Lecture 14 - Doubly-Linked Lists and Pointers to Pointers

Question 1

pointers to pointers

Answer 1

• in the same way that we can create a pointer that points to an integer or a structure, we can also create a pointer that points to another pointer
• ptr_ptr->ptr->next

Question 2

why use pointers to pointers?

Answer 2

• in some cases, we haven’t been able to get a single function to do everything we want
• ex. we’d like to have a function free() a memory location and set the pointer to NULL
• how can we create a function to do both of these operations?
• we need something that can modify the pointer in addition to what is pointed to

Question 3

passing a pointer to a pointer?

Answer 3

• consider a function called my_free()
  void my_free(struct double_l **ptr_ptr) {
  struct double_l *ptr = NULL;
  assert(ptr_ptr != NULL);ptr = *ptr_ptr;
  free(ptr);
  *ptr_ptr = NULL;
  }
• call it like my_free(&ptr)

Question 4

other uses of passing a pointer to a pointer

Answer 4

• the main() function is passed a pointer to pointers to char
  int main(int arc, char **argv) {
  char *temp = NULL;
  if (argc > 1) {
  temp = arg[1];
  printf(“argument 1 is: %s\n”, temp);
  }
  }

Question 5

rules for using pointers to pointers

Answer 5

• issue of pointer type becomes more important
• cannot assign pointers to each other that are not the right type
• now you have more types to choose from
• you need to be sure what you are pointing to is something real (and that it’s still there)
• more NULL conditions to check for

Question 6

pointer problems

Answer 6

• trying to dereference a null pointer will lead to undefined behavior
• will cause program to crash if you try to use the value or return a dereferenced NULL pointer

Question 7

rules of thumb for multilevel pointers

Answer 7

• don’t user more levels of indirection than you need
• use multilevel pointers only when not doing so would be very inefficient or error prone
• you can triple-level pointers (but is probably wrong)

Question 8

list operations using pointers to pointers

Answer 8

• look at another situation where using pointers to pointers makes sense
  void prepend_to_head(struct item **head_ptr, struct item *new_ptr);
• call like prepend_to_head_(&head, item_ptr);

Question 9

prepend_to_head()

Question 10

list elements containing pointers to other things

Question 11

using internal pointers incorrectly…

Question 12

why was the previous function incorrect?

Question 13

using internal

Question 14

pointers to pointers - again

Question 15

the many faces of ‘zero’

Question 16

the mark of zero

Question 17

what to do about zero..

Question 18

function pointers

Question 19

declaring a function pointer

Question 20

using a function pointer

Question 21

passing a pointer to function

Question 22

what’s passing a pointer to a function good for?

Question 23

newton’s method

Question 24

pointers to functions and linked-lists

Question 25

new list_search

Question 26

strings