Page 114 - Programming Microcontrollers in C

Structures 99

this program. This function receives a pointer to a Tnode and a pointer
to a character string as arguments. It returns a pointer to a Tnode. If
the Tnode pointer argument is a NULL on entry to the function, the
first if loop is executed. Within that loop, talloc() returns a pointer
to a new Tnode. The function strsave() copies the string into a
safe place and returns a pointer to this location. This pointer is put into
the word pointer location in the new Tnode. A value of 1 is put into
the count location, and the pointers to the left and right child Tnodes
are set to NULL. At this time, the word has been put into the Tnode ,
and the pointer to this Tnode is returned to the calling program.

/* addtree: add a node with w, at or below p */

Tnode *addtree(Tnode *p, char *w)
{
int cond;

if(p == NULL) /* new word has arrived */
{
p=talloc(); /* make a new node */
p->word=strsave(w);
p->count=1;
p->left=p->right=NULL;
}
else if((cond = strcmp(w,p->word)) == 0 )
p->count++; /* repeated word */
else if(cond <0) /* less than into left subtree*/
p->left = addtree(p->left,w);
else /* greater than into right subtree */
p->right = addtree(p->right,w);
return p;
}
Suppose now that at a later time addtree() is called and this
time p is no longer a NULL. In this case, a string compare test will be
executed to determine if the word that has been passed is equal to that
in the Tnode pointed to by p. If it is equal, it is a repeated word for
that node, so the word count is incremented and control is returned to
the calling program. If it is not equal (say, it is lexically less than the