Generate a xml from a R list

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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

xml2 r xslt xml
2 Answers

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>
user_168531140

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)
  }
}
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