On Wed, 23 Aug 2006, Thomas Lumley wrote:
> On Wed, 23 Aug 2006, Dominick Samperi wrote:
>
> > In for loops of the form:
> > for(m in seq(beg,end))
> > ...
> >
> > when beg > end, the default behavior is
> > to decrement, and the action described
> > by ... is executed at least once.
> >
> > On the other hand, if you view this
> > construction as a translation of the C code:
> > for(m = beg; m < end; m++)
> > ...
Your R translates more closely to
if(beg <= end) for(m = beg; m <= end; m++) {}
else for(m = beg; m >= end; m--) {}
which is quite a lot different (and the <= vs < mistake seems to explain
why you think it should not be executed at least once).
> > then the semantics of C/C++ is not
> > respected,
or to put it another way, your translation was unfaithful to the original
and
for(m in beg + seq(length=max(0,end-beg)) )
is a better translation.
> > because the code in ... is
> > not executed when beg > end in
> > the case of C/C++.
>
> There is another important way in which the C loop is different
> from the R loop. If you modify m or end inside the loop the number of
> iterations of the C loop changes but the number of iterations of the R
> loop doesn't. R's for() loop just isn't the same as C's.
I was told only yesterday that one should try to think in the (human)
language you are going to be writing in. That I have never needed to
write such constructs in years of S/R programming suggests to me that
thinking R changes one's perspective over thinking C (or Fortran or ...).
--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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