R How to Check if XPath Exists

Question

hoping someone more knowledgeable than me can throw some light here.

As part of a larger web-scraper I want to pull meta data out of a set of pages. When I ran this it

Answer 1

Assuming the error comes when you try and process the empty list...

> parsed <- htmlParse("http://www.coindesk.com/information")
> meta <- xpathApply(parsed, "//meta[starts-with(@property, \"og:description\")]", xmlGetAttr,"content")
> meta
list()
> length(meta)==0
[1] TRUE

Then test for length(meta)==0 - which is TRUE if the element is missing. Otherwise its FALSE - as in this example of extracting the title property:

> meta <- xpathApply(parsed, "//meta[starts-with(@property, \"og:title\")]", xmlGetAttr,"content")
> meta
[[1]]
[1] "Beginner's guide to bitcoin - CoinDesk's Information Center"

> length(meta)==0
[1] FALSE