How to compute rowSums in rcpp - r

Question

I'm converting an R function into Rcpp, where I have used the R function rowSums, which appears to not be a valid sugar expression in Rcpp. I found code for an Rcpp version of rowSums here. But I'm getting
error: use of undeclared identifier
when I use rowSumsC() in my main Rcpp function.
Is there a easy fix?
Edit: The 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 in 0.12.8. Therefore, there is no need to use rowSumsC function devised by Hadley in Advanced R.
Having said this, there are a few issues with the code.
Rcpp presently does not support Matrix to Vector or Matrix to Matrix computations. (Support for the later may be added per #583, though if needed one should consider using RcppArmadillo or RcppEigen). Therefore, the following line is problematic:
w = w / rowSums(w)
To address this, first compute the rowSums and then standardize the matrix using a traditional for loop. Note: Looping in C++ is very fast unlike R.
NumericVector summed_by_row = rowSums(w);
for (int i = 0; i < k; ++i) {
w(_,i) = w(_,i) / summed_by_row[i];
}
Next, C++ indices begin at 0 not 1. Therefore, the following for loop is problematic:
for (int i=1; i<k; ++i)
The fix:
for (int i=0; i<k; ++i)
Lastly, the parameters of the function can be reduced as some of the values are not relevant or are overridden.
The function declaration goes 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);
.....
Putting all of the above feedback together, 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)
Output
Before:
0.0470993 0.125384
0.0285671 0.160996
rowSum: 0.172483 0.189563
After:
0.273066 0.661436
0.165623 0.849300