C question from Dan Garcia

Currently, I’m following Dan Garcia’s CS61C course (which is freely available online). On lesson 6, there’s an interesting puzzle (which, btw, I got it wrong – I’ll tell you why on the next post). The question is: which snippets prints always 5?

1,,)
main() {
   int *a-ptr; *a-ptr = 5; printf(“%d”, *a-ptr);
}

2)

main() {
  int *p, a = 5;
  p = &a; …
  /* code; a & p NEVER on LHS of = */
  printf(“%d”, a);
}

3)

main() {
int *ptr;
ptr = (int *) malloc (sizeof(int));
*ptr = 5;
printf(“%d”, *ptr);
}

Option 2 says that there’s code between p = &a; and the printf line, but a and p are never on the left hand sign of an equals operation. With this data, which option (if any) do you think will always print 5?

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~ by Luis Abreu on May 27, 2009.

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