[Rcpp-devel] Ranges and Casting

[Balamuta, James Joseph]

Francisco,

Based on the opening post, I’d probably push you more toward Armadillo as Dirk was in the previous message. Using linspace<>(), may be a drop-in replacement for seq() with the appropriate helper functions. Long ago (~4+ years now) I wrote a few, see:

https://github.com/coatless/r-to-armadillo/blob/master/src/seq.cpp

That said, Range is definitely preferred inside of subsets operations into Rcpp-like data structures. However, the subset and assign operation is a bit problematic with template expansion. So, the given example needs to have an intermediary:

  // Initialization:
  Rcpp::NumericVector my_vec = Rcpp::NumericVector::create(1, 2, 3, 4, 5, 6);

  // Subset and Assign to 5:
  Rcpp::IntegerVector subset_idx = Rcpp::Range(0, 3);
  my_vec[subset_idx] = 20;

  // Subset Positions:
 Rcpp::NumericVector subset_result = my_vec[Rcpp::Range(4, 5)];

Hope it helps.

Best,

JJB

From: Rcpp-devel <rcpp-devel-bounces at lists.r-forge.r-project.org> on behalf of Francisco Bischoff <fbischoff at med.up.pt>
Date: Friday, January 15, 2021 at 8:05 PM
To: Dirk Eddelbuettel <edd at debian.org>
Cc: "rcpp-devel at lists.r-forge.r-project.org" <rcpp-devel at lists.r-forge.r-project.org>
Subject: Re: [Rcpp-devel] Ranges and Casting

Thanks for the tips.

Still, Range() is the correct way to write:

NumericVector my_vec(200);

my_vec(Range(0,10)) = 10;

?

Thanks in advance




--
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On Sat, Jan 16, 2021 at 1:54 AM Dirk Eddelbuettel <edd at debian.org<mailto:edd at debian.org>> wrote:

On 16 January 2021 at 01:35, Francisco Bischoff wrote:
| About the Range() function, I think it should handle decreasing ranges
| too...
| But, idk if using Matlabs approach or R approach:
|
| R's:
|
| a <- 1
| b <- 10
|
| print(a:b)
| 1 2 3 4 5 6 7 8 9 10
| print(b:a)
| 10 9 8 7 6 5 4 3 2 1
|
| Matlab's
| a = 1;
| b = 10;
| disp(a:b);
| 1 2 3 4 5 6 7 8 9 10
| disp(b:a);
| numeric(0)
| disp(b:-1:a)
| 10 9 8 7 6 5 4 3 2 1
|
| I think that we are in the R domain, so we should use Range(b, a) normally
| (my humble opinion).

Hm but I think the Range class in Rcpp is not the same as R's seq(). It is
used internally in a few places and may not generalize well to the 'b:-1:a'
use.

I had a quick look, and Armadillo may not have anything directly relevant
either. linspace() is close but not quite the same.

I would probably just write myself a little helper function.

| About Casting, this is a good example:
|
| Rcout << Range(0, 10) << std::endl;
|
| This does not compute...
|
| What should I do?
|
| Rcout << as<IntegerVector>(Range(0, 10)) << std::endl;
| or
| Rcout << (IntegerVector)(Range(0, 10)) << std::endl;

That can happen as template expressions can get in the way. Alternatives are
maybe using Armadillo types for, again, just using a quick loop and printing
elements.

Rcpp has a lot of functions. But it doesn't have "all of R's functions" and
should not be seen as aiming for that.

Dirk

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