R：在 Rvest 中使用管道链命令抓取多个 url

Question

我有一个包含多个 url 的 chr 列表。我想从这些网址中的每一个下载内容。

为了避免写出数百条命令，我希望使用 lapply 循环自动执行该过程。

但是我的命令返回错误。是否可以从多个网址中抓取？

目前的方法

长方法：有效，但我希望自动化

urls <-c("https://en.wikipedia.org/wiki/Belarus","https://en.wikipedia.org/wiki/Russia","https://en.wikipedia.org/wiki/England")

library(rvest)
library(httr) # required for user_agent command

uastring <- "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36"
session <- html_session("https://en.wikipedia.org/wiki/Main_Page", user_agent(uastring))
session2<-jump_to(session, "https://en.wikipedia.org/wiki/Belarus")
session3<-jump_to(session, "https://en.wikipedia.org/wiki/Russia")
writeBin(session2$response$content, "test1.txt") 
writeBin(session3$response$content, "test2.txt")

自动/循环：不起作用。

urls <-c("https://en.wikipedia.org/wiki/Belarus","https://en.wikipedia.org/wiki/Russia","https://en.wikipedia.org/wiki/England")

library(rvest)
library(httr) # required for user_agent command

uastring <- "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36"
session <- html_session("https://en.wikipedia.org/wiki/Main_Page", user_agent(uastring))
lapply(urls, .%>% jump_to(session))
Error: is.session(x) is not TRUE

总结

我希望自动化以下两个过程，jump_to() 和 writeBin()，如下面的代码所示

session2<-jump_to(session, "https://en.wikipedia.org/wiki/Belarus")
session3<-jump_to(session, "https://en.wikipedia.org/wiki/Russia")
writeBin(session2$response$content, "test1.txt") 
writeBin(session3$response$content, "test2.txt")

Answer 1

你可以这样做：

urls <-c("https://en.wikipedia.org/wiki/Belarus","https://en.wikipedia.org/wiki/Russia","https://en.wikipedia.org/wiki/England")
require(httr)
require(rvest)
uastring <- "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36"
session <- html_session("https://en.wikipedia.org/wiki/Main_Page", user_agent(uastring))

outfile <- sprintf("%s.html", sub(".*/", "", urls))

jump_and_write <- function(x, url, out_file){
  tmp = jump_to(x, url)
  writeBin(tmp$response$content, out_file) 
}

for(i in seq_along(urls)){
  jump_and_write(session, urls[i], outfile[i])
}