2.6 Lists and Iterators

Transcription

1 Sequences and Urns 2.6 Lists and erators Sequence: ordered collection of items. Urn: unordered collection of items. Key operations: insert an element, iterate over the elements. Design challenge. Support iteration by client, without revealing the internal representation of the collection. next Robert Sedgewick and Kevin ayne Copyright erator Interface erator Client java.util.erator! hasnext Are there more items in the list?! next Return the next item in the list.! remove Delete the last item returned by next. java.util.erator! hasnext Are there more items in the list?! next Return the next item in the list.! remove Delete the last item returned by next. public interface erator<em> { boolean hasnext(); em next(); void remove(); // optional next Sequence<String> list = new Sequence<String>(); erator<string> i = list.iterator(); while (i.hasnext()) { String s = i.next(); 3 4

2 erable Interface nhanced For Loop java.lang.erable! iterator Return an iterator. Java 1.5 shorthand. public interface erable<em> { erator<em> iterator(); x: Sequence, Urn, java.util.arraylist, arrays. Sequence<String> list = new Sequence<String>(); for (String s : list) implements erable! can iterate using foreach construct 5 6 Sequence: Linked List Implementation Sequence: Linked List Implementation (cont) import java.util.erator; import java.util.nosuchlementxception; public class Sequence<em> implements erable<em> { private Node first, last; private class Node { em item; Node mext; public void add(em item) { Node x = new Node(); x.item = item; if (first == null) first = x; else last.next = x; last = x; same as queue private class Seqerator implements erator<em> { Node current = first; public boolean hasnext() { return current!= null; public void remove() { throw new UnsupportedOperationxception(); em item = current.item; current = current.next; return item; return new Seqerator(); next slide current 7 8

3 Sequence: Array Implementation Sequence: Array Implementation (cont) import java.util.erator; import java.util.nosuchlementxception; private class Seqerator implements erator<em> { int i = 0; public class Sequence<em> implements erable<em> { private Object[] a = new Object[8]; private int N = 0; public void add(em item) { if (N >= a.length) resize(); a[n++] = item; return new Seqerator(); private class Seqerator // see next slide public boolean hasnext() { return i < N; public void remove() { throw new UnsupportedOperationxception(); return (em) a[i++]; i N Load alancing Server.java Load balancing. N users want to choose among N identical file shares. he goal is to balance users across file shares. Assume it's too hard to coordinate (or query) all resources to see how empty they are. Random assignment. Assign each user to a resource at random. % java Loadalance 10 0: 1: 2: user7 3: user1 user2 user8 4: user0 user9 5: 6: user3 user6 7: 8: user4 9: user5 max load = 3 public class Server { private Sequence<String> list = new Sequence<String>(); private int load; public void add(string user) { list.add(user); load++; public String tostring() { String s = ""; for (String user : list) s += user + " "; return s; 11 12

4 Load alancing Load alancing public class Loadalance { int N = Integer.parseInt(args[0]); Server[] servers = new Server[N]; for (int i = 0; i < N; i++) servers[i] = new Server(); // assign N users to N servers at random for (int j = 0; j < N; j++) { String user = "user" + j; int i = (int) (ath.random() * N); servers[i].add(user); Load balancing. N users want to choose among N identical file shares. he goal is to balance users across file shares. Assume it's too hard to coordinate (or query) all resources to see how empty they are. Coordinated assignment. Assign user i to server i. Result. ax load = 1. Random assignment. Assign each user to a resource at random. heory. ax load " log N / log log N. // print results for (int i = 0; i < N; i++) System.out.println(i + ": " + servers[i]); est of two. Choose two resources at random. Assign user to least busy one. heory. ax load " log log N Java List Libraries: ArrayList and LinkedList inary ree erator java.util.arraylist! add Add item to end of list. inary tree. Create an iterator for a binary tree. (and avoid using extra space)! iterator Return an iterator to the list.! size, remove, set, clear, indexof, toarray,. public class inaryree<em> { private Node root; import java.util.arraylist; public class est { ArrayList<String> list = new ArrayList<String>(); for (String s : list) private class Node { em item; Node l, r; return new reorder(); goal: implement this 15 16

5 reorder raversal inary ree erator: reorder raversal reorder traversal. isit a node before its two children. private class reorder implements erator<em> { Stack<Node> stack = new Stack<Node>(); private void preorder(node x) { if (x == null) return; System.out.println(x.item); preorder(x.l); preorder(x.r); Q. ow to implement an iterator for preorder traversal? preorder: reorder() { if (root!= null) stack.push(root); public void remove() { // throw exception as before public boolean hasnext() { return!stack.ismpty(); Node x = stack.pop(); em item = x.item; if (x.r!= null) stack.push(x.r); if (x.l!= null) stack.push(x.l); return item; inary ree erator: Level Order Level order. xamine nodes in order of distance from root. Q. ow to implement an iterator for level order traversal? level order: 19