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POwhich(x, arr.ind=T) in RCPP
 

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  1. POwhich(x, arr.ind=T) in RCPP
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    <p>I couldn't find the cool <code>which(x,arr.ind=T)</code> feature in Rcpp or RcppArmadillo. So I decided to quickly code that up on my own.</p> <pre><code>// [[Rcpp::export]] arma::umat whicha(arma::mat matrix, int what ){ arma::uvec outp1; int n = matrix.n_rows; outp1 = find(matrix==what); int nf = outp1.n_elem; arma::mat out(nf,2); arma::vec foo; arma::uvec foo2; foo = arma::conv_to&lt;arma::colvec&gt;::from(outp1) +1; foo2 = arma::conv_to&lt;arma::uvec&gt;::from(foo); for(int i=0; i&lt;nf; i++){ out(i,0) = ( foo2(i) %n); out(i,1) = ceil(foo(i) / n ); if(out(i,0)==0) { out(i,0)=n; } } return(arma::conv_to&lt;arma::umat&gt;::from(out)); } </code></pre> <p>The code seems quite inefficient, but <code>microbenchmark</code> reveals that it can be faster than R's <code>which</code> function.</p> <p>Question: Can I further change this function to actually exactly reproduce R's <code>which</code> function, i.e. pass <code>MATRIX == something</code> to it? Right now I need a second argument for that. I just like to have this for convenience.</p> <hr> <p>Update: fixed a bug - needed ceil instead of floor</p> <p>How to check:</p> <pre><code>ma=floor(abs(rnorm(100,0,6))) testf=function(k) {all(which(ma==k,arr.ind=T) == whicha(ma,k))} ; sapply(1:10,testf) </code></pre> <p>Benchmark:</p> <pre><code>&gt; microbenchmark(which(ma==k,arr.ind=T) , whicha(ma,k)) Unit: microseconds expr min lq median uq max neval which(ma == k, arr.ind = T) 10.264 11.170 11.774 12.377 51.317 100 whicha(ma, k) 3.623 4.227 4.830 5.133 36.224 100 </code></pre>
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    1. COWell, a straight up improvement if you want performance is to not copy data around in arma types. Having an `arma::mat` object as input in your `whicha` function means you are copying data from an R object into an armadillo object. This has a cost. Returning an `arma::umat` will convert this object to an R object which also means data copies.
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      1. USRomain Francois
 

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