[Rcpp-devel] Understanding the behaviour of const CharacterVector as a function parameter

[Simon Zehnder]

On Sep 29, 2013, at 3:24 PM, Romain Francois <romain at r-enthusiasts.com> wrote:

> Le 29/09/13 14:06, Simon Zehnder a écrit :
>> Dear Rcpp::Users and Rcpp::Devels,
>> 
>> I would like to understand a certain behaviour of my code I encountered lately.
>> 
>> I am working with CharacterVector and the following behaviour occurred:
>> 
>> void test1 (Rcpp::CharacterVector &charv)
>> {
>> 	Rprintf("test1: %s\n", (char*) charv(0));
>> }
>> 
>> void test2 (const Rcpp::CharacterVector &str)
>> {
>> 	Rprintf("test2: %s\n", (char*) charv(0));
>> }
> 
> Try actually using the variable you pass in, as in:
> 
> void test2 (const Rcpp::CharacterVector &str)
> {
> 	Rprintf("test2: %s\n", (char*) str(0));
> }

My fault! :)
I tried to post a simpler example than my code and made this silly error.
> Although it still exposes the bug.
> 
> You can use something like this in the meantime:
> 
> void test2 (const Rcpp::CharacterVector& charv)
> {
>    String x = charv[0] ;
>    Rprintf("test2: %s\n", x.get_cstring());
> }
> 
This was my first approach. I write a date parser that converts strings to seconds since 1970 for Rcpp. Then I decided to convert my code to use char pointers, which I assumed to be much faster. It works with nonconst parameters and is indeed on 10e+6 values 1 second faster (takes 0.836 secs in total). Regarding the better performance of my code in case of using char I want to stick to it and accept here using nonconst parameters. 

> It looks like the bug is about converting the result of charv(0) to a char*. Probably worth looking at the string_proxy class.
> 
I took a look at the string_proxy class and other related classes. It takes some more time for me to understand what is going on, but step by step I get a better insight, what is going on under the hood. 

Thank you for the quick response. 

Best

Simon

> Romain
> 
>> Using a string like "2013-05-04 20:23:21" for the Rcpp::CharacterVector gives the following outputs:
>> 
>> test1: 2013-05-04 20:23:21
>> 
>> test2:  `
>> 
>> This does also not change if I use a cast to const char* in test2. I tried something similar with strings and printing the c_str() of them, there the 'const' keyword does not make a difference - it always prints the correct string.
>> 
>> Is this something specific to the Rcpp::CharacterVector, that uses a string_proxy for its elements returned by the operator ()? Is there a way to use const Rcpp::CharacterVector and get the behaviour of test1?
>> 
>> 
>> Best
>> 
>> Simon
> 
> -- 
> Romain Francois
> Professional R Enthusiast
> +33(0) 6 28 91 30 30
> 
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