Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!nrl-cmf!cmcl2!phri!marob!djs!samperi
From: samperi@djs.UUCP (Dominick Samperi)
Newsgroups: comp.lang.fortran
Subject: Re: Fortran versus C for numerical analysis
Message-ID: <183@djs.UUCP>
Date: 13 Sep 88 16:59:35 GMT
References: <410@marob.MASA.COM> <3445@lanl.gov>
Reply-To: samperi@djs.UUCP (Dominick Samperi)
Organization: Village Software
Lines: 44

In article <3445@lanl.gov> jlg@lanl.gov (Jim Giles) writes:
|that's why dynamically allocated multi-
|dimensional arrays don't optimize well in C - they are implemented as
|an array of pointers which _can_ point to aliased areas of memory).

The emphasis on _can_ is important here, for C permits many freedoms
that you need not use for routine numerical applications. Arrays can be
declared or created at runtime that are no different from FORTRAN arrays
(except that the rows/columns are swapped); there is no need to use any
tricks that result in obscure aliasing. It is true though that because of
the freedoms, the compiler may not be able to optimize the way it does
for FORTRAN arrays (how does it do this, by the way?). The use of the
autoincrement/decrement operators might be classified along with these
aliasing tricks and other lower-level C constructs as "advanced" features
of the language, and avoided entirely in numerical applications. Then
obscure code like

	while(*dest++ = *source++) ;

could be written like this instead:

	i = 0 ;
	while(source[i] != 0)
	{
		dest[i] = source[i] ;
		i = i + 1 ;
	}
	dest[i] = 0 ;

	This would also rule out those tricky expressions of the form
b[i] = a[i++], for example.

	On the other hand, using some of the low-level features might be
useful at times. For example, a lower-triangular matrix a[i][j], j <= i,
can be created, where memory is not allocated for the zero elements above
the diagonal. A more exotic example might be one where it is possible
to arrange for the various rows of a matrix to be allocated in different
memory modules/banks. This functionality could easily be added to FORTRAN
by simply providing a standardized macro facility.
-- 
Dominick Samperi
    samperi@acf8.nyu.edu             uunet!hombre!samperi
    cmcl2!acf8!samperi               rutgers!acf8.nyu.edu!samperi
      (^ ell)