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

[Alexandre Bujard]

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

Great idea!!!


On Wed, Nov 21, 2012 at 2:00 PM, Romain Francois
<romain at r-enthusiasts.com>wrote:

> Le 21/11/12 13:22, Darren Cook a écrit :
>
>  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 <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.
>>
>
> Nope. The NumericVector::operator=( sugar expression ) is used and memory
> for p is allocated just once, when p is constructed with this lne:
>
>  NumericVector p(age.size());
>
> None of the "p = ..." lines allocate memory for p.
>
> The differences in timing don't come from additional memory allocations.
>
> Although this thread allows me to identify something that can be improved
> in the operator=.
>
>
>  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);
>>
>
>
> I like "clip".
>
>
>
>  This is such a common operation that I'm surprised R doesn't already
>> have a function like this!
>>
>> Darren
>>
>>
>>
>
> --
> Romain Francois
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