Mastering Algorithms with C

Questions and Answers

Q: One of the difficulties with pointers is that often when we misuse them, our errors are not caught by the compiler at compile time; they occur at runtime. Which of the following result in compile-time errors? Which of the following result in runtime errors? Why?

a) char *sptr = "abc",*tptr; *tptr = sptr;	b) char *sptr = "abc",*tptr; tptr = sptr;
c) char *sptr = "abc",*tptr; *tptr = *sptr;	d) int *iptr = (int *)10; *iptr = 11;
e) int *iptr = 10; *iptr = 11;	f ) int *iptr = (int *)10; iptr = NULL;

A: a) A compile-time error occurs because when we dereference tptr, we get a character, whereas sptr is a pointer to a character. Thus, the code is trying to assign a character pointer to a character, which is a type conflict. b) No error occurs because both tptr and sptr are character pointers. c) A runtime error is likely to occur because no storage has been allocated for tptr. When we dereference tptr, we cannot be sure where it points. d) A runtime error is likely to occur because assigning an integer pointer a fixed address is dangerous. When dereferencing iptr, we try to write 11 at address 10, which is probably invalid. e) A compile-time error or warning occurs because the code is trying to initialize an integer pointer to an integer, which is a type conflict. f ) No error occurs because although the code first performs the dangerous step of initializing iptr to a fixed address, it is then immediately reset to NULL, which is valid.

Q: Recall that calculations with ...