[Rcpp-devel] R vectorisation vs. C++ vectorisation

[Darren Cook]

I wrote:
>>> P.S. I don't think the sugar versions can be made any quicker, because
>>> they have to allocate intermediate vectors, and do more memory copies.

By "sugar versions" I meant vacc4() vs. vacc3()
(https://gist.github.com/4111256) not pmin() and friends. The vacc4()
code looks like:

  NumericVector p(age.size());
  p = 0.25 + 0.3 * 1 / (1 - exp(0.04 * age)) + 0.1 * ily;
  p = p * ifelse(female, 1.25, 0.75);
  p = pmax(0,p);
  p = pmin(1,p);

Each operation copies the whole NumericVector each time, each of which
needs a memory allocation.

vacc3a() does the same pipeline of operations on a single double, which
is therefore likely to be a single CPU register, and the whole of
vacc3a() will be inlined in vacc3().


> Also, I'm probably goinf to add a function that does the equivalent of :
> 
> pmax( b, pmin( a, x ) )
> 
> I'm thinking of naming it pminmax. Any better idea ?

I'd expect a function called minmax() to return two values, the min and
max function. The "p" prefix is too subtle for me :-)

How about clip():
   p = clip(p, 0.0, 1.0);

This is such a common operation that I'm surprised R doesn't already
have a function like this!

Darren


-- 
Darren Cook, Software Researcher/Developer

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