Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #164706

Re: on looking up a word in a table, pointer question

Show all headers | View raw

Lew Pitcher <lew.pitcher@digitalfreehold.ca> writes:

> On Fri, 28 Jan 2022 22:27:22 +0000, Lew Pitcher wrote:
>
>> On Fri, 28 Jan 2022 18:16:15 -0300, Meredith Montgomery wrote:

>>> struct kw * lookup(char *word, struct kw * table, int n) {
>>>   struct kw *p;
>>> 
>>>   for ( p = table; p->word != NULL; p++) {
>>>     if (strcmp(word, p->word) == 0)
>>>       return p;
>>>   }
>>> 
>>>   return NULL; /* not found */
>>> }

> Gawd, I shouldn't post on an empty stomach. And, I'm too used to using 
> char pointers that I forgot about array entry length computations.
> (Excuses, excuses... mea culpa)
>
> I meant
>   for ( p = table; p < (struct kw *)((char *)table + sizeof table); p++) {
> because
>   1) pointers increment by the size of the thing they are pointing to
>      so, here, we have to make (table + sizeof table) increment in
>      char rather than struct kw, so we coerce <<table>> into char
>   2) pointers compare to pointers of the same type, so we must coerce
>      the results of ((char *)table + sizeof table) /back/ into a
>      struct kw * before we compare to <<p>>

But, as has been pointed out, table is a pointer here, not an array, so
sizeof table is of no use at all, no matter how you scale it.

However, when you really /do/ have an array called table, the usual way
to get its size is to write sizeof table / sizeof *table.  That's
clearer, in my opinion, than casting pointer types.

Second however, since I'm a bit odd, I would write (&table)[1] to get a
pointer just past the end of the array:

  for (struct kw *p = table; p < (&table)[1]; p++) ...

To the OP: remember, this does not work inside a function where you
have a pointer to the first element, rather than an array proper: table
is the name of an array outside the lookup function, but it's a pointer
inside it.

I've been caught out by this, so I always avoid parameter names that
shadow array names.  gcc even has a warning you can turn on for this,
though it is more general than I'd like.

-- 
Ben.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

on looking up a word in a table, pointer question Meredith Montgomery <mmontgomery@levado.to> - 2022-01-28 18:16 -0300
  Re: on looking up a word in a table, pointer question scott@slp53.sl.home (Scott Lurndal) - 2022-01-28 21:29 +0000
  Re: on looking up a word in a table, pointer question Meredith Montgomery <mmontgomery@levado.to> - 2022-01-28 18:49 -0300
  Re: on looking up a word in a table, pointer question Kaz Kylheku <480-992-1380@kylheku.com> - 2022-01-28 22:07 +0000
    Re: on looking up a word in a table, pointer question Meredith Montgomery <mmontgomery@levado.to> - 2022-01-28 22:03 -0300
  Re: on looking up a word in a table, pointer question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-01-28 22:23 +0000
    Re: on looking up a word in a table, pointer question Meredith Montgomery <mmontgomery@levado.to> - 2022-01-28 22:05 -0300
    Re: on looking up a word in a table, pointer question James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-28 21:10 -0500
  Re: on looking up a word in a table, pointer question Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2022-01-28 22:27 +0000
    Re: on looking up a word in a table, pointer question Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2022-01-28 23:18 +0000
      Re: on looking up a word in a table, pointer question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-01-29 02:32 +0000
        Re: on looking up a word in a table, pointer question Meredith Montgomery <mmontgomery@levado.to> - 2022-01-30 10:23 -0300
          Re: on looking up a word in a table, pointer question Bart <bc@freeuk.com> - 2022-01-30 13:44 +0000
    Re: on looking up a word in a table, pointer question Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2022-01-28 23:23 +0000

csiph-web