<font face="arial,helvetica,sans-serif">Hi Xiao,</font><div><font face="arial,helvetica,sans-serif"><br></font></div><div><font face="arial,helvetica,sans-serif">The problem is that sourceCpp can only accept arguments of types that can be converted to R using Rcpp::as (this is detailed in the help topic for sourceCpp and in the Rcpp vignettes). Plain C function pointers aren't convertible in this fashion so the compilation fails.</font></div>
<div><font face="arial,helvetica,sans-serif"><br></font></div><div><font face="arial,helvetica,sans-serif">If your intention is to pass an R function to C++ you can however use the Rcpp::Function type in your signature. This is explained in more detail in this Rcpp Gallery article:</font></div>
<div><font face="arial,helvetica,sans-serif"><br></font></div><div><font face="arial,helvetica,sans-serif"><a href="http://gallery.rcpp.org/articles/r-function-from-c++/">http://gallery.rcpp.org/articles/r-function-from-c++/</a></font></div>
<div><font face="arial,helvetica,sans-serif"><br></font></div><div><font face="arial,helvetica,sans-serif">J.J.<br></font><br><div class="gmail_quote">On Sat, May 18, 2013 at 1:31 PM, Xiao He <span dir="ltr"><<a href="mailto:praguewatermelon@gmail.com" target="_blank">praguewatermelon@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi everyone, I have two questions regarding passing a function name as an argument.<div><br></div><div>(1). Suppose that I have a function foo() shown below. The function takes a NumericVector and a function pointer that points to a function that takes a NumericVector and returns a double. Note that the function pointer will only point to functions not exposed to R.</div>
<div><br></div><div><div><font face="courier new, monospace">double foo(NumericVector x, double (*f)(NumericVector x) ){</font></div><div><font face="courier new, monospace"><span style="white-space:pre-wrap"> </span>double output;</font></div>
<div><font face="courier new, monospace"><span style="white-space:pre-wrap"> </span>output=(*f)(x);</font></div><div><font face="courier new, monospace"><span style="white-space:pre-wrap"> </span>return output;</font></div>
<div><font face="courier new, monospace">}</font></div><div><br></div></div><div>When I try to compile it as is using sourceCpp(), it's fine, but when I add "// [[Rcpp::export]]" above the function definition, I get an error message:</div>
<div><br></div><div><div><font face="courier new, monospace">Error in sourceCpp("foo.cpp") : </font></div><div><font face="courier new, monospace"> Error 1 occurred building shared library.</font></div><div><font face="courier new, monospace">foo.cpp:229: error: expected initializer before ‘SEXP’</font></div>
</div><div><br></div><div>So I wonder how I can fix this mistake.</div><div><br></div><div><br></div><div>(2). Imagine a more complex scenario: suppose there are two functions available to be passed to foo(). But the two functions differ in the number of arguments each has (see fun1() and fun2() below). I wonder if there is any way to deal with this.</div>
<div><br></div><div><div><font face="courier new, monospace">double fun1(NumericVector x){</font></div><div><font face="courier new, monospace"><span style="white-space:pre-wrap"> </span>double total=0;</font></div>
<div><font face="courier new, monospace"><span style="white-space:pre-wrap"> </span>for(int i=0;i<x.size();i++)</font></div><div><font face="courier new, monospace"><span style="white-space:pre-wrap"> </span>total+=x(i);</font></div>
<div><font face="courier new, monospace"><span style="white-space:pre-wrap"> </span>return total/10;</font></div><div><font face="courier new, monospace">}</font></div><div><font face="courier new, monospace"><br>
</font></div><div><font face="courier new, monospace"><br></font></div><div><font face="courier new, monospace">double fun2(NumericVector x, int n){</font></div><div><font face="courier new, monospace"><span style="white-space:pre-wrap"> </span>double total=0;</font></div>
<div><font face="courier new, monospace"><span style="white-space:pre-wrap"> </span>for(int i=0;i<x.size();i++)</font></div><div><font face="courier new, monospace"><span style="white-space:pre-wrap"> </span>total+=x(i)+add;</font></div>
<div><font face="courier new, monospace"><span style="white-space:pre-wrap"> </span>return total/n;</font></div><div><font face="courier new, monospace">}<span style="white-space:pre-wrap"> </span></font></div>
<div><span style="white-space:pre-wrap"><font face="courier new, monospace"> </font></span></div></div><div>Thank you in advance!</div><div><br></div><div><br></div><div>Best,</div><div>-Xiao</div><div>
<br>
</div>
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