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Carnegie Mellon Worcester Polytechnic Institute

Iterators

Professor Hugh C. Lauer CS-2303, System Programming Concepts (Slides include materials from The C Programming Language, 2nd 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, 5th and 6th editions, by Deitel and Deitel)

Iterators CS-2303, A-Term 2012 1

Carnegie Mellon Worcester Polytechnic Institute Worcester Polytechnic Institute

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



Recommended implementation in C++ class TreeNode { public: int incr(); int getCount(); string getWord(); int compare(const string &w2); TreeNode *setleft(TreeNode *t); TreeNode *setright(TreeNode *t); TreeNode *getleft(); TreeNode *getright();


TreeNode(const string &w); ~TreeNode(); private: const string word; int count; TreeNode *left, *right; }; // class TreeNode

TreeNode.h


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





BinaryTree.h

There is something very unsatisfying about this implementation.

What is it?





These two functions are problem specific — not about binary trees

Output format is built into Binary Tree class!


Preference

Would like to make tree traversal part of Binary Tree class …

… while printing details (and other problem-specific issues) are handled outside of Binary Tree class



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



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



Binary tree iterators

Pre-order

Post-order

In-order



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



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; }

return; }

iterator::operator++(){ current = node; if (node -> right) { node = node -> right; while (node -> left) { stack.push(node); node = node-> left; } } else node = stack.pop();

return current; }






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






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



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!=



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.



Questions?



Reading

Absolute C++, §17.3 & 19.1

Iterators also exist in Java Similar in concept Different in detail



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 !=



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; } // for (it = ...)


Iterator acts like a pointer begin() method set “pointer” to first element

end() method points to one after last element!


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 “[ ]"



Uses of Iterators

To help you sequence through objects of a container — e.g.,

Arrays, vectors Lists (singly- doubly-linked) Trees Input Output …



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>


The following is legal in C++

class C { class D { … } // class C } // class C

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++ §7.2 (end)


You can also declare classes inside of functions!


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?


CS-2303, A-Term 2012 Iterators 27

Questions?

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