[Rcpp-devel] convert to unsigned char* variable

[Dirk Eddelbuettel]

Hi Gustaf,

On 21 October 2014 at 16:05, Gustaf Granath wrote:
| Thanks for the replies. After some testing it seems like I dont need the
| unsigned stuff and the functions seems to work fine with just a regular int
| variable. 

Sounds good.

| The functions I wanted to use were just small parts of a program and
| in that program bitmap images were loaded and image data stored as a unsigned
| char* (for bitmaps unsigned char are often used due to the 0-255 property I
| believe). I thought the simpliest solution was to stick to the original set up
| (so replicating the variables) but it seems that is not necessary which make
| things easier.
| 
| However, I got curious how to handle this if I need it.
| 
| First, I still dont get the difference between R raw and unsigned char (if
| there is one). E.g. 
| this:
| unsigned char a = 'b';
| cout << hex << a << endl;
| equals
| > charToRaw("b")
| [1] 62
| So can unsigned char be used within Rcpp with RawVector?

Yes, what you get from something like charToRaw() should work seamlessly in
RawVector.  [ But in all those years with R, I have found very little use for
'raw' outside of serialization. ]

| For example, is it possible to go from int/char to raw using Rcpp? Or is the

Rcpp is a priori for mapping R types to the closest corresponding C++ types.
So R 'raw' to Rcpp 'RawVector'.  The conversion from raw to unsigned char is
not something R does so Rcpp does not immediately spring to mind either.

R has fewer basic types than C++ as the signed/unsigned duality does not
exist, and for numeric types R only knows one precision.  So Rcpp can't
really translate between all combinations.

| recommended way to use unsigned char to go outside Rcpp as Tim suggested?

You can do that. Once you have a C++ object (and know its storage type), you
can operate on it via standard casts.

That said, Knuth's dictum about premature optmisation comes to mind. I'd try
to get it working first, and only then worry about efficiency.

Dirk

  
| Again, thanks for the input,
| 
| Gustaf
| 
| 
| On 2014-10-21 10:27, Tim Keitt wrote:
| 
| 
| 
|     On Mon, Oct 20, 2014 at 2:17 PM, Gustaf Granath <gustaf.granath at gmail.com>
|     wrote:
| 
|         Hi
|         Im trying to use some C++ functions in R. However, these functions are
|         build around unsigned char* variables (1D array). I have for example a
|         large matrix in R that represents black (e.g. 1-bit image, 0 and 1s). I
|         cant find a solution to convert a character vector into unsigned char*
|         within Rcpp. Im new to Rcpp and C++ so I might have missed something
|         obvious here. Please direct me to resources on this topic (google did
|         not help much).
| 
|         A start:
| 
|         SEXP test(SEXP cM){
|           CharacterVector Vc(cM);
|           int n = Vr.size();
|           RawVector x(n); //is this the same as unsigned char* ?
|                                     //I tried to fill it with
|         char*(Mr) but I didnt succeed
|         }
| 
| 
|     I tend to go through the standard library as its less of a black box. Try
|     creating a std::string and appending your chars to that string. Then call
|     obj.c_str() to get the C-style array of chars. Or push the chars onto a
|     std::vector and do something like &obj[0] to get a 1-D array.
| 
|     THK
|      
| 
| 
|         Thanks,
| 
|         Gustaf
| 
|         --
|         Gustaf Granath (PhD)
|         Post doc
|         McMaster University
|         School of Geography & Earth Sciences
| 
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|    
| 
| 
| 
|     --
|     http://www.keittlab.org/
| 
| 
| --
| Gustaf Granath (PhD)
| Post doc
| McMaster University
| School of Geography & Earth Sciences
| 

-- 
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org