How to calculate rowSums in rcpp

Question

I convert the R function to Rcpp, where I used the R rowSums function, which does not seem to be a valid sugar expression in Rcpp. I found the code for the Rcpp version of rowSums here . But I get

error: use of undeclared identifier

when I use rowSumsC() in my main Rcpp function.

Is there an easy fix?

Edit: Code

 cppFunction( "NumericMatrix Expcpp(NumericVector x, NumericMatrix w, NumericVector mu, NumericVector var, NumericVector prob, int k) { for (int i=1; i<k; ++i){ w(_,i) = prob[i] * dnorm(x,mu[i], sqrt(var[i])); } w = w / rowSums(w) return w; }") 

Answer 1

Rcpp officially added rowSum support to 0.12.8 . Therefore, there is no need to use the rowSumsC function developed by Hadley in Advanced R.

Having said that, there are several problems with the code.

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Rcpp currently does not support Matrix calculations to Vector or Matrix to Matrix . (Support for a later version may be added in # 583 , although RcppArmadillo or RcppEigen should be considered if necessary). Therefore, the following line is problematic:

 w = w / rowSums(w) 

To fix this, first compute rowSums and then standardize the matrix using the traditional for loop. Note: The C ++ loop is very different from R.

 NumericVector summed_by_row = rowSums(w); for (int i = 0; i < k; ++i) { w(_,i) = w(_,i) / summed_by_row[i]; } 

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Next, C ++ indices start at 0 not 1 . Therefore, for the loop, there is the following:

 for (int i=1; i<k; ++i) 

Correction:

 for (int i=0; i<k; ++i) 

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Finally, function parameters can be reduced, as some of the values ​​are not relevant or overridden.

Function declaration comes from:

 NumericMatrix Expcpp(NumericVector x, NumericMatrix w, NumericVector mu, NumericVector var, NumericVector prob, int k) 

To:

 NumericMatrix Expcpp(NumericVector x, NumericVector mu, NumericVector var, NumericVector prob) { int n = x.size(); int k = mu.size(); NumericMatrix w = no_init(n,k); ..... 

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Combining all the above feedbacks, we get the desired function.

 Rcpp::cppFunction( 'NumericMatrix Expcpp(NumericVector x, NumericVector mu, NumericVector var, NumericVector prob) { int n = x.size(); int k = mu.size(); NumericMatrix w = no_init(n,k); for (int i = 0; i < k; ++i) { // C++ indices start at 0 w(_,i) = prob[i] * dnorm(x, mu[i], sqrt(var[i])); } Rcpp::Rcout << "Before: " << std::endl << w << std::endl; NumericVector summed_by_row = rowSums(w); Rcpp::Rcout << "rowSum: " << summed_by_row << std::endl; // normalize by column to mimic R for (int i = 0; i < k; ++i) { w(_,i) = w(_,i) / summed_by_row[i]; } Rcpp::Rcout << "After: " << std::endl << w << std::endl; return w; }') set.seed(51231) # Test values n <- 2 x <- seq_len(n) mu <- x var <- x prob <- runif(n) mat <- Expcpp(x, mu, var, prob) 

Exit

 Before: 0.0470993 0.125384 0.0285671 0.160996 rowSum: 0.172483 0.189563 After: 0.273066 0.661436 0.165623 0.849300