Date: Tue, 28 Nov 2000 14:40:07 -0800 (PST)
From: Hannah C. Tang <hctang@cs.washington.edu>
To: cse142-section-bf@cs.washington.edu
Subject: [Section BF] Off-by-one

In section, I've discussed how one of the two most common C programming
errors (uninitialized pointers/variables) caused a fatal bug in a program
which was being developed at the company I spent my internship at.

As you're working on HW 5, please keep in mind the second most common
error: iterating past the end of an array.  Keep an eye on your loop
conditions, and remember that arrays are zero-indexed.  That is, if you're
trying to initialize an array to zero, your loop conditions should look
like:

  for(index = 0; index < ARRAY_SIZE; index++) {
    myArray[index] = 0;
  }

Or:

  for(index = 1; index <= ARRAY_SIZE; index++) {
    myArray[index - 1] = 0;
  }

Note the loop initialization and termination conditions.  The two other
possible combinations of these initialization/termination conditions are
off by one, and will result in a bug in your program.

  /* This will write a zero at the memory location immediately
   * following the array (who knows what might be there?) -- which 
   * is an off-by-one error */
  for(index = 0; index <= ARRAY_SIZE; index++) {
    myArray[index] = 0;
  }

  /* This will NOT write a zero at the last index of the
   * array, leaving you with an uninitialized value -- which 
   * is again an off-by-one error */
  for(index = 1; index < ARRAY_SIZE; index++) {
    myArray[index - 1] = 0;
  }
  

Hannah

(Side note: Just last week, it took me 2 days to chase down a combination
of the two errors.  I wrote past the end of an array -- and the next
memory location contained a pointer)