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NNTP-Posting-Date: Sun, 27 May 2012 13:11:02 -0500
Message-ID: <4FC26E34.56C@mindspring.com>
Date: Sun, 27 May 2012 14:11:00 -0400
From: pete <pfiland@mindspring.com>
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Subject: Re: Array addressing styles: arr[i].d[j] versus pArr->d[j], etc.
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Xref: csiph.com comp.lang.c:21419

John Reye wrote:
> 
> For a concrete example, consider the following:
> Do you prefer fill_array1() or fill_array2()
> 
> #define ARR_ELEMS 4
> 
> struct st1
> {
>   char * name;
>   int i;
>   int ringbuf[ARR_ELEMS];
>   unsigned ringbuf_current_index;
> };
> 
> #define ST1_ELEMS 10
> 
> struct st1       st[ST1_ELEMS];
> unsigned statistics[ST1_ELEMS];
>                  /*     |        */
>                  /* common index */
> 
> char *gen_name()
> {
>   static char a[] = "asdf";
>   return a;
> }
> 
> int gen_value()
> {
>   static int a = 0;
>   return a++;
> }
> 
> unsigned inc_ringbuf_index(unsigned i)
> {
>   if (++i == ARR_ELEMS)
>     return 0U;
>   return i;
> }
> 
> void fill_array1(unsigned i)
> {
>   st[i].i = i;
>   st[i].name = gen_name(i);
>   st[i].ringbuf[ st[i].ringbuf_current_index  ] = gen_value();
>   st[i].ringbuf_current_index =
> inc_ringbuf_index( st[i].ringbuf_current_index );
>   statistics[i]++;
> }
> 
> void fill_array2(unsigned i)
> {
>   struct st1 * const pst = &st[i]; // st + i
>   unsigned idx = pst->ringbuf_current_index;
> 
>   pst->i = i;
>   pst->name = gen_name(i);
>   pst->ringbuf[ idx ] = gen_value();
>   pst->ringbuf_current_index = inc_ringbuf_index( idx );
>   statistics[i]++;
> }

The array addressing style that I would use
depends on the code is doing.
If (i) remains unchanged in a function,
such as you have shown in your fill_array functions,
then I might be inclined to go with fill_array2.

But, if I were cycling through the array

    for (i = 0; ST1_ELEMS > i; ++i) {
        puts(st[i].name);
    }

then I would use the first array addressing style instead.

-- 
pete