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From: keinert@IASTATE.EDU (Keinert Fritz)
Newsgroups: comp.lang.fortran
Subject: Summary: pointers in Fortran
Message-ID: <1991May17.115453@IASTATE.EDU>
Date: 17 May 91 16:54:53 GMT
Sender: news@news.iastate.edu (USENET News System)
Reply-To: keinert@IASTATE.EDU (Keinert Fritz)
Organization: Iowa State University
Lines: 70

Thanks to all who replied to my question about pointers in Fortran.
Our Internet connection was down for a couple of days, so e-mail was
delayed, and this newsgroup has not been downloaded yet. I hope 
this posting makes it out, and does not duplicate too many other
postings.

According to the replies I got, pointers are documented in the Fortran
for RISC Installation Guide, rather than the manual, and also in Sun,
SGI and Cray Fortran.  I found them in a Cray manual.

I thought the declaration for a pointer to real was

	pointer (p,real)

This is incorrect, or rather, it defines p as a pointer to a
variable called "real". This is not what I intended.

The correct notation is

	real     r
	pointer (p,r)

which defines a variable p of type "pointer to real". As far as
calculating and printing goes, p is treated as an integer.  There is
no storage reserved for the variable r, even though it appears that
way.  Rather, r is the name that is given to the contents of p. In C
notation, r would be called *p.

There is a referencing operator loc(), but no dereferencing operator.
Instead, the dereferenced version of a pointer has its own name.

A fancy way to copy the value of y to x, in C:

	float x, y, *p;
	
	y = 3.;
	p = &y;
	x = *p;

The same in Fortran:

	real  x, y, r
	pointer (p, r)

	y = 3.
	p = loc(y)
	x = r

This way of handling pointers is really clumsy. In C:

	float x[5], *p;
	for (p = x; p < x+5; p++) *p = 0.;

becomes in Fortran

	real x(5), r
	pointer (p,r)
	p = loc(x(1))
	do i = 1, 5
	    r = 0.
	    p = p + 4
	enddo

The only good use I found for pointers in Fortran at the moment 
is dynamic memory allocation (see separate posting).
--
Fritz Keinert                             phone:  (515) 294-5128
Department of Mathematics                 fax:    (515) 294-5454
Iowa State University                     e-mail: keinert@iastate.edu
Ames, IA 50011