Retrieve number of citations of a paper in Google Scholar

ncites_scholar

#' Retrieve number of citations of a paper in Google Scholar
#' 
#' This function retrieves the number of citations of a given paper in Google Scholar.
#' 
#' @param user character vector. The user ID in Google Scholar Citations. Obtain from author's profile website in google scholar (http://scholar.google.com/citations?user=...)
#' @param paper character vector. The paper ID in Google Scholar Citations. Copy from publication list in author's profile website.
#' @author F. Rodriguez-Sanchez
#' @examples
#' ncites_scholar(user="B7vSqZsAAAAJ", paper="d1gkVwhDpl0C")  
#' 
ncites_scholar <- function(user, paper){
  
  require(XML)
  require(stringr)
  
  URL <- paste0("http://scholar.google.com/citations?user=", user, 
                "&view_op=view_citation&hl=en&citation_for_view=", user, ":", paper)
    
  doc <- htmlParse(URL, encoding="UTF-8")
  citeschunk<- paste(xpathSApply(doc, "//div[@class='gsc_value']", xmlValue), collapse=" ")
  cites <- unlist(str_extract_all(citeschunk, "Cited by \\d+"))[2]
  ncites <- ifelse (!is.na(cites), as.integer(str_extract(cites, "\\d+")), 0)
    
  return(ncites)
  
  
}