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From: fouts@lemming.nas.nasa.gov.nas.nasa.gov (Marty Fouts)
Newsgroups: comp.lang.fortran
Subject: Re: Fortran versus C for numerical analysis
Message-ID: <984@amelia.nas.nasa.gov>
Date: 14 Sep 88 23:28:56 GMT
References: <1484@ficc.uu.net> <3502@lanl.gov>
Sender: news@amelia.nas.nasa.gov
Reply-To: fouts@lemming.nas.nasa.gov.nas.nasa.gov (Marty Fouts)
Organization: NASA Ames Research Center
Lines: 98

In article <3502@lanl.gov> jlg@lanl.gov (Jim Giles) writes:
>
>No, I've never seen a C dynamically allocated multidimensional array
>that wasn't implemented as an array of pointers.  For a two dimensional
>array, each pointer in the array of pointers points to a row of the array.
>Higher dimensions are implementes as pointers_to_pointers_to_....
>the number of indirect levels is one less than the number of dimensions.
>
>- This is _NOT_ 'like any other array'.

Jim,  here's one:

main()
{
  float x[10][10];
  fprintf(stdout,"Hi Jim!\n");
  exit(0);
}

Most C compilers will generate the same kind of 2d array code for this
as any other language with adjacently spaced arrays.  The array
qualifies as dynamic, since it is local/automatic and gets allocated
on the stack when the procedure is run. (;-)

I think various people are quibbling over poor definitions at this
point.  Fortran doesn't support dynamic arrays -- in the sense of
space allocated at runtime -- at all. (Well, it didn't before the
"SAVE" statement introduced the possiblility of recursion.  Now it
does.)   C supports two different kinds of dynamic arrays:  Call stack
allocated and memory allocator returned.  Fortran doesn't support
memory allocator returned. (No pointers) The above is an example of
call stack allocated.  memory allocator returned is slightly
different.  Fortran does support conformant arrays (sort of) in
subroutine references This

      SUBROUTINE A(X,N,M)
      DIMENSION X(N,M)

is legal, but this

      SUBROUTINE A(X,N,M)
      DIMENSION X(N,M), Y(N,M)

is not.  This has been the cause of considerable grief to Fortran
programers over the years. C only sort of has conformant arrays.  The
C declaration 

suba(x)
 real x[][N];

is a sort of conformant C array.  C lets you get away with one
dimension being unspecified, but requires you specify the others.
Conformant local arrays are also quite illegal in C.

It is true that C compilers can't generate efficent code in
the general case of memory allocated dynamic arrays, but Fortran
compilers can't generate any code at all.  (;-) Further, C compilers
can, and some do, generate good code when they detect explicit dope
vectors.  In order to avoid the math of index calculation, many
compilers generate offset index vectors.  There is a C idiom (See
Numerical Recipes in C's appendices for an example) which looks
exactly like a dope vector.  At least one compiler I am aware of can
sometimes detect and always be told that a dynamic array is of this
form, allowing the compiler to generate exactly the code you would
expect for a contiguously allocated array.

Neither language has all of the features that I could use for arrays
in numerical work.  A partial list (off the top of my head) includes:

  o Range specification, ala Pascal's [n..m] format.
  o The ability to query an array for its shape.  By shape, I mean
    the number of indices and the ranges of each index.
  o not having to specify the shape of an array when it is passed
    as an argument.
  o True dynamic (all senses) multiple dimensioned arrays in
    subroutines.
  o good checking for uninitialized reference errors
    and index out of bound errors which I can selectively enable
    at runtime.
  o reduction operators (;-)

With respect to array semantics, Fortran is very good in the presences
of vector machines and naive compiler technology.  C is very good for
dynamically varying requirements in memory tight environments.  I can
easily make C behave like Fortran for many applications; I have a
great deal of trouble going the other way.

Marty






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