Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!mailrus!ncar!tank!oddjob!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.fortran
Subject: Re: Fortran versus C for numerical analysis
Message-ID: <13567@mimsy.UUCP>
Date: 15 Sep 88 01:16:51 GMT
References: <1457@valhalla.ee.rochester.edu> <3570@lanl.gov>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 51

>In article <1457@valhalla.ee.rochester.edu> davis@galaxy.ee.rochester.edu
>(Al Davis) writes:
>>Well, what is wrong with an array of pointers?  Having worked with both, I
>>believe the array of pointers is the superior method.  It eliminates the
>                                      ^^^^^^^^^^^^^^^
>>conformant array fudge.  It is usually faster.  It is more general.  It
>                          ^^^^^^^^^^^^^^^^^^^^
>>works for strangely shaped sparse matrices.  Etc.

In article <3570@lanl.gov> jlg@lanl.gov (Jim Giles) writes:
>I agree with everything above except the marked portions.

Good so far.

>If I want an array of pointers to pointers to ..., I should do something
>different from just declaring a two dimensional array - arrays of pointers
>aren't the same!

You have just described C declarations.

>For one thing, they are almost always slower - they can _never_
			 ^^^^^^^^^^^^^^^^^^^^
>be faster because a conformant array _can_ be implemented that way if
>necessary.  An array of pointer to pointer to ... _can't_ be treated
>as a conformant array though - some of the pointers might be aliased!

It is true that they can never be faster.  Saying that they are `almost
always slower', however, shows just as much bias as Al Davis's claim.

>The fact that they usually aren't doesn't help, the optimizer must assume
>that it's possible.  Someone claimed that interprocedural alias detection
>will soon be simple - I haven't seen it yet.

Of course not.  When I said `soon' I meant `on the order of 10 years'.
(This is certainly not `soon enough' for those doing this sort of work now.)

>...  Indeed,
>it's possible to extend Fortran to include pointers (the Fortran I use
>has them), it's very difficult to get conformant arrays in C - even
>when that's what you really want!

Indeed, it is possible to extend C to include conformant arrays (the C
I use has them%); it is very difficult to get pointers in Fortran---even
when that is what you really want!

-----
% Well, actually, I do not use it.  It is GCC, available for free from
prep.ai.mit.edu and other places on the Internet.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris