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Path: utzoo!watmath!clyde!rutgers!sri-unix!hplabs!sdcrdcf!trwrb!aero!venera.isi.edu!lmiller
From: lmiller@venera.isi.edu.UUCP
Newsgroups: comp.lang.c
Subject: Re: pointers to arrays
Message-ID: <1783@venera.isi.edu>
Date: Fri, 21-Nov-86 14:48:00 EST
Article-I.D.: venera.1783
Posted: Fri Nov 21 14:48:00 1986
Date-Received: Sun, 23-Nov-86 03:27:30 EST
References: <273@bms-at.UUCP>
Reply-To: lmiller@venera.isi.edu.UUCP (Larry Miller)
Organization: ISI, University of So. Calif.
Lines: 64
Keywords: What? Why? How?

In article <273@bms-at.UUCP> stuart@bms-at.UUCP (Stuart D. Gathman) writes:
>I am still confused about several things concerning pointers to arrays.
>
>There does seem to be such a type, even in K & R.
>
>1) How does one get such an animal?  The only methods I can figure are
>
>	a) a cast: ( type (*)[] ) array_of_type
>	b) a function returning such a type (but the return must use a cast!)
>

Paraphrasing myself ("Programming in C", John Wiley 1986):

   The distinction between an array, and a pointer to its first element,
   is, for the most part, not useful, IF YOU'RE DEALING WITH A 1D ARRAY.
   For a 2D array, however, the distinction is important.

   An example is a 2D array of test score data, the first index selects
   the student, the second selects the indidividual test:

   int  scores[MAX_STUDENTS][MAX_TESTS];

   A routine for computing the average of a particular test (a column),
   using conventional array subscripting:

   ---------------------------------------------------------------------- 

   double test_avg(scores, max, n)
   int  scores[][MAX_TESTS], max, n;
   {
     long  sum = 0;
     int   i;

     for (i = 0; i < max; i++)
       sum += scores[i][n];

     return (double) sum / max;
   }
   ---------------------------------------------------------------------- 

   Now the same routine using a pointer.  row_ptr will point to an
   individual ROW of the matrix (and is thus a pointer to an array),
   and we'll index off of that to get to the column:

   double test_avg (scores, max, n)
   int  scores[][MAX_TESTS], max, n;
   {
     long  sum = 0,
	   i,
	   (*row_ptr)[MAX_TESTS] = scores;	/* ptr to first ROW of scores */

     for (i = 0; i < max; i++)
       sum += (*row_ptr++)[n];

     return (double) sum / max;
   }
   ---------------------------------------------------------------------- 

What's the difference?  The second version was about 25% faster on a VAX
750 using an array with 10,000 rows.  The SLOWER the machine, the GREATER
this difference will be.

Larry Miller
lmiller@venera.isi.edu