Question

I’m learning queues and I came across this piece of code. It’s from a book, so I can’t post the whole code here but what I’m posting will be sufficient. More than a problem, I just want to confirm that whether my understanding of this code is correct or not.

In the function, delete_ap(), the ‘if’ statement calls qretrieve() function and stores its return value in pointer ‘p’.
My problem is: If the value returned is not NULL, then too the ‘if’ statement is executed, isn’t it?
So a value is still stored in ‘p’ and we can just print this value without using an ‘else’ statement as used in this example.

Thanks!

/* Delete an appointment from the queue. */
void delete_ap(void)
{
  char *p;
  if((p=qretrieve()) ==NULL) return;
  printf("%s\n", p); <--Problem is in this line and the one above it.
}

/* Retrieve an appointment. */
char *qretrieve(void)
{
  if(rpos==spos) /* spos:holds the index of the next free storage location.
                    rpos:holds the index of the next item to retrieve.*/

   { 
    printf("No more appointments.\n");
    return NULL;
   }
  rpos++;
  return p[rpos-1];
}

Answer 1

This is the same as:

char *p = qretreive(); // <-- will always execute
if(p==NULL) 
  return; // <-- will not execute the next line if p is invalid
printf("%s\n", p);