> names = new HashSet<String>(); names.add( John ); names.add( Jane ); for ( String name : names ) { System.out.println(name); For each item in the names collection, do what is inside of this loop. Declaring the for-each item variable named name. Within the loop, name is assigned the next item in the collection.

19 These Loops are Equivalent Set<String> names = new HashSet<String>(); names.add( John ); names.add( Jane ); for ( String name : names ) { System.out.println(name); Iterator<String> iter = names.iterator(); while (iter.hasnext()) { String name = iter.next(); System.out.println(name); Under the hood, Java is actually translating this for-each loop into this Iterator based loop.

20 Follow-Along: For-Each Through a Set public static void iteratethroughaset() { System.out.println("*** iteratethroughaset ***"); Set<String> aset = new HashSet<String>(); aset.add("unc"); aset.add("dean Dome"); aset.add("basketball"); aset.add("roy"); System.out.println("The Set: " + aset); for (String word : aset) { System.out.println(word);

21 Hands-On: Iterate through a List using For-Each 1. Uncomment the static method call to the iteratethroughalist method. 2. After the TODO comment, write a for-each loop that prints out each item in alist. The general form: for ( T item : Iterable<T> ) { // for-each body 3. Check in on PollEv.com/ comp110

22 public static void iteratethroughalist() { System.out.println("*** iteratethroughaset ***"); List<String> alist = new ArrayList<String>(); alist.add("unc"); alist.add("basketball"); alist.add("basketball"); alist.add("basketball"); alist.add("dean Dome"); alist.add("roy"); System.out.println("The List: " + alist); for (String word : alist) { System.out.println(word);

23 Follow-Along: For-Each Through a Map public static void iteratethroughamap() { System.out.println("*** iteratethroughamap ***"); Map<String, String> amap = new HashMap<String, String>(); amap.put("kris", "Kris Jordan"); amap.put("meggie", "Meggie Cruser"); amap.put("karen", "Karen Cheng"); amap.put("jeffrey", "Jeffrey Young"); System.out.println("The Map: " + amap); for (String key : amap.keyset()) { System.out.println(aMap.get(key));

24 For-Each over Arrays For Each loops work with collections that implement the Iterable interface. Great news: for-each also works with native arrays. Let s try it

25 public static void iteratethroughanarray() { int[] numbers = new int[] { 1, 2, 3, 4 ; // TODO: Iterate through numbers using a for-each loop for (int number : numbers) { System.out.println(number);

26 Loops while Loops Most general form of a loop. Needed when you are uncertain of exactly how many times you will loop. for Loops Most useful for looping some specific # of times. Conventionally uses a counting variable. for-each Loops Useful when working item-by-item through a collection or array. Limitation: sometimes you need the index to do important things. Challenge: Try writing ArrayUtil s interleave method using for-each loops.

27 Follow-Along: Reading a Text File

28 Set Operations

29 Set: Union A B Notation: A B All of the elements in either A or B. Emphasis on the OR. You can use Set s addall method to perform a union: a.addall(b); // Set a will now have all of Set a and Set b s elements

30 Set: Intersection A B Notation: A B Only the elements in both A and B. Emphasis on the AND. You can use Set s retainall method to perform an intersection: a.retainall(b); // Set a will now have the intersection of a and b

31 Set: Difference or Complement A B Notation: A \ B All of the elements in A that do not exist in B. You can use Set s retainall method to perform complement: a.removeall(b);