[R] R vs Fortran

Robin Hankin r.hankin at auckland.ac.nz
Wed Oct 2 23:38:22 CEST 2002

Dear R experts

I work in computational fluid dynamics in 2D: I have a 200-by-200
array of fluid properties such as density and velocity and these
evolve in time (the precise equations depend on the problem).

Up to now, I've been using Fortran and the code is very very messy.
It works, but a professional programmer friend of mine saw the source
code once, and had to be strapped down for his own (and my!) safety.
It's time to rewrite it in a more object-oriented manner.

I have a couple of options: port the thing to c++, or deal directly in
R.  The R route has a number of advantages.  For example, I deal with
fluxes between adjacent chessboard-square-style elements.  The Fortran
idiom is:

    do 10 i=1,200
      do 20 j=1,200
        flux(i,j) = ( r(i,j)*u(i+1,j) + r(i+1,j)*u(i,j) ) /2
20    continue
10  continue

where r is the density and u the x-component of velocity.  I might
need to do this or similar-looking things such as u(i+1,j)*v(i,j+1)
perhaps a hundred times in my Fortran code.  But in R we could have

flux =  ( r*right(u) + right(r)*u ) /2

[where right <- function(x){cbind(x[,-1],NA)} ]

To my mind, the functional form is much better: it's vectorized and
clear and terse.  My question is, is it fast? (or more precisely, how
much slower would this nice approach be than my clunky old Fortran).
I guess I could tolerate a factor of two or three, and wait for a
shiny next-generation PC.

any comments anyone?


Robin Hankin, Lecturer,
School of Geography and Environmental Science
Tamaki Campus
Private Bag 92019 Auckland
New Zealand

r.hankin at auckland.ac.nz
tel 0064-9-373-7599 x6820; FAX 0064-9-373-7042

as of: Thu Oct  3 09:30:01 NZST 2002
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