1. 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)
CS-2303, A-Term 2012
Iterators

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
CS-2303, A-Term 2012
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3. 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
CS-2303, A-Term 2012
Iterators

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
CS-2303, A-Term 2012
Iterators

5. 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
There is something very unsatisfying about this implementation.
What is it?
BinaryTree.h
CS-2303, A-Term 2012
Iterators

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
These two functions are problem specific — not about binary trees
Output format is built into Binary Tree class!
CS-2303, A-Term 2012
Iterators

7. 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
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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
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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
CS-2303, A-Term 2012
Iterators

10. Binary tree iterators Pre-order Post-order In-order
CS-2303, A-Term 2012
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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
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Iterators

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; } 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; }
CS-2303, A-Term 2012
Iterators

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
CS-2303, A-Term 2012
Iterators

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
CS-2303, A-Term 2012
Iterators

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!=
CS-2303, A-Term 2012
Iterators

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.
CS-2303, A-Term 2012
Iterators

17. Questions?
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18. Reading Absolute C++, §17.3 & 19.1 Iterators also exist in Java
Similar in concept
Different in detail
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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 !=
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20. end() method points to one after last element!
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 = ...)
begin() method set “pointer” to first element
Iterator acts like a pointer
end() method points to one after last element!
CS-2303, A-Term 2012
Iterators

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 “[ ]"
CS-2303, A-Term 2012
Iterators

22. Uses of Iterators
To help you sequence through objects of a container — e.g.,
Arrays, vectors
Lists (singly- doubly-linked)
Trees
Input
Output …
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23. Iterator Example (again)
In this case, it appears that iterator is a member class or struct of class list<class T>
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 = ...)
CS-2303, A-Term 2012
Iterators

24. The following is legal in C++
class C { class D { … } // 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!
CS-2303, A-Term 2012
Iterators

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?
CS-2303, A-Term 2012
Iterators

26. Questions?
CS-2303, A-Term 2012
Iterators