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[CHAP. 13
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template<class T> int ListIter<T>::operator!() { if (current == NULL) // reset current pointer if (previous == NULL) current = list.first; else current = previous->next; return (current != NULL); // returns TRUE if current exists } If the current and previous pointers are both NULL, then either the list is empty or it has only one item. So setting current equal to the list s first pointer will either make current NULL or leave it pointing to the first item in the list. If current is NULL but previous is pointing to a node, then we simply reset current to point to the item that follows that node. Finally, the function returns 0 or 1 according to whether current is NULL. This allows the function to be invoked in the form if (!it) . . . where it is an iterator. The expression (!it) is read a current item exists, because the function will return 1 (i.e., true ) if current is not NULL. We use this function to check the status of the current pointer before invoking an insertion or deletion function that requires using the pointer. The operator++() increments the iterator by advancing its current pointer to the next item in the list after advancing its previous pointer. It precedes this action with the same resetting procedure that the operator!() function performed if it finds the current pointer NULL: template<class T> int ListIter<T>::operator++() { if (current == NULL) // reset current pointer if (previous == NULL) current = list.first; else current = previous->next; else { previous = current; // advance current pointer current = current->next; } return (current != NULL); // returns TRUE if current exists } This operator allows for easy traversal of the list: for (it.reset(); !it; ++it) . . . just like an ordinary for loop traversing an array. It resets the iterator to locate the first item in the list. Then after visiting that item, it increments the iterator to advance and visit the next item. The loop continues as long as !it returns true , which means that there is still an item to be visited. The insert(t) function creates a new node for t and then inserts that node immediately after the current node: template<class T> void ListIter<T>::insert(T t) { ListNode<T>* p = list.newNode(t,0); if (list.isEmpty()) list.first = p; else { p->next = current->next; current->next = p; } }
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The insert operation can be visualized like this:
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data next
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Bowen, Van
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data next
Dixon, Tom
data next
Mason, Joe
ListNode<string>
ListNode<string>
ListNode<string>
friends first
it current previous list
ListIter<string>
data next
White, Ann
ListNode<string>
White, Ann
string
After:
data next
Bowen, Van
data next
Dixon, Tom
data next
Mason, Joe
ListNode<string>
ListNode<string>
ListNode<string>
friends first
it current previous list
ListIter<string>
data next
White, Ann
ListNode<string>
Note that the operation leaves the current and previous pointers unchanged. The preInsert() function is similar to the insert() function, except that it inserts the new node in front of the current node:
template<class T> void ListIter<T>::preInsert(T t) { ListNode<T>* p = list.newNode(t,current); if (current == list.first) list.first = previous = p; else previous->next = p; }
TEMPLATES AND ITERATORS
[CHAP. 13
The preInsert operation can be visualized like this:
Before:
data next
Bowen, Van
data next
Davis, Jim
data next
White, Ann
ListNode<string>
ListNode<string>
ListNode<string>
friends first
it current previous list
ListIter<string>
data next
Morse, Sam
ListNode<string>
Morse, Sam
string
After:
data next
Bowen, Van
data next
Davis, Jim
data next
White, Ann
ListNode<string>
ListNode<string>
ListNode<string>
friends first
it current previous list
ListIter<string>
data next
Morse, Sam
ListNode<string>
Note that like insert, this operation also leaves the current and previous pointers unchanged. The remove() function deletes the current node: template<class T> void ListIter<T>::remove() { if (current == list.first) list.first = current->next; else previous->next = current->next; delete current; current = 0; } It leaves the previous pointer unchanged and the current pointer NULL .
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CHAP. 13]
TEMPLATES AND ITERATORS
The remove operation can be visualized like this:
Before:
data next
Bowen, Van
data next
Dixon, Tom
data next
Mason, Joe
ListNode<string>
ListNode<string>
ListNode<string>
friends first
it current previous list
ListIter<string>
data next
White, Ann
ListNode<string>
After:
data next
Bowen, Van
data next
Dixon, Tom
ListNode<string>
ListNode<string>
friends first
it current previous list
ListIter<string>
data next
White, Ann
ListNode<string>
Here is a test driver for the list iterator: #include "ListIter.h" int main() { List<string> friends; ListIter<string> it(friends); it.insert("Bowen, Van"); ++it; it.insert("Dixon, Tom"); ++it; it.insert("Mason, Joe"); ++it; it.insert("White, Ann"); ++it;
// sets current to first item // sets current to second item // sets current to third item // sets current to fourth item
TEMPLATES AND ITERATORS
[CHAP. 13
friends.print(); it.reset(); // sets current to first item ++it; // sets current to second item it = "Davis, Jim"; // replace with new name ++it; // sets current to third item it.remove(); // removes third item friends.print(); if (!it) it.preInsert("Morse, Sam"); friends.print(); for (it.reset(); !it; ++it) // traverses entire list it = "[" + it() + "]"; friends.print(); } Bowen, Van -> Dixon, Tom -> Mason, Joe -> White, Ann -> * Bowen, Van -> Davis, Jim -> White, Ann -> * Bowen, Van -> Davis, Jim -> Morse, Sam -> White, Ann -> * [Bowen, Van] -> [Davis, Jim] -> [Morse, Sam] -> [White, Ann] -> * The for loop changes each data value in the list by prepending a left bracket and appending a right bracket to each string. Note that the assignment it = "[" + it() + "]" calls the operator()() and operator=() functions of the ListIter<string> class as well as the constructor string(const char*) and operator+=() function defined in the string class. To give ListIter objects the access to the protected members of List objects that they need to do their job, we need to declare the ListIter class a friend of the List class: template<class T> class List { friend class ListIter<T>; public: // other members protected: ListNode<T>* first; // other members }; List iterators also need the access to the protected members of ListNode objects: template<class T> class ListNode { friend class List<T>; friend class ListIter<T>; public: ListNode(T& t, ListNode<T>* p) : data(t), next(p) { } protected: T data; // data field ListNode* next; // points to next node in list };
An iterator acts like a window, allowing access to one item at a time in the container. Iterators are sometimes called cursors because they locate a specific element among the entire structure, the same way that a cursor on your computer screen locates one character location. A structure may have more than one iterator. For example, one could declare three iterators on a list like this:
List<float> list; ListIter<float> it1(list), it2(list), it3(list);
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