Re: gets() and "string" variable type
Pete Lomax wrote:
>
> Andy Drummond wrote:
> >
> > What I would like to see addressed too is the possibility
> > of a type "string", which is a string of bytes rather than
> > a sequence of 32-bit words. The memory saving is significant,
> > and the possibility of strings containing non-characters is
> > completely removed.
> >
> > So - comments?
> The s[i] op is of critical importance, which I now know from practical
> experience,
> and supporting both 8-bit strings and 4-byte sequences would inevitably lead
> to an overhead of about 20%. Still want?
>
> Regards,
> Pete
> PS [OT] Can someone explain the last line of this snippet from be_execute.e:
>
> case L_RHS_SUBS_CHECK:
> if (!IS_SEQUENCE(*(object_ptr)pc[1])) {
> goto subsfail;
> }
> /* FALL THROUGH */
> case L_RHS_SUBS: /* rhs subscript of a sequence */
> top = *(object_ptr)pc[2]; /* the subscript */
> obj_ptr = (object_ptr)SEQ_PTR(*(object_ptr)pc[1]);/* the sequence */
> if ((unsigned long)(top-1) >= ((s1_ptr)obj_ptr)->length) {
> tpc = pc;
> top = recover_rhs_subscript(top, (s1_ptr)obj_ptr);
> }
> top = (object)*(top + ((s1_ptr)obj_ptr)->base);
>
> My grasp of C is limited and I do not actually understand how it knows to get
> "top" from "base+top*4" rather than just "base+top". Not that it does not seem
> logical, more "how does it know?" Specifically, which parts of
>
> typedef long object;
> typedef object *object_ptr;
>
> are applied/applicable when and where? Is it the earlier *(object_ptr) or the
> latter (object)* or both or what?
Your question confuses me, but then again, so does the code.
To me, the last line of code is saying this:
top = -- self explanatory
(object) -- cast the next value as if it were an object type (the value of the
expression)
* -- the value pointed to by the expression of
(top +
((s1_ptr) -- cast the next value as an s1_ptr type
obj_ptr) -- the name of the structure pointer variable actually being used
->base); -- the member of the structure
Does that make more sense? I don't think there is any multiplication going on
(unless I misunderstand your question), just pointer dereferencing and casting.
I'm no C expert either, at least where it comes to very complicated expressions.
There are a lot of gotchas there. And I haven't yet studied the C source enough
to really understand what's going on in detail.
As for your typedef question, again I'm not sure what you are asking. The
typedef statement says that object_ptr is a pointer to an object (which is really
a long). The part where the code says (object)* is a cast to an object, not an
object pointer. The asterisk binds to the right, not the left, I believe.
Clear as mud? Or is that why we use Euphoria in the first place?
--
A complex system that works is invariably found to have evolved from a simple
system that works.
--John Gall's 15th law of Systemantics.
"Premature optimization is the root of all evil in programming."
--C.A.R. Hoare
j.
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