How to refer to an XML node with specific id

Question

I am trying to parse XML returned by API as below:

library(httr2)
library(xml2)
library(tidyverse)

resp_xml <- request("https://data-api.ecb.europa.eu/service/data/CBD2/A..W0.11._Z._Z.A.A.A0000._X.ALL.CA._Z.LE._T.EUR") %>%
  req_perform() %>%
  resp_body_xml()

xml2::xml_find_all(resp_xml, "//generic:SeriesKey") %>% 
  xml2::xml_find_all("//generic:Value")

How can I extract only the values for id="REF_ARA"? I want to get the list just with country codes like "AT", "BE" etc.. I've tried something like this but I get only NAs:

xml2::xml_find_all(resp_xml, "//generic:SeriesKey") %>% 
  xml2::xml_find_all("//generic:Value") %>% 
  xml2::xml_attr("REF_AREA")

Answer 1

If you're trying to get the values of value for nodes where id="REF_AREA", you can do so using XPath:

xml_find_all(resp_xml, "//generic:SeriesKey") %>% 
  xml_find_all("//generic:Value[@id='REF_AREA']") %>% 
  xml_attr("value")

#>  [1] "AT" "B0" "BE" "BG" "CY" "CZ" "DE" "DK" "EE" "ES" "FI" "FR" "GB" "GR" "HR"
#> [16] "HU" "IE" "IT" "LT" "LU" "LV" "MT" "NL" "PL" "PT" "RO" "SE" "SI" "SK" "U2"

Basically, you are trying to find nodes where the value of id (i.e. an attribute) is equal to "REF_AREA". The code below could help you to better understand this concept:

xml_find_all(resp_xml, "//generic:SeriesKey") %>% 
  xml_find_all("//generic:Value") %>% 
  .[xml_attr(., "id") == "REF_AREA"]

#> {xml_nodeset (30)}
#>  [1] <generic:Value id="REF_AREA" value="AT"/>
#>  [2] <generic:Value id="REF_AREA" value="B0"/>
#>  [3] <generic:Value id="REF_AREA" value="BE"/>
#>  [4] <generic:Value id="REF_AREA" value="BG"/>
#>  [5] <generic:Value id="REF_AREA" value="CY"/>
#>  [6] <generic:Value id="REF_AREA" value="CZ"/>
#>  [7] <generic:Value id="REF_AREA" value="DE"/>
#>  [8] <generic:Value id="REF_AREA" value="DK"/>
#>  [9] <generic:Value id="REF_AREA" value="EE"/>
#> [10] <generic:Value id="REF_AREA" value="ES"/>
#> [11] <generic:Value id="REF_AREA" value="FI"/>
#> [12] <generic:Value id="REF_AREA" value="FR"/>
#> [13] <generic:Value id="REF_AREA" value="GB"/>
#> [14] <generic:Value id="REF_AREA" value="GR"/>
#> [15] <generic:Value id="REF_AREA" value="HR"/>
#> [16] <generic:Value id="REF_AREA" value="HU"/>
#> [17] <generic:Value id="REF_AREA" value="IE"/>
#> [18] <generic:Value id="REF_AREA" value="IT"/>
#> [19] <generic:Value id="REF_AREA" value="LT"/>
#> [20] <generic:Value id="REF_AREA" value="LU"/>
#> ...

^{Created on 2023-12-06 with reprex v2.0.2}