Extracting data from XML in R

r, xml

Solution

`purrr` and `xml2` go well together:

library(xml2)
library(purrr)

txt <- '<Doc name="Doc1">
    <Lists Count="1">
        <List Name="List1">
            <Points Count="3">
                <Point Id="1">
                    <Tags Count ="1">"a"</Tags>
                    <Point Position="1"  /> 
                </Point>
                <Point Id="2">
                    <Point Position="2"  /> 
                </Point>
                <Point Id="3">
                    <Tags Count="1">"c"</Tags>
                    <Point Position="3"  /> 
                </Point>
            </Points>
        </List>
    </Lists>
</Doc>'

doc <- read_xml(txt)
xml_find_all(doc, ".//Points/Point") %>% 
  map_df(function(x) {
    list(
      Point=xml_attr(x, "Id"),
      Tag=xml_find_first(x, ".//Tags") %>%  xml_text() %>%  gsub('^"|"$', "", .),
      Position=xml_find_first(x, ".//Point") %>% xml_attr("Position")
    )
  })
## # A tibble: 3 × 3
##   Point   Tag Position
##   <chr> <chr>    <chr>
## 1     1     a        1
## 2     2  <NA>        2
## 3     3     c        3

Problem

I need to extract certain data from XML that looks like this (simplified for brevity) ``` <Doc name="Doc1"> <Lists Count="1"> <List Name="List1"> <Points Count="3"> <Point Id="1"> <Tags Count ="1">"a"</Tags> <Point Position="1" /> </Point> <Point Id="2"> <Point Position="2" /> </Point> <Point Id="3"> <Tags Count="1">"c"</Tags> <Point Position="3" /> </Point> </Points> </List> </Lists> </Doc> ``` The output should be a data frame that matches a tag and position to each point Id ``` Point Tag Position 1 1 a 1 2 2 <NA> 2 3 3 c 3 ``` I am new to XML, I was playing with xml2 package. So far, I could extract each variable separately, but since some points may not have a Tag data , I can't find a way to match between the three parameters. ``` > library(xml2) > xml_data<-read_xml(...) > xml_data %>% xml_find_all("//Point") %>% xml_attr("Id") [1] "1" "2" "3" > xml_data %>% xml_find_all("//Vertical") %>% xml_attr("Position") [1] "1" "2" "3" > xml_data %>% xml_find_all("//Tags") %>% xml_text() [1] "\"a\"" "\"c\"" ```

Original source