Binary Trees - Chapter 10

Binary Trees and C Traps and Pitfalls

• A binary tree is a data structure in which each node contains links to two successor nodes, so that the whole structure is shaped like a branching tree.

  typedef char Data; struct node { Data d; struct node * left; struct node * right; }; typedef struct node Node; typedef Node * Bin_tree; extern Bin_tree new_node(void); extern Bin_tree init_node(Data d, Bin_tree p1, Bin_tree p2); extern Bin_tree create_tree(Data a[], int i, int size); extern void preorder(Bin_tree root); extern void inorder(Bin_tree root); extern void postorder(Bin_tree root);

• Output

  PREORDER g d b a c f e i h j INORDER a b c d e f g h i j POSTORDER a c b e f d h j i g

• main.c
• bin_tree.c
• bin_tree.h
• makefile

Riddle

• Find three ways to change one character in the code bellow so that the resulting code will print 20 times the minus sign ('-').
  • Remember, you can change one character only for each solution.

  int i, n=20; for (i = 0; i < n; i--) printf("-");

Greedy Lexer

• The lexer is a part of the compiler that breaks the code text into tokens
• It is greedy - bites off the biggest possible piece of text
• Division by a pointer vs. start of comment

  double res, x, * p; res = x/*p;

• Equality and pre-decrement

  == is a single token Whereas = = are two tokens a---b is the same as a -- - b But not a - -- b

Operator Precedence

• Works but uses multiplication

  i = hi*16+lo;

• Probably a bug (Bug: i = hi << (4 + low))

  i = hi << 4 + low;

• 2 options

  i = (hi << 4) + low i = hi << 4 | low;

if - else Statements

• Watch Out for Semicolons

  int calc_max(int *x, int size) { for ? { if (x[i] > big); big = x[i]; } }

• The compiler will happily digest the semicolon to:

  if (x[i] > big) {} big = x[i];

• Dangling else

  if (x == 0) if (y == 0) error(); else { z = x + y; f(z); }

• However else is associated with the closest unmatched if.

  if (x == 0) if (y == 0) error(); else { z = x + y; f(z); }

• Always use curly braces {} with if-else statements

Macros

• Macros are not functions
  • Even when written with all the parentheses

  #define MAX(a,b) (((a)>(b)) ? (a) : (b)) int a[4]; int biggest; biggest = x[0]; i = 1; while (i < 4) biggest = MAX(biggest, x[i++]);

• Expands to:

  #define MAX(a,b) (((a)>(b)) ? (a) : (b)) int a[4]; int biggest; biggest = x[0]; i = 1; while (i < 4) biggest = (biggest > x[i++]) ? biggest : x[i++];

• Would have worked if MAX were a function

The assert Macro

• Example:

  assert(x > y); /* should print info only if x <= y */

• Lets try to write it

  #define assert(e) if (!(e)) assert_error(__FILE__,__LINE__)

• Not safe

  if ((x > 0) && (y > 0)) assert(x > y); else assert(y > x);

• We can try and add braces

  #define assert(e) { if (!e) assert_error(__FILE__,__LINE__) }

• This will solve the dangling else problem but will cause a problem with the semicolon:

  if ((x > 0) && (y > 0)) { if (!(x > y)) assert_error(__FILE__,__LINE__) }; else { if (!(y > x)) assert_error(__FILE__,__LINE__) };

• One correct solution is:

  #define assert(e) ((void)((e) || assert_error(__FILE__,__LINE__)))

Scope

• Return the address of a local variable

  void fnc(int * pi) { int i; return &i; }

Overflow and underflow

• ch is never greater than UCHAR_MAX.

  unsigned char ch; for (ch = 0; ch <= UCHAR_MAX; ch++)

• size cannot be smaller than 0

  unsigned int size = 10; while (--size >= 0) {

Deallocating memory

• Referring to garbage memory

  while (ptr != NULL) { free(ptr); ptr = ptr->next; }

Strings

• This could be incorrect for three reasons:
  • Malloc might fail to allocate the required memory
  • It is important to free the memory allocated
  • We did not allocate enough memory

  char *r; r = malloc(strlen(s)+strlen(t)); strcpy(r, s); strcat(r, t);

• The correct code

  char *r; r = malloc(strlen(s)+strlen(t)+1); if (!r) { /* print some error and exit */ exit(1); } strcpy(r, s); strcat(r, t); /* later */ free(r); r = NULL; /* Try to reset free'd pointers to NULL */