Implementation of Linked Lists

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30D. Implementation of Linked Lists

Function definitions

Now it is just a matter of defining the required functions and constant (emptyList) for the list ADT.

The empty list is represented by a null pointer. Here are implementations of the head, tail and isEmpty functions and the emptyList constant. Remember that a list is a pointer, so we use -> notation to select a part of the structure to which L points.


  const ListCell* emptyList = NULL;

  int head(const ListCell* L)
  {
    return L->head;
  }

  ListCell* tail(ListCell* L)
  {
    return L->tail;
  }

  bool isEmpty(const ListCell* L)
  {
    return L == emptyList;
  }

The : operator will need to be renamed since we cannot define : to be an operator in C++. Let's use cons(h, t) for h : t. (Cons is short for construct.)

  ListCell* cons(int h, ListCell* t)
  {
    return new ListCell(h, t);
  }

Using C++ notation

When we discuss list in conceptual terms, we must use conceptual notation. But now that you have seen how the list functions are implemented, there is nothing wrong with using C++ notation directly in a C++ program. You can write

NULL instead of emptyList,
L->head instead of head(L),
L->tail instead of tail(L),
L == NULL instead of isEmpty(L),
new ListCell(h,t) instead of h : t or cons(h,t).

Since cons(h,t) is considerably shorter then new ListCell(h,t), these notes use cons.

Building a linked list

To build up a linked list corresponding to [2, 4, 6], just notice that [2, 4, 6] = 2 : 4 : 6 : [ ]. Replacing h : t by cons(h, t) gives

  ListCell* twoFourSix = cons(2, cons(4, cons(6, NULL)));

Pay attention to types. An attempt to compute [1,2] by

  ListCell* oneTwo = cons(1,2);

cannot be right because the second parameter of cons is required to be a list, not an integer. A list with n members needs n list cells. Since cons creates one list cell, you need to use cons twice to create a list of length 2.

Exercises

Write a statement that creates variable W of type ListCell* and makes it hold list [5, 3]. Answer
What are the values of variables X, Y and k after performing the following sequence of statements? Express your answers in conceptual notation.
```
  ListCell* X = cons(2, cons(4, cons(6, NULL)));
  ListCell* Y = tail(X);
  int  k = head(Y);
  tail(X);
```
Answer

What happens if you do the following statements?

  ListCell* X = emptyList;
  ListCell* Y = tail(X);

Answer

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