1. Lists - I
The List ADT

2. List ADT (expanded from Weiss)
A list is a dynamic ordered tuple of homogeneous elements
A1, A2, A3, …, AN
where Ai is the ith element of the list
Definition: The position of element Ai is i; positions range from 1 to N inclusive
Definition: The size of a list is N ( a list of NO elements is called “an empty list”)
dynamic means the list can change over time
homogeneous means the elements are all of the same type
ordered means that A1+1 follows (or succeeds) Ai and Ai-1 precedes Ai

3. Other considerations
What design considerations are there regarding this list.
Will the list hold an “infinite” number of elements, or will it have limited capacity? If so, what’s the maximum number of elements?
How will the list handle insertion of duplicate elements?
If the list allows insertion of duplicates, how does it handle deletion of a duplicated element?

4. Operations on a List List( ) -- construct an empty list
List(const List &rhs) -- construct a list as a copy of rhs
~List( ) -- destroy the list
const List &operator= (const List &rhs)
make this list contain copies of the elements of rhs in the same order
elements are deep copied from rhs, not used directly. If L1 = (A1, A2, A3) and L2 = (B1, B2) before the assignment, then L2 = L1 causes L2 = (A1, A2, A3)

5. Operations on a List (cont)
bool isEmpty( ) const -- returns true if the list size is zero
void makeEmpty( ) -- causes the list to become empty
void remove (const Object &x)
the first occurrence of x is removed from the list, if it is present. If x is not present, the list is unchanged.
an occurrence of x is an element Ai of the list such that Ai == x
Also:
insert
find
findPrevious
What about ‘size’? -- there is no operator that returns the size of a list
Also no retrieve(int) that gives the ith element of the list.
Not possible
for (i=1; i <=L.size(); i++)
cout << L.retrieve(i);
How do you scan a list?
How do you retrieve an element from a list?

6. What’s Missing?
There is no size( ) method that returns the size of the list
There is no retrieve( int i) or operator[ int i] method that access the ith element in the list
So, it’s NOT possible to write code like this: for (int i = 1; j < L.size( ); i++) cout << L.retrieve ( i );
How do we “scan” a list and look at all the elements, one at a time?

7. Iterators
An iterator is an object that provides access to the elements of a collection (in a specified order) without exposing the underlying structure of the collection.
order dictated by the iterator
collection provides iterators on demand
each iterator on a collection is independent
iterator operations are generic
C++ tradition (as in the STL) is to think of iterators as generalizations of pointers.
An iterator “points” to an element of the collection and can be made to “point” to the previous or next element by operating on the iterator itself, not on the collection.

8. Iterator Operations
bool isPastEnd( ) -- returns true if the iterator is past the end of the list
void advance( ) -- advances the iterator to the next position in the list. If iterator already past the end, no change.
const Object &retrieve( ) -- returns the element in the list at the current position of the iterator. It is an error to invoke “retrieve” on an iterator that isPastEnd
Scanning in pseudocode
Iterator iter = collection.first();
while (!iter.isPastEnd()) {
do something with iter.retrieve();
iter.advance(); }
No knowledge about the structure of the collection is needed.
Collection.first() provides an iterator pointing to the first element of the collection
If collection is empty, the iterator is already pastEnd.

9. List Operations
ListIter<Object> first( ) -- returns an iterator representing the first element on the list
ListIter<Object> zeroth( ) -- returns an iterator representing the header of a list
ListIter<Object> find(const Object &x) -- returns an iterator representing the first occurrence of x in the list. If x not present, the iterator isPastEnd.
ListIter<Object> findPrevious(const Object &x) -- returns an iterator representing the element before x in the list. If x is not in the list, the iterator represents the last element in the list. If x is first element (or list is empty), the iterator returned is equal to the one returned by zeroth( ).
On first() -- what if list is empty? OK, the iterator isPastEnd
More about header nodes later – implementation may or may not have one

10. “scanning” a Collection
Iterator iter = collection.first ( );
while (! iter.isPastEnd ( ) ) {
Object x = iter.retrieve( ) ;
// do something with x
iter.advance ( ); }

11. List Operators (cont)
void insert (const Object &x, const ListIter<Object> &p)
inserts a copy of x in the list after the element referred to by p
if p isPastEnd, the insertion fails without an indication of failure.

12. Ex: Building a List List<int> list; // empty list of int
ListIter<int> iter = list.zeroth( );
for (int i=0; i < 5; i++) {
list.insert(i, iter);
iter.advance( ); }
Builds list: 0, 1, 2, 3, 4

13. Ex: Building a List #2 List<int> list; // empty list of int
ListIter<int> iter = list.zeroth( );
for (int i=0; i < 5; i++) {
list.insert(i, iter); }
Builds list: 4,3, 2, 1, 0
Why different from previous slide??
This one doesn’t advance the iterator so all inserts at the front, resulting in nodes
appearing in “reverse” order from insertion order
Can you build a list (from empty) using the iterator returned by first() instead of zeroth()?
Ans: no -- first() isPastEnd on an empty list.

14. More List Operations
Find an element in the list and return it’s position
Tell me the value of the Nth element
Determine if the list is full
Determine if the list is empty
Print the list