[Rcpp-devel] Unintentional modification of vector in place?

[Evan Cortens]

Wonderful! Thanks again!

On Mon, Mar 20, 2017 at 12:13 PM, Dirk Eddelbuettel <edd at debian.org> wrote:

>
> Evan,
>
> On 20 March 2017 at 12:02, Evan Cortens wrote:
> | Thanks very much! Apologies for posting about something that has been
> covered
> | elsewhere, I did search, but clearly not well enough. (Indeed, I see,
> this is
> | almost exactly the same issue: https://stackoverflow.
> com/questions/11300048/
> | rcpp-pass-by-reference-vs-by-value )
>
> No worries. I think this should be in the R FAQ. I need to add something...
>
> | On (3), apologies for being unclear, of course casting from int to
> double isn't
> | an issue. I meant to say it could be an issue the other way around,
> namely,
> | double to int. If you instead pass, for example, c(1.1, 2.2, 3.3, 4.4,
> 5.5,
> | 6.6) to my example function, it'll silently cast that to an integer
> vector c(1,
> | 2, 3, 4, 5, 6) (i.e., truncating). That said, this isn't really
> unexpected
> | behaviour, rather exactly what you'd expect if you did {double x = 1.1;
> int y =
> | (int) x;} in C. I'll pass on writing my own compiler, at least for now :)
>
> I know. I was just being snarky -- some of these things we cannot test for
> as
> the compiler gets in the way. As in should, because most of the time it
> does
> know better.
>
> | Finally, just to close the loop on this and to make sure I understand...
> if I
> | don't want to modify the original vector, I should do this, yes?
> |
> | cppFunction('IntegerVector test_int_vec(IntegerVector x) {
> |   IntegerVector y = clone(x);
> |   for ( R_xlen_t i = 0; i < x.size(); i++ )
> |     y[i] = y[i] * 2;
> |   return y;
> | }')
>
> Yes, pretty much.   And the simpler code here works too:
>
> R> cppFunction("IntegerVector test_int_vec(IntegerVector x) { return 2*x;
> }")
> R> vec <- c(1L, 2L, 3L)
> R> test_int_vec(vec)
> [1] 2 4 6
> R> vec
> [1] 1 2 3
> R>
>
> Dirk
>
> --
> http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org
>
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