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

[Evan Cortens]

Hi folks,

First, thanks for an amazing package! This is my first post to the Rcpp
listserv, so please forgive me if I'm missing something obvious.

I've just noticed what, to me at least, is rather unexpected behaviour, as
follows:

# Here's the Rcpp function
library(Rcpp)

cppFunction('IntegerVector test_int_vec(IntegerVector x) {
  for ( R_xlen_t i = 0; i < x.size(); i++ )
    x[i] = x[i] * 2;
  return x;
}')

# This works as expected
> a <- c(1, 2, 3, 4, 5)
> a
[1] 1 2 3 4 5
> test_int_vec(a)
[1]  2  4  6  8 10
> a
[1] 1 2 3 4 5

# But this doesn't
> b <- c(1L, 2L, 3L, 4L, 5L)
> b
[1] 1 2 3 4 5
> test_int_vec(b)
[1]  2  4  6  8 10
> b
[1]  2  4  6  8 10


You'll see that in the first example, in which I pass a numeric vector, the
function acts as I'd expect it, namely, it returns the result of the
calculation, leaving the original vector unmodified. However, in the second
example, when I pass it an integer vector, it modifies the vector in place,
and the original vector is changed.

I can only imagine this has something to do with implicit coercion: when
the reals are coerced to integers in the first example, the vector must be
cloned, whereas in the second example, in which no coercion is necessary,
it's a pointer to the original data. Another thing that leads me to believe
this is that the same thing happens in reverse if you change the
return/variable type of the function to NumericVector--reals end up being
modified in place and integers aren't.

In my mind, one of the following things should happen:
1) In both situations, the vector is modified in place.
2) In both situations, the vector is copied--probably this isn't right, as
it makes it impossible to modify in place.
3) Allow modification in place, but throw an error if the wrong type is
passed.

Is this a bug? Am I just misunderstanding? Any help would be appreciated!

Thanks in advance and all best,

Evan

=====

> sessionInfo()
R version 3.3.2 (2016-10-31)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 7 x64 (build 7601) Service Pack 1

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United
States.1252    LC_MONETARY=English_United States.1252 LC_NUMERIC=C

[5] LC_TIME=English_United States.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] Rcpp_0.12.10

loaded via a namespace (and not attached):
[1] tools_3.3.2     withr_1.0.2     memoise_1.0.0   digest_0.6.12
devtools_1.12.0


-- 
Evan Cortens, PhD
Institutional Analyst - Office of Institutional Analysis
Mount Royal University
403-440-6529
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