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From: gl8f@astsun.astro.Virginia.EDU (Greg Lindahl)
Newsgroups: comp.lang.fortran
Subject: Re: Fortran vs. C for numerical work (SUMMARY)
Message-ID: <1990Nov28.065242.10154@murdoch.acc.Virginia.EDU>
Date: 28 Nov 90 06:52:42 GMT
References: <9458:Nov2721:51:5590@kramden.acf.nyu.edu> <7097@lanl.gov> <17680:Nov2806:04:1090@kramden.acf.nyu.edu>
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In article <17680:Nov2806:04:1090@kramden.acf.nyu.edu> brnstnd@kramden.acf.nyu.edu (Dan Bernstein) writes:

>Huh? A double-pointer array, as we were discussing, is a
>single-dimensional array of pointers to single-dimensional arrays. To
>access a random element of the array takes two additions and two memory
>references. In contrast, to access a random element of a flat array
>takes two additions, a multiplication, and a memory reference. On most
>widely used machines, a multiplication is quite a bit slower than a
>memory reference, particularly a cached memory reference. That's why
>double-pointer arrays are better than flat arrays for so many
>applications.

Except that inside loops, the multiplication is strength-reduced to an
addition. That's why flat arrays are faster than double-pointer arrays
for so many applications. The totally flexible language would hide
this little detail from the programmer and would pick the right one.
If I have to choose, I'd use flat arrays in my numerical programs.
If I have to use C, a few macros give me what I really want.