converting double * to std::vector gives me error

Question

I have seen other posts regarding this issue. I followed them.

The code I am using to convert a double array to a double vector is-

cppFunction("


             std::vector<double> ArrayToVector(double *arr){

               std::vector<double> vec(arr, arr+ sizeof(arr)/sizeof(arr[0]));

              return vec;
             }



            ")

But I am getting this error:

file52e20f14d34.cpp:28:52: error: cannot initialize a variable of type 'Rcpp::traits::input_parameter<double>::type *' (aka 'InputParameter<double> *') with an lvalue of type 'SEXP' (aka 'SEXPREC *')
    Rcpp::traits::input_parameter< double >::type *arr(*arrSEXP);
                                                   ^   ~~~~~~~~
1 error generated.
make: *** [file52e20f14d34.o] Error 1
clang++  -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -I"/Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include" -I"/private/var/folders/4w/v4pl36r9475cb2fkspc3qxkm0000gp/T/RtmpSXqByM/sourceCpp-x86_64-apple-darwin15.6.0-1.0.1" -I/usr/local/include   -fPIC  -Wall -g -O2  -c file52e20f14d34.cpp -o file52e20f14d34.o
Error in sourceCpp(code = code, env = env, rebuild = rebuild, cacheDir = cacheDir,  : 
  Error 1 occurred building shared library.

Can anyone explain me why this is happening? Any help would be much appreciated.

This is because the pointer has lost the knowledge that it came from an array. `sizeof(arr)` is nonsense when the type is not a fixed array. You can fix this with templates, or by passing either the end of the array of doubles or its length as a second parameter. — Gem Taylor, May 14 '19 at 11:12
`sizeof(arr)` provide size of the pointer not size of the array it point to. So this is a bug. Since `sizeof(arr[0])` can be bigger then size of pointer then result is zero. — Marek R, May 14 '19 at 11:13
This is clearly [XY problem](http://xyproblem.info/). You have some issue with bridging between `R` and `C++` which you didn't described. — Marek R, May 14 '19 at 11:15
@GemTaylor I tried putting the length of the array as a second parameter; it did not work either.:( — gultu, May 14 '19 at 11:18
error message is completely unrelated to shown code. Please read about XY problem and restate your question. — Marek R, May 14 '19 at 11:19
Please read at least [Extending R with C++: A Brief Introduction to Rcpp](https://cloud.r-project.org/web/packages/Rcpp/vignettes/Rcpp-introduction.pdf). It is not fair to come here with half-baked question that you expect us volunteers to fix for you _when is there is a decade-worth of documentation and earlier questions_. — Dirk Eddelbuettel, May 14 '19 at 11:23
"But I am getting this error!" ... but not with the code you posted — 463035818_is_not_an_ai, May 14 '19 at 11:28
@formerlyknownas_463035818; yes with this code I was getting this error. Anyway, reading the file provided by Dirk — gultu, May 14 '19 at 11:36
tell me where in included code you see anything related to type: `'Rcpp::traits::input_parameter::type *'` or `'SEXP'`? — Marek R, May 14 '19 at 15:58
@MarekR The crucial point is `cppFunction()` from the R package [Rcpp](http://dirk.eddelbuettel.com/code/rcpp.html). This function does some code generation and compilation to integrate C++ functions with the R language and works fine for `double` many other argument types, but not for `double*`. — Ralf Stubner, May 15 '19 at 12:09
@I basically use R vigorously for my analysis. Sometimes I have to use C++ for defining some functions. Anyway, it was a mistake to test some C++ codes in Rcpp. I have to use C++ compiler. — gultu, May 15 '19 at 12:23

score 3 · Answer 1 · answered May 14 '19 at 11:27

You cannot infer the length of the array from double* (unless there is a special double value used as an array terminator). The array size must be passed into the function as well:

std::vector<double> ArrayToVector(double* arr, size_t arr_len) {
    return std::vector<double>(arr, arr + arr_len);
}

converting double * to std::vector gives me error

1 Answers1