Generate a xml from a R list - r

Question

I'm new to xml and processing it in R.
I've been able to read and retrieve info from xml files using the xml2 package, but creating xml files from R objects has proven to be more challenging.
In particular, I'd like to generate a xml file from a R list. Consider the example below:
library(reprex)
library(xml2)
r_list <- list(person1 = list(starts = letters[1:3], ends = letters[4:6]), person2 = list(starts = LETTERS[1:4], ends = LETTERS[5:8]))
str(r_list)
#> List of 2
#> $ person1:List of 2
#> ..$ starts: chr [1:3] "a" "b" "c"
#> ..$ ends : chr [1:3] "d" "e" "f"
#> $ person2:List of 2
#> ..$ starts: chr [1:4] "A" "B" "C" "D"
#> ..$ ends : chr [1:4] "E" "F" "G" "H"
test1 <- xml2::as_xml_document((r_list))
#> Error: Root nodes must be of length 1
new_xml <- xml_new_root(.value = "category", name = "personList")
for(person in names(r_list)){
xml_add_child(new_xml, as_xml_document(r_list[person]))
}
new_xml
#> {xml_document}
#> <category name="personList">
#> [1] <person1>ad</person1>
#> [2] <person2>AE</person2>
Created on 2021-11-25 by the reprex package (v2.0.1)
I tried to directly coerce the list to xml using the as_xml_document function, but I get the error Root nodes must be of length 1.
Following the idea on this question, I tried to create the xml document with a root node and xml_add_child() to this document, but I did not get the expected result (see code output). In that question, they transform from an R data frame and not a list.
I'd also like to have personalized tag names and add attributes to these tags. The wished output would be:
<category name="personList">
<pers name="person1">
<starts>
<value>a</value>
<value>b</value>
<value>c</value>
</starts>
<ends>
<value>d</value>
<value>e</value>
<value>f</value>
</ends>
</pers>
<pers name="person2">
<starts>
<value>A</value>
<value>B</value>
<value>C</value>
<value>D</value>
</starts>
<ends>
<value>D</value>
<value>E</value>
<value>F</value>
<value>G</value>
</ends>
</pers>
</category>
Thanks for your help and have a nice day

Answer 1

R list attributes can be mapped to XML attributes:
library(xml2)
library(tidyverse)
r_list <- list(person1 = list(starts = letters[1:3], ends = letters[4:6]), person2 = list(starts = LETTERS[1:4], ends = LETTERS[5:8]))
r_list
new_xml <- xml_new_root(.value = "category", name = "personList")
for (person in names(r_list)) {
p <- list()
p[["pers"]] <- list(
starts = r_list[[person]]$starts %>% map(~list(value = list(.x))),
ends = r_list[[person]]$ends %>% map(~list(value = list(.x)))
)
attr(p[["pers"]], "name") <- person
xml_add_child(new_xml, as_xml_document(p))
}
write_xml(new_xml, "foo.xml")
output:
<?xml version="1.0" encoding="UTF-8"?>
<category name="personList">
<pers name="person1">
<starts>
<value>a</value>
<value>b</value>
<value>c</value>
</starts>
<ends>
<value>d</value>
<value>e</value>
<value>f</value>
</ends>
</pers>
<pers name="person2">
<starts>
<value>A</value>
<value>B</value>
<value>C</value>
<value>D</value>
</starts>
<ends>
<value>E</value>
<value>F</value>
<value>G</value>
<value>H</value>
</ends>
</pers>
</category>

Answer 2

Following the comment by #Limey (to see this question), I could generate the wished output with the following code (posted as answer just for completeness, as #danlooo answer also produces the same output).
library(XML)
r_list <- list(person1 = list(starts = letters[1:3], ends = letters[4:6]), person2 = list(starts = LETTERS[1:4], ends = LETTERS[5:8]))
str(r_list)
category = newXMLNode("category", attrs = c(name="personList"))
for(person in names(r_list)){
pers <- newXMLNode("pers", attrs = c(name = person), parent = category)
startsn <- newXMLNode("starts", parent = pers)
for(value in seq_along(r_list[[person]][["starts"]])){
svalue <- newXMLNode("value", r_list[[person]][["starts"]][[value]], parent = startsn)
}
endsn <- newXMLNode("ends", parent = pers)
for(value in seq_along(r_list[[person]][["ends"]])){
evalue <- newXMLNode("value", r_list[[person]][["ends"]][[value]], parent = endsn)
}
}
category