Self-modifying code

[Steven Ryan]

In article <1075 at garth.UUCP> smryan at garth.UUCP (Steven Ryan) writes:
>>If the machine allows a change to one variable (an array, say) to
>>affect some other unrelated variable, then it is not conforming to
>>the intent of my program.  In fact, it is not conforming to anything
>>useful at all, since nobody would argue that it is useful programming
>>practice to ever do such a thing on purpose (I hope?).
>
>This particular example is done all the time in handling recursive data
>structures.

What do I mean by that? [I do not know how to reply by private mail--student
driver.]

A simple example is reversing a singly linked list and, just to make it
interesting, inserting the list ordinal:

p := nil; q := first_element; n := 1;
while q isnt nil
 do
  r := q.link;     -- r and q.link are variables pointing at the location.
  q.link := p;     -- p and q.link are now pointing at the same location.
  p := q;          -- now p and q.
  p.ordinal := n;  -- as a sideeffect, also modifies q.ordinal.
  q := r;          -- now r and q.
  n +:= 1
 od