R WebCrawler – XML内容似乎不是XML：

最初运行时我得到：

> WebCrawler(url = "www.bikeforums.net")
[1] "www.bikeforums.net"
[1] "www.bikeforums.net"

Warning message:
XML content does not seem to be XML: 'www.bikeforums.net'

这是代码：

require("XML")

# cleaning workspace
rm(list = ls())

# This function recursively searches for links in the given url and follows every single link.
# It returns a list of the final (dead end) URLs.
# depth - How many links to return. This avoids having to recursively scan hundreds of links. Defaults to NULL,which returns everything.
WebCrawler <- function(url,depth = NULL,verbose = TRUE) {

  doc <- XML::htmlParse(url)
  links <- XML::xpathSApply(doc,"//a/@href")
  XML::free(doc)
  if(is.null(links)) {
    if(verbose) {
      print(url)
    }
    return(url)
  } else {
    urls.out <- vector("list",length = length(links))
    for(link in links) {
      if(!is.null(depth)) {
        if(length(unlist(urls.out)) >= depth) {
          break
        }
      }
      urls.out[[link]] <- WebCrawler(link,depth = depth,verbose = verbose)
    }
    return(urls.out)
  }
}


# Execution
WebCrawler(url = "www.bikeforums.net")

任何建议我做错了什么？

UPDATE

大家好,

我开始这个赏金,因为我认为在R社区需要这样一个功能,可以抓取网页.赢得赏金的解决方案应该显示一个需要两个参数的函数：

WebCrawler(url = "www.bikeforums.net",xpath = "title" )

>作为输出我想有一个数据框架与两列：网站链接,如果示例xpath表达式匹配列与匹配的表达式.

我非常感谢你的回复