Iterators An iterator permits you to examine the elements of a data structure one at a time. C++ iterators Input iterator Output iterator Forward iterator Bidirectional iterator Reverse iterator Quite often, an application needs to examine all elements of an instance. The elements of a linear list may be examined by doing examine(get(i)), 0 <= i < size(). This technique has much unnecessary overhead associated with it. For example, the index i is checked each time get() is invoked. What if the data structure we are dealing with doesn’t have a get(index) method? Iterators provide a uniform way to examine all elements of a data structure instance, and often with much smaller overhead than using other techniques.

Bidirectional Iterator Allows both forward and backward movement through the elements of a data structure.

Bidirectional Iterator Methods iterator(T* thePosition) Constructs an iterator positioned at specified element dereferencing operators * and -> Post and pre increment and decrement operators ++ and – Equality testing operators == and != hasNext, next, and remove are the methods of Java’s interface Iterator.

Iterator Class Assume that a bidirectional iterator class iterator is defined within the class arrayList. Assume that methods begin() and end() are defined for arrayList. begin() returns an iterator positioned at element 0 of list. end() returns an iterator positioned one past last element of list.

Using An Iterator arrayList<int>::iterator xHere = x.begin(); arrayList<int>::iterator xEnd = x.end(); for (; xHere != xEnd; xHere++) examine( *xHere); vs for (int i = 0; i < x.size(); i++) examine(x.get(i));

Merits Of An Iterator it is often possible to implement the ++ and -- operators so that their complexity is less than that of get many data structures do not have a get by index method iterators provide a uniform way to sequence through the elements of a data structure

Linked Representation list elements are stored, in memory, in an arbitrary order explicit information (called a link) is used to go from one element to the next

Layout of L = (a,b,c,d,e) using an array representation. Memory Layout Layout of L = (a,b,c,d,e) using an array representation. a b c d e The figures show computer memory. An array uses a contiguous chunk of memory. A linked representation uses an arbitrary layout. c a e d b

Linked Representation c a e d b firstNode pointer (or link) in e is NULL use a variable firstNode to get to the first element a

Normal Way To Draw A Linked List b c d e NULL firstNode link or pointer field of node data field of node

Chain firstNode NULL a b c d e A chain is a linked list in which each node represents one element. There is a link or pointer from one element to the next. The last node has a NULL pointer.

Node Representation template <class T> struct chainNode { // data members T element; chainNode<T> *next;   // constructors come here }; next element

Constructors Of chainNode ? chainNode() {} ? element chainNode(const T& element) {this->element = element;} chainNode(const T& element, chainNode<T>* next) {this->element = element; this->next = next;} next element

get(0) a b c d e firstNode checkIndex(0); NULL firstNode checkIndex(0); desiredNode = firstNode; // gets you to first node return desiredNode->element; Do a checkIndex first. Then move to desired node. Once you are at the desired node, you can return the element in that node as in return desiredNode.element; When firstNode = null, there is no element whose index is 0.

get(1) a b c d e firstNode checkIndex(1); NULL firstNode checkIndex(1); desiredNode = firstNode->next; // gets you to second node return desiredNode->element;

get(2) a b c d e firstNode checkIndex(2); NULL firstNode checkIndex(2); desiredNode = firstNode->next->next; // gets you to third node return desiredNode->element;

get(5) a b c d e firstNode checkIndex(5); // throws exception NULL firstNode checkIndex(5); // throws exception desiredNode = firstNode->next->next->next->next->next; // desiredNode = NULL return desiredNode->element; // NULL.element Note that checkIndex would throw an exception. Other two statements executed only when checkIndex not done.

Erase An Element erase(0) deleteNode = firstNode; b c d e NULL firstNode erase(0) First do a checkIndex. deleteNode = firstNode; firstNode = firstNode->next; delete deleteNode;

first get to node just before node to be removed erase(2) a b d e NULL firstNode c b beforeNode c first get to node just before node to be removed beforeNode = firstNode->next;

save pointer to node that will be deleted erase(2) firstNode null a b c d e beforeNode save pointer to node that will be deleted deleteNode = beforeNode->next;

now change pointer in beforeNode erase(2) firstNode null a b c d e beforeNode now change pointer in beforeNode beforeNode->next = beforeNode->next->next; delete deleteNode;

insert(0,’f’) firstNode newNode a b c d e NULL firstNode f newNode Step 1: get a node, set its data and link fields First, do a checkIndex. newNode = new chainNode<char>(theElement, firstNode);

insert(0,’f’) firstNode newNode Step 2: update firstNode b c d e NULL firstNode f newNode Step 2: update firstNode firstNode = newNode;

firstNode = new chainNode<char>(‘f’, firstNode); One-Step insert(0,’f’) a b c d e NULL firstNode f newNode firstNode = new chainNode<char>(‘f’, firstNode);

first find node whose index is 2 insert(3,’f’) a b c d e NULL firstNode f newNode beforeNode c first find node whose index is 2 next create a node and set its data and link fields chainNode<char>* newNode = new chainNode<char>( ‘f’, beforeNode->next); finally link beforeNode to newNode beforeNode->next = newNode;

Two-Step insert(3,’f’) beforeNode = firstNode->next->next; a b c d e NULL firstNode f newNode beforeNode beforeNode = firstNode->next->next; beforeNode->next = new chainNode<char> (‘f’, beforeNode->next);

To-do Practice: Read more on iterators: Read more on linked lists: https://www.hackerrank.com/domains/cpp/introduction Read more on iterators: http://www.cplusplus.com/reference/iterator/ http://www.cplusplus.com/reference/iterator/iterator/ Read more on linked lists: https://en.wikipedia.org/wiki/Linked_list