Iterators. Professor Hugh C. Lauer CS-2303, System Programming Concepts

Transcription

1 Iterators Professor Hugh C. Lauer CS-2303, System Programming Concepts (Slides include materials from The C Programming Language, 2 nd edition, by Kernighan and Ritchie, Absolute C++, by Walter Savitch, The C++ Programming Language, Special Edition, by Bjarne Stroustrup, and from C: How to Program, 5 th and 6 th editions, by Deitel and Deitel) 1

2 Recall Programming Assignment #4 Read words from input Store in binary tree Print out words in alphabetical order, along with number of occurrences of each word 2

3 Recommended implementation in C++ class TreeNode { public: int incr(); int getcount(); string getword(); int compare(const string &w2); TreeNode(const string &w); ~TreeNode(); private: const string word; int count; TreeNode *left, *right; TreeNode *setleft(treenode *t); TreeNode *setright(treenode *t); TreeNode *getleft(); TreeNode *getright(); }; // class TreeNode TreeNode.h 3

4 Recommended implementation (continued) class BinaryTree { public: TreeNode *AddNode(const string &word); int gettotalnodes(); void PrintTree(ostream &output); BinaryTree(); //Default constructor ~BinaryTree(); //Destructor private: static int totalnodes; void PrintTree(TreeNode *subtree, ostream &output); TreeNode *AddNode(TreeNode *subtree, const string &word); TreeNode *root; }; // class BinaryTree BinaryTree.h 4

5 Recommended implementation (continued) class BinaryTree { public: TreeNode *AddNode(const string &word); int gettotalnodes(); void PrintTree(ostream &output); BinaryTree(); //Default constructor ~BinaryTree(); //Destructor There is something very unsatisfying about this implementation. What is it? private: static int totalnodes; void PrintTree(TreeNode *subtree, ostream &output); TreeNode *AddNode(TreeNode *subtree, const string &word); TreeNode *root; }; // class BinaryTree BinaryTree.h 5

6 Recommended implementation (continued) class BinaryTree { public: TreeNode *AddNode(const string &word); int gettotalnodes(); void PrintTree(ostream &output); BinaryTree(); //Default constructor ~BinaryTree(); //Destructor private: static int totalnodes; void PrintTree(TreeNode *subtree, ostream &output); TreeNode *AddNode(TreeNode *subtree, const string &word); TreeNode *root; }; // class BinaryTree Output format is built into Binary Tree class! These two functions are problem specific not about binary trees 7

7 Preference Would like to make tree traversal part of Binary Tree class while printing details (and other problemspecific issues) are handled outside of Binary Tree class 8

8 Iterator A construct that allows cycling through objects of a data structure in some useful order so program can operate on those objects in sequence 9

9 Characteristics of an Iterator Has state So it can keep track of where you are in the data structure Has begin(), end(), ==, and!= operations So program can start iterator, compare iterator objects, and know when it has finished iterating Has ++ operator (and possibly -- operator) So program can step to next object (and possibly previous object) May have [], *, -> operators So program can access specific objects from iterator 10

10 Binary tree iterators Pre-order Post-order In-order 11

11 Binary tree in-order iterator /* This iterator uses a stack object from Standard Template Library*/ #include <stack> class iterator { TreeNode *node; // pointer to current iteration stack<treenode *> stack; // record of how we got there public: iterator(); //constructor TreeNode *begin(); TreeNode *end(); TreeNode *operator++(); bool operator==(iterator &); bool operator!=(iterator &); } //class iterator 12

12 Binary tree in-order iterator (continued) iterator::iterator() { stack.push(null); // initialize stack node = root; if (node!= NULL) while (node->left) { stack.push(node); node = node->left; } iterator::operator++(){ current = node; if (node -> right) { node = node -> right; while (node -> left) { stack.push(node); node = node-> left; } } else node = stack.pop(); } return; } return current; 13

13 Binary tree in-order iterator (continued) /* This iterator uses a stack object from Standard Template Library*/ #include <stack> class iterator { TreeNode *node; // pointer to current iteration stack <TreeNode *> stack; // record of how we got there public: iterator(); //constructor TreeNode *begin(); TreeNode *end(); TreeNode *operator++(); bool operator==(iterator &); bool operator!=(iterator &); } //class iterator 14

14 Binary tree in-order iterator (continued) /* This iterator uses a stack object from Standard Template Library*/ #include <stack> class iterator { TreeNode *node; // pointer to current iteration stack <TreeNode *> stack; // record of how we got there public: iterator(); //constructor TreeNode *begin(); TreeNode *end(); TreeNode *operator++(); bool operator==(iterator &); bool operator!=(iterator &); } //class iterator For these methods to work, class iterator must be a friend of class TreeNode Private members of TreeNode must become protected 15

15 Binary tree in-order iterator (continued) iterator::treenode *begin(){ } while (!stack.empty()) stack.pop(); stack.push(null); // re-initialize stack node = root; if (node!= NULL) while (node->left) { stack.push(node); node = node->left; } return node; iterator::treenode *end(){ return NULL } //iterator::treenode *end bool iterator::operator==(iterator it &){ return node == it.node; } //iterator::treenode *end bool iterator::operator!=(iterator it &){ return node!= it.node; } //bool iterator::operator!= 16

16 Binary tree in-order iterator (concluded) An iterator that steps through the binary tree in the appropriate order Needs access to protected members of the TreeNode class Pre-order and post-order traversal are easy and obvious extensions Allows the design of a Binary Tree independent of what we want to do with it. 17

17 Questions? 18

18 Reading Absolute C++, 17.3 & 19.1 Iterators also exist in Java Similar in concept Different in detail 19

19 Iterator An Abstraction Generalization of pointer into an array Key concepts Access to current element: * and > (Sometimes also random access: [ ] ) Next element: ++ (Sometimes also previous element: -- ) Equality: == and!= 20

20 Iterator Example list<event> EventQueue; list<event>::iterator it; for (it=eventqueue.begin(); it!= EventQueue.end(); it++) { if (it > time > newevent.time) EventQueue.insert(it, newevent); break; end() method points to one after last element! } // for (it =...) Iterator acts like a pointer begin() method set pointer to first element 21

21 More on Iterators Can build own iterators for own container classes Different iterators provide different operations E.g., linked list does not offer "--" or [ ]" string and vector containers do offer "--" or [ ]" 22

22 Uses of Iterators To help you sequence through objects of a container e.g., Arrays, vectors Lists (singly- doubly-linked) Trees Input Output 23

23 Iterator Example (again) list<event> EventQueue; list<event>::iterator it; for (it=eventqueue.begin(); it!= EventQueue.end(); it++) { if (it > time > newevent.time) EventQueue.insert(it, newevent); break; } // for (it =...) In this case, it appears that iterator is a member class or struct of class list<class T> 24

24 The following is legal in C++ class C { class D { } // class C } // class C You can also declare classes inside of functions! Class D is a nested class in Class C aka local class Member functions of class D can access members of class C Class D may be public, protected, private See Absolute C (end) 25

25 Recall Programming Assignment #4 Read words from input Store in binary tree Print out words in alphabetical order, along with number of occurrences of each word What other iterator is suggested by this problem? 26

26 Questions? 27