Why is my list empty after execution? - r

Question

I managed to fill my doi_list but it does not work if I encapsulate the code into a function. From a tutorial I've seen I assume that this should be possible but doi_list is empty after get_doi_from_category() finishes.
library(aRxiv)
get_doi_from_category <- function(category, doi_list) {
arxiv_rec <- arxiv_search(category)
arxiv_doi_list <- arxiv_rec[13]
by(arxiv_doi_list, 1:nrow(arxiv_doi_list),
function(row) {
if(nchar(row) > 0) {
doi_list <<- c(doi_list, row)
}
})
}
doi_list <- list()
get_doi_from_category('cat:stat.ML', doi_list)
for(doi in doi_list)
{
print(doi)
}
get_doi_from_category('cat:stat.CO', doi_list)
get_doi_from_category('cat:stat.ME', doi_list)
get_doi_from_category('cat:stat.TH', doi_list)
PS: First day with R.

Answer 1

Here's a better way to do what you want in R:
categ <- c(CO = "cat:stat.CO", #I'm naming these elements so
ME = "cat:stat.ME", # that the corresponding elements
TH = "cat:stat.TH", # in the list are named as well.
ML = "cat:stat.ML") # Could also just set 'names(doi_list)' to 'categ'.
doi_list <-
lapply(categ, function(ctg)
(doi <- arxiv_search(ctg)$doi)[nchar(doi) > 0])
I sort of threw you in the deep end on the last line with in-line assignment of doi; a more step-by-step approach would be:
lapply(categ, function(ctg){
arxiv.df <- arxiv_search(ctg)
doi <- arxiv.df$doi
doi[nchar(doi) > 0]})