Rcpp: Calculation in loop stops with error "Not a matrix" - r

Question

in an R script I source a cpp file to make some calculations. In that R script, a function defined in the cpp file is called and a matrix and an integer is provided. After a few rounds through the loop it gives the error "Not a matrix" (in line of code resid = (x(_,j) - x(_,i))*(x(_,j) - x(_,i));), even though for the rounds before it worked.
R script:
## all together
# rm(list=ls())
library(RcppArmadillo)
library(Rcpp)
sourceCpp("~/test.cpp",verbose = FALSE)
cat("start loop")
for(n in c(45:46)){
cat("\n", n, "\n")
p_m <- matrix(data=rnorm(n^2,1,1),nrow = n, ncol=n)
print(class(p_m))
print(some_function(p_m,nosamples=10))
}
cat("finished")
I start this R script via the command line. R version R-4.1.0. In R-Studio it crashes with a fatal error.
The cpp file:
// [[Rcpp::depends(RcppArmadillo)]]
#include <RcppArmadillo.h>
using namespace Rcpp;
// [[Rcpp::export]]
NumericVector some_function(NumericMatrix x,int nosamples) {
int ncol = x.ncol();
NumericVector out2(nosamples);
int loops;
int loops2;
double result=0;
NumericVector::iterator it;
double acc = 0;
NumericVector resid(ncol);
NumericVector out(ncol*(ncol-1)/2);
loops2=0;
std::cout << nosamples << std::endl;
std::cout << (ncol-1) << std::endl;
std::cout << ncol*(ncol-1)/2 << std::endl;
while(loops2 < (nosamples)){
std::cout << "loops2:" << std::endl;
std::cout << loops2 << std::endl;
loops=0;
int i;
int j;
for(j=0;j<(ncol-1);++j){
std::cout << " j: " << j << std::endl;
for (i = (j+1); i < (ncol); ++i) {
std::cout << " i: " << i << std::endl;
resid = (x(_,j) - x(_,i))*(x(_,j) - x(_,i)); //here it stops
std::cout << " i: " << i << std::endl;
for(int ii=0; ii<ncol;++ii){
acc += resid[i];
}
result=sqrt(acc);
loops += 1;
out[loops] = result;
std::cout << " i: " << i << std::endl;
}
}
std::cout << "loops:" << std::endl;
std::cout << loops << std::endl;
out = out[out > 0];
it = std::min_element(out.begin(), out.end());
out= *it;
std::cout << out << std::endl;
loops2 += 1;
out2[loops2]=out[0];
}
std::cout << "cpp finished" << std::endl;
return(out2);
}
Can someone explain what the problem is about?
Thanks and kind regards
Edit
I adapted some things in the cpp file (shown below) and the error disappeared. First I thought, everything is fine. But when I increase the number of loops, another problem occurs: the function breaks, but no error is shown. It breaks after loop number 543 ("loop2: 543"). At least it does the same in each round of the while loop with the same data.
I adapted the R-script and the ccp file to make this problem (at least on my machine) reproducible.
I know this code seems to be somehow meaningless, but it is part of a bigger program and I wanted to give here a minimum example.
The R script:
## all together
# rm(list=ls())
library(RcppArmadillo)
library(Rcpp)
sourceCpp("~/test.cpp",verbose = FALSE)
cat("start loop")
for(n in c(100:101)){
cat("\n", n, "\n")
p_m <- matrix(data=rnorm(n^2,1,1),nrow = n, ncol=n)
print(class(p_m))
print(some_function(p_m,nosamples=800))
}
cat("finished")
The cpp file:
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::depends(RcppEigen)]]
#include <RcppArmadillo.h>
#include <RcppEigen.h>
using namespace Rcpp;
using Eigen::Map;
using Eigen::VectorXd;
typedef Map<VectorXd> MapVecd;
// [[Rcpp::export]]
NumericVector some_function(NumericMatrix x,int nosamples) {
int ncol = x.ncol();
NumericVector out(ncol*(ncol-1)/2);
NumericVector out2(nosamples);
NumericVector out3(ncol*(ncol-1)/2);
NumericVector resid(ncol);
int loops;
int loops2;
double result=0;
double acc = 0;
int show_cout=0;
loops2=0;
std::cout << nosamples << std::endl;
std::cout << (ncol-1) << std::endl;
std::cout << ncol*(ncol-1)/2 << std::endl;
while(loops2 < (nosamples)){
std::cout << "loops2:" << loops2 << std::endl;
loops=0;
int i;
int j;
for(j=0;j<(ncol-1);++j){
// std::cout << " j: " << j << std::endl;
for (i = (j+1); i < (ncol); ++i) {
if(show_cout==1){
std::cout << " i: " << i << std::endl;
}
resid = (x(_,j) - x(_,i))*(x(_,j) - x(_,i));
if(show_cout==1){
std::cout << " i: " << i << std::endl;
}
for(int ii=0; ii<ncol;++ii){
acc += resid[ii];
}
result=sqrt(acc);
loops += 1;
out[loops] = result;
if(show_cout==1){
std::cout << " i: " << i << std::endl;
}
}
}
// std::cout << "loops:" << loops << std::endl;
//
out = out[out > 0];
const MapVecd xy(as<MapVecd>(out));
out3=xy.minCoeff();
out2[loops2]=out3[0];
loops2 += 1;
}
std::cout << "cpp finished" << std::endl;
return(out2);
}

Answer 1

Two things here:
Use out[loops++] = result; instead of loops += 1; out[loops] = result; because you were starting at 1, and probably accessing the last element outside of the range of this vector.
Use
for(int ii=0; ii<ncol;++ii){ double eps = x(ii, j) - x(ii, i); acc += eps * eps; }
instead of relying on this resid vector.