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POMultiplying a column vector by a numeric scalar in RcppArmadillo
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  1. POMultiplying a column vector by a numeric scalar in RcppArmadillo
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    copied!<p>I am having some trouble compiling this simple <code>c++</code> code using <code>Rcpp</code> and the <code>RcppArmadillo</code> package. Take the following simple example to multiply each column of a matrix by a numeric scalar:</p> <pre><code>code &lt;- 'arma::mat out = Rcpp::as&lt;arma::mat&gt;(m); for(int i = 0; i &lt; out.n_cols; ++i){ out.col(i) *= v; } return Rcpp::wrap( out );' </code></pre> <p>Trying to compile this using...</p> <pre><code>require( RcppArmadillo ) armMult &lt;- cxxfunction( signature( m = "numeric" , v = "numeric" ), code , plugin = "RcppArmadillo" ) </code></pre> <p>Results in the compile error....</p> <pre><code>#error: no match for 'operator*=' in 'arma::Mat&lt;eT&gt;::col(arma::uword) [with eT = double, arma::uword = unsigned int](((unsigned int)i)) *= v' </code></pre> <p>However, if we swap the <code>numeric</code> variable <code>v</code> for <code>2.0</code> as below....</p> <pre><code>code &lt;- 'arma::mat out = Rcpp::as&lt;arma::mat&gt;(m); for(int i = 0; i &lt; out.n_cols; ++i){ out.col(i) *= 2.0; //Notice we use 2.0 instead of a variable } return Rcpp::wrap( out );' </code></pre> <p>It compiles just fine....</p> <pre><code>armMult &lt;- cxxfunction( signature(m="numeric"), code,plugin="RcppArmadillo") </code></pre> <p>And we can then do...</p> <pre><code>m &lt;- matrix( 1:4 , 2 , 2 ) armMult( m ) [,1] [,2] [1,] 2 6 [2,] 4 8 </code></pre> <p>What am I missing here? How can I make this work with a simple numeric scalar. I would like to be able to pass a scalar like...</p> <pre><code>armMult( m , 2.0 ) </code></pre> <p>And return the same result as above.</p>
 

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