Error using rcpp armadillo when making R package - r

Question

I am trying to get into using C++ code in my R packages. I created a test package using
usethis::create_package("~/Documents/ptest")
and I then set up the C++ necessaries using
usethis::use_rcpp_armadillo("sum_func")
I then created R/test-package.R and pasted in
## usethis namespace: start
#' #useDynLib ptest, .registration = TRUE
## usethis namespace: end
NULL
## usethis namespace: start
#' #importFrom Rcpp sourceCpp
## usethis namespace: end
NULL
In sum_func.cpp I then put
//sum.cpp
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
double rcpp_sum(NumericVector v){
double sum = 0;
for(int i=0; i<v.length(); ++i){
sum += v[i];
}
return(sum);
}
I then created R/sum__func.R and put in
#' My Sum Function
#'
#' #param vec A vector of values
#'
#' #return The sum
#' #export
#'
#' #examples
#' my_sum(1:10)
my_sum <- function(vec) {
rcpp_sum(vec)
}
But then when I run devtools::load_all(".") I get
> ℹ Loading ptest Exports from
> /Users/hwarden/Documents/ptest/src/sum_func.cpp: double
> rcpp_sum(NumericVector v)
>
> /Users/hwarden/Documents/ptest/src/RcppExports.cpp updated.
> /Users/hwarden/Documents/ptest/R/RcppExports.R updated. Re-compiling
> ptest ─ installing *source* package ‘ptest’ ... ** using staged
> installation ** libs clang++ -mmacosx-version-min=10.13
> -std=gnu++11 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I'/Library/Frameworks/R.framework/Versions/4.1/Resources/library/Rcpp/include'
> -I'/Library/Frameworks/R.framework/Versions/4.1/Resources/library/RcppArmadillo/include'
> -I/usr/local/include -fPIC -Wall -g -O2 -UNDEBUG -Wall -pedantic -g -O0 -fdiagnostics-color=always -c RcppExports.cpp -o RcppExports.o clang++ -mmacosx-version-min=10.13 -std=gnu++11
> -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I'/Library/Frameworks/R.framework/Versions/4.1/Resources/library/Rcpp/include'
> -I'/Library/Frameworks/R.framework/Versions/4.1/Resources/library/RcppArmadillo/include'
> -I/usr/local/include -fPIC -Wall -g -O2 -UNDEBUG -Wall -pedantic -g -O0 -fdiagnostics-color=always -c sum_func.cpp -o sum_func.o clang++ -mmacosx-version-min=10.13 -std=gnu++11 -dynamiclib
> -Wl,-headerpad_max_install_names -undefined dynamic_lookup -single_module -multiply_defined suppress -L/Library/Frameworks/R.framework/Resources/lib -L/usr/local/lib -o ptest.so RcppExports.o sum_func.o
> -L/Library/Frameworks/R.framework/Resources/lib -lRlapack -L/Library/Frameworks/R.framework/Resources/lib -lRblas -L/usr/local/gfortran/lib/gcc/x86_64-apple-darwin18/8.2.0 -L/usr/local/gfortran/lib -lgfortran -lquadmath -lm -F/Library/Frameworks/R.framework/.. -framework R -Wl,-framework -Wl,CoreFoundation ld: warning: directory not found for option '-L/usr/local/gfortran/lib/gcc/x86_64-apple-darwin18/8.2.0' ld:
> warning: directory not found for option '-L/usr/local/gfortran/lib'
> ld: library not found for -lgfortran clang: error: linker command
> failed with exit code 1 (use -v to see invocation) make: ***
> [ptest.so] Error 1 ERROR: compilation failed for package ‘ptest’ ─
> removing
> ‘/private/var/folders/n4/lwxlchrn2s7gvc878rw7ln1m0000gn/T/RtmpjJw195/devtools_install_c9e2c847f68/ptest’
> Error in `(function (command = NULL, args = character(),
> error_on_status = TRUE, …`: ! System command 'R' failed
> --- Exit status: 1 stdout & stderr: <printed>
> ---
I get an error but after googling I can't figure out how to solve it.

Answer 1

Thank you #dirk-eddelbuettel,
This article was the most useful.
I'm not an expert, but for me this worked (or at least got me to my next error).
I ran
brew reinstall gcc
to make sure I had the most up to date gfortran. Then I made a ~/.R/Makevars (as I did not have one already). I then wrote this in there
FC = usr/local/opt/gcc/bin/gfortran
F77 = /usr/local/opt/gcc/bin/gfortran
FLIBS = -L/usr/local/opt/gcc/lib
I restarted R and the package then loaded successfully! Many thanks!