r - download.file 错误 : scheme not supported

Question

我需要从“http://www.elections.state.md.us”下载一些 csv 文件。

这是我的代码。

url <- "http://www.elections.state.md.us/elections/2012/election_data/index.html"
# recognize the links
links <- getHTMLLinks(url)
filenames <- links[str_detect(links,"_General.csv")]
filenames_list <- as.list(filenames)
filenames
# create a function
downloadcsv <- function(filename,baseurl,folder){
  dir.create(folder,showWarnings = FALSE)
  fileurl <- str_c(baseurl,filename)
  if(!file.exists(str_c(folder,"/",filename))){
    download.file(fileurl,
                  destfile = str_c(folder,"/",filename))
    # 1 sec delay between files
    Sys.sleep(1)
  }
}
library(plyr)
l_ply(filenames_list,downloadcsv,
      baseurl = "www.elections.state.md.us/elections/2012/election_data/",
      folder = "elec12_maryland")

错误如下:

Error in download.file(fileurl, destfile = str_c(folder, "/", filename)) : scheme not supported in URL 'www.elections.state.md.us/elections/2012/election_data/State_Congressional_Districts_2012_General.csv'

但是，当我尝试将 url 粘贴到 IE 中时，它确实起作用了。那么我的代码有什么问题呢？

任何想法都会有所帮助，谢谢。

Answer 1

事实证明，url 必须以 http://、https://、ftp://或 file://等方案开头。所以在最后一行，我把代码改成了

l_ply(filenames_list,downloadcsv,
      baseurl = "http://www.elections.state.md.us/elections/2012/election_data/",
      folder = "elec12_maryland")

而且它有效。