[Rcpp-devel] A question in slides from Rcpp workshop
edd at debian.org
Fri Aug 5 18:21:44 CEST 2011
On 5 August 2011 at 04:05, Zhongyi Yuan wrote:
| Hi Douglas,
| Thank you for the reply. That clarifies quite a bit.
| But still, it does not explain the different behavior of those examples, does
| it? One moment ago, I was thinking that the L suffix might have made it
| constant and hence can't be modified. Looks like I am wrong. (And fun(1:3)
| gives the same result too.)
Don't take this the wrong way but you man know too little R to get to what
this is about. But please consider this difference:
In particular, both '1' and '1.0' get you a double.
We added the example to show that when you use 1L:3L (eg an integer vector)
and assign to a Rcpp::NumericVector you _do get a separate copy_ due to C++
casting semantics. The proper lightweight call is seq(1.0, 3.0, by=1.0)
which then exhibits the shallow copy vs deep copy issue the rest of the
example is about.
This is somewhat advanced corner-case stuff so maybe you should not fret too
too much about it now but come back to it later.
| On Fri, Aug 5, 2011 at 3:49 AM, Douglas Bates <bates at stat.wisc.edu> wrote:
| On Fri, Aug 5, 2011 at 5:27 AM, Zhongyi Yuan <yuanzygoso at gmail.com> wrote:
| > Dear useR's,
| > After I found Rcpp a few days ago, I've been very excited collecting
| > documents for learning. But still I find myself understand little.
| > Here's a question I want you to help me with.
| > In page 17 of Dirk and Romain's slides from part2 of the Apr 28 Rcpp
| > workshop (here's a link:
| > http://dirk.eddelbuettel.com/papers/rcpp_workshop_part_2_details.pdf), I
| > can't figure out why the two examples behave differently.
| > And also, why are people using 1L:3L? What not just 1:3?
| It happens that they are the same but only because 1:3 generates an
| integer sequence by default. Most of the time 1 gives a double
| precision floating point number whereas 1L is an integer. Those with
| long-time experience in writing R code tend to use the L when they
| know that an integer is wanted, just to bypass the conversion step.
| > Maybe I am asking silly question? But please do help me. I couldn't find
| > answer on google.
| > Thank you.
| > Best,
| > Zhongyi
| > _______________________________________________
| > Rcpp-devel mailing list
| > Rcpp-devel at lists.r-forge.r-project.org
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