RcppParallel RVector push_back or something similar? - r

Question

I am using RcppParallel to speed up some calculations. However, I am running out of memory in the process, so I would like to save results within the Parallel loop that are pass some relevance threshold. Below is a toy example to illustrate my point:
#include <Rcpp.h>
#include <RcppParallel.h>
using namespace Rcpp;
// [[Rcpp::depends(RcppParallel)]]
// [[Rcpp::plugins(cpp11)]]
struct Example : public RcppParallel::Worker {
RcppParallel::RVector<double> xvals, xvals_output, yvals;
Example(const NumericVector & xvals, NumericVector & yvals, NumericVector & xvals_output) :
xvals(xvals), xvals_output(xvals_output), yvals(yvals) {}
void operator()(std::size_t begin, size_t end) {
for(std::size_t i=begin; i < end; i++) {
double y = xvals[i] * (xvals[i] - 1);
// if(y < 0) {
// xvals_output.push_back(xvals[i]);
// yvals.push_back(y);
// }
xvals_output[i] = xvals[i];
yvals[i] = y;
}
}
};
// [[Rcpp::export]]
List find_values(NumericVector xvals) {
NumericVector xvals_output(xvals.size());
NumericVector yvals(xvals.size());
Example ex(xvals, yvals, xvals_output);
parallelFor(0, xvals.size(), ex);
List L = List::create(xvals_output, yvals);
return(L);
}
The R code would be:
find_values(seq(-10,10, by=0.5))
The commented out code is what I would like to do.
That is, I would like to initialize an empty vector, and append only the y-values that pass a certain threshold and also the associated x-values.
In my real usage, I am calculating a MxN matrix, so memory is an issue.
What is the correct way to approach this issue?

Answer 1

If anyone ever comes across a similar problem, here's a solution using "concurrent_vector" from TBB (which RcppParallel uses under the hood and is available as a header).
#include <Rcpp.h>
#include <RcppParallel.h>
#include <tbb/concurrent_vector.h>
using namespace Rcpp;
// [[Rcpp::depends(RcppParallel)]]
// [[Rcpp::plugins(cpp11)]]
struct Example : public RcppParallel::Worker {
RcppParallel::RVector<double> xvals;
tbb::concurrent_vector< std::pair<double, double> > &output;
Example(const NumericVector & xvals, tbb::concurrent_vector< std::pair<double, double> > &output) :
xvals(xvals), output(output) {}
void operator()(std::size_t begin, size_t end) {
for(std::size_t i=begin; i < end; i++) {
double y = xvals[i] * (xvals[i] - 1);
if(y < 0) {
output.push_back( std::pair<double, double>(xvals[i], y) );
}
}
}
};
// [[Rcpp::export]]
List find_values(NumericVector xvals) {
tbb::concurrent_vector< std::pair<double, double> > output;
Example ex(xvals,output);
parallelFor(0, xvals.size(), ex);
NumericVector xout(output.size());
NumericVector yout(output.size());
for(int i=0; i<output.size(); i++) {
xout[i] = output[i].first;
yout[i] = output[i].second;
}
List L = List::create(xout, yout);
return(L);
}
Output:
> find_values(seq(-10,10, by=0.5))
[[1]]
[1] 0.5
[[2]]
[1] -0.25