Chapter: Dynamic data structures

A major use for pointers is to enable dynamic structures like linked lists and trees. Recall from earlier courses how these are essentially recursive data structures:

A linked list is either

• empty, or
• consists of a node containing data and a (reference to another) linked list

A binary tree is either

• empty, or
• consists of a node containing data and two (references to other) other binary trees

Notice two things: The recursion in the definitions. And the fact that we need to have references to our own type of node in order to define our node! structs exist in C and are essentially objects containing several fields of data. Unlike objects in proper OO languages, C's structs are simply data-- we cannot encapsulate methods within objects as we can in, say, Java. typedef in C allows for definitions of recursive data structures.

Here's code to enable linked lists of words:

typedef struct listnode {char *word; struct listnode *next;} List, *ListP;
ListP malloc(int);

ListP construct(char *data, ListP n) {
  ListP new = malloc(sizeof(List));
  new->word = data;
  new->next = n;
  return new;
}

char *head(ListP lp) {
  return lp->word;
}

ListP rest(ListP lp) {
  return lp->next;
}

head(l) returns the first word in the list l. rest(l) returns the whole of list l less the first node. construct(w, l) constructs a larger list by adding a node containing word w to the front of the existing list l.

Exercise 3

This file contains most of the code you need to read a file, store the unique words in a linked list, reverse the list, print it. It contains an even terser version of strcomp().

Exercise 4

Exercise 5

Before we leave immutable lists, try to figure out the memory allocation that is taking place. Here's a suggestion:

• replace each occurrence of malloc in the program by mymalloc.
• write the procedure mymalloc to work just like malloc (in fact it will call malloc) but will also keep track of the total number of bytes that have been allocated. (Hint: Remember what static variables can do for you?)
• insert code to display results collected by mymalloc

Exercise 6