Hard Test Analysis

Introduction

The hard test pushes the boundaries of XML parsing with the xml2 package, focusing on the conversion of an XML document into a structured R list. This test not only tests the package’s parsing capabilities but also its ability to transform XML data into a format that is easily manipulable within R. It involves creating a custom function to recursively parse the XML document, demonstrating the xml2 package’s flexibility and power in handling complex XML structures. The code snippet in this section provides a comprehensive example of how to leverage the xml2 package to parse XML documents into R lists, showcasing the package’s robustness and versatility in XML data manipulation.

Section 1: Loading Libraries and XML Content

library(xml2)
library(stringr)
library(rlist)

z <- '
<CATALOG>
 <CD>
    <TITLE>Empire Burlesque</TITLE>
    <ARTIST>Bob Dylan</ARTIST>
    <COUNTRY>USA</COUNTRY>
    <COMPANY>Columbia</COMPANY>
    <PRICE>10.90</PRICE>
    <YEAR>1985</YEAR>
 </CD>
 <CD>
    <TITLE>Hide your heart</TITLE>
    <ARTIST>Bonnie Tylor</ARTIST>
    <COUNTRY>UK</COUNTRY>
    <COMPANY>CBS Records</COMPANY>
    <PRICE>9.90</PRICE>
    <YEAR>1988</YEAR>
 </CD>
</CATALOG>'

Explanation

• The xml2, stringr, and rlist libraries are loaded to handle XML data and list manipulation in R.

• An XML content string representing a catalog of CDs is defined, including details like title, artist, country, company, price, and year.

Section 2: Parsing XML to List Using rlist

res <- rlist::list.parse(z, type='xml')
res

## $CD
## $CD$TITLE
## [1] "Empire Burlesque"
## 
## $CD$ARTIST
## [1] "Bob Dylan"
## 
## $CD$COUNTRY
## [1] "USA"
## 
## $CD$COMPANY
## [1] "Columbia"
## 
## $CD$PRICE
## [1] "10.90"
## 
## $CD$YEAR
## [1] "1985"
## 
## 
## $CD
## $CD$TITLE
## [1] "Hide your heart"
## 
## $CD$ARTIST
## [1] "Bonnie Tylor"
## 
## $CD$COUNTRY
## [1] "UK"
## 
## $CD$COMPANY
## [1] "CBS Records"
## 
## $CD$PRICE
## [1] "9.90"
## 
## $CD$YEAR
## [1] "1988"

Explanation

• The list.parse function from the rlist package is used to parse the XML string into an R list.

• The type=‘xml’ argument specifies that the input is XML content.

• The parsed list is stored in the variable res.

Section 3: Custom Function to Parse XML to List

parse_xml_to_list <- function(xml_string) {
 xml_doc <- read_xml(xml_string)
  
 xml_to_list <- function(node) {
    
    if (xml_length(node) == 0) {
      return(xml_text(node))
    } 
    
    else {
      children <- xml_children(node)
      list_result <- lapply(children, xml_to_list)
      return(setNames(list_result, xml_name(children)))
    }
 }
    
 result <- xml_to_list(xml_doc)
  
 return(result)
}

Explanation

• A custom function parse_xml_to_list is defined to parse an XML string into an R list.

• The function uses recursion to traverse the XML document. If a node has no children, it returns the text content of the node. Otherwise, it creates a list with the node’s name as the key and the children’s list as the value.

• The xml_length function is used to check if a node has children.

• The xml_children function is used to get the children of a node.

• The xml_name function is used to get the name of a node.

• The xml_text function is used to get the text content of a node.

Section 4: Using the Custom Function

res2 <- parse_xml_to_list(z)

print(res2)

## $CD
## $CD$TITLE
## [1] "Empire Burlesque"
## 
## $CD$ARTIST
## [1] "Bob Dylan"
## 
## $CD$COUNTRY
## [1] "USA"
## 
## $CD$COMPANY
## [1] "Columbia"
## 
## $CD$PRICE
## [1] "10.90"
## 
## $CD$YEAR
## [1] "1985"
## 
## 
## $CD
## $CD$TITLE
## [1] "Hide your heart"
## 
## $CD$ARTIST
## [1] "Bonnie Tylor"
## 
## $CD$COUNTRY
## [1] "UK"
## 
## $CD$COMPANY
## [1] "CBS Records"
## 
## $CD$PRICE
## [1] "9.90"
## 
## $CD$YEAR
## [1] "1988"

identical(res, res2) 

## [1] TRUE

Explanation

• The custom function parse_xml_to_list is used to parse the XML string z into an R list, which is stored in res2.

• The print function is used to display the parsed list.

• The identical function checks if the list parsed by rlist::list.parse is identical to the list parsed by the custom function, demonstrating the equivalence of the two methods.