LISTS

• Find out what is the first item on a list
• Find out what is the nth item on a list
• Insert an item at the front of the list
• Insert an item at the back of the list
• Insert an item in between two other items on the list
• Check for membership: Is item x on list L?
• Remove an item from the list
• Sort the list
• etc.

Lists are extremely useful:

• as the underlying implementations for other abstract datatypes
• in database and spreadsheet application
• for handling dynamic data - i.e. data that needs frequent update

Implementations vary - depend on intended use.

We have and will look at lists throughout the semester. You can view the documentation and implementation of the StringList class implemented with arrays in string-list.h and string-list.C.

Lists are like Bags... they have similar interfaces: insert, isEmpty, etc. The same basic implementation can be used (for example, arrays), but they are NOT the same. Lists have no grab (although they could). Bags have no order. List insert is substantially different, since order must be preserved:

List::insert(int entry)
{
  assert(!isFull());

  for(size_t i=used; i > 0; i--)
    data[i] = data[i-1];
  data[0] = s;
  used++;
}

Lists need not just store data in order. They can also keep track of where in the data are we looking. This can be accomplished by maintaining a cursor. (Your cursor in your screen indicates where you are in a window; a cursor in a list indicates at which item you are in the list.) Operations to support the cursor include:

• current(): return the item in the list at which the cursor currently points
• reset(): place the cursor at the front of the list
• atEnd(): place the cursor at the end of the list
• isFront(): check if the cursor is at the front of the list
• isEnd(): check if the cursor is at the end of the list
• advance(): move the cursor to next item on list
• back(): move the cursor to the previous item on list
• etc.

What are list cursors good for?

• As a place holder - they let us keep track of where we are in the list
• Support iteration through list. For example, "for each item in list, do ___"
• Allow insertion at arbitrary points in the list.

Not all list operations involve the cursor. Does "insert at front of the list" involve the cursor? Maybe. To fully specify our list interface, we need to indicate what happens to the cursor at each operation. Our goal is to avoid ambiguity.

Consider the following insertion into a list of integers:

3 7
^

   insert(4);


4 3 7          // cursor still points to same item
  ^

_
^             // cursor points to end

   insert(4);

4 
  ^           // cursor still points to end

              // OR

4
^             // cursor now points to first item

You can read the specification of the remaining list functions in string-list.h.

Some examples of the use of the cursor:

3 4 7 
  ^

    remove();  // remove the item currently pointed to

3 7            // isEnd() is false
  ^

    remove();

3              // isEnd() is true
  ^

    remove();

???            // precondition violated!



_              // the empty list: 
^              //   isEmpty, isEnd, isFront all true

    insert(1);

1              // isFront() is true
^

    addAfterCurrent(5);

1 5
^

    addBeforeCurrent(7);

7 1 5
  ^

    advance();

7 1 5
    ^

    addAfterCurrent(3);

7 1 5 3
    ^