R dplyr 与HTML（或XML）文档、节点或节点集发生变异

Question

我有一个包含多个 HTML link 的文件，现在我想使用 dplyr 和 rvest 为每个 link 的图像获取 link每行。

当我手动执行时它工作正常并且 returns 该行但是当在一个函数中调用相同的代码时它失败并出现以下错误：

Error: no applicable method for 'xml_find_all' applied to an object of class "factor"

我不知道我做错了什么。任何帮助表示赞赏。为了使我的问题更清楚，我添加了（在评论中）一些示例行并展示了手动方法。

library(rvest)
library(dplyr)
library(httr) # contains function stop_for_status()

#get html links from file
#EXAMPLE

# "_id",url

# 560fc55c65818bee0b77ec33,http://www.seriouseats.com/recipes/2011/01/sriracha-ceviche-recipe.html
# 560fc57e65818bee0b78d8b7,http://www.seriouseats.com/recipes/2008/07/pasta-arugula-tomatoes-recipe.html
# 560fc57e65818bee0b78dcde,http://www.seriouseats.com/recipes/2007/08/cook-the-book-minty-boozy-chic.html
# 560fc57e65818bee0b78de93,http://www.seriouseats.com/recipes/2010/02/chipped-beef-gravy-on-toast-stew-on-a-shingle-recipe.html
# 560fc57e65818bee0b78dfe6,http://www.seriouseats.com/recipes/2011/05/dinner-tonight-quinoa-salad-with-lemon-cream.html
# 560fc58165818bee0b78e65e,http://www.seriouseats.com/recipes/2010/10/dinner-tonight-spicy-quinoa-salad-recipe.html

#
#load into SE
#
SE <- read.csv("~/Desktop/SeriousEats.csv")

#
#function to retrieve imgPath per URL
#using rvest
#      
getImgPath <- function(x) {

  imgPath <- x %>% html_nodes(".photo") %>% html_attr("src")
  stop_for_status(res)
  return(imgPath)
}

#This works fine
#UrlPage <- read_html ("http://www.seriouseats.com/recipes/2011/01/sriracha-ceviche-recipe.html")
#imgPath <- UrlPage %>% html_nodes(".photo") %>% html_attr("src")

#
#This throws an error msg
#
S <- mutate(SE, imgPath = getImgPath(SE$url))

Answer 1

这个有效：

library(rvest)
library(dplyr)

# SE <- data_frame(url = c(
#    "http://www.seriouseats.com/recipes/2011/01/sriracha-ceviche-recipe.html",
#    "http://www.seriouseats.com/recipes/2008/07/pasta-arugula-tomatoes-recipe.html"
# ))

SE <- read.csv('/path/to/SeriousEats.csv', stringsAsFactors = FALSE)

getImgPath <- function(x) {
    # x must be "a document, a node set or a single node" per rvest documentation; cannot be a factor or character
    imgPath <- read_html(x) %>% html_nodes(".photo") %>% html_attr("src")
    # httr::stop_for_status(res) OP said this is not necessary, so I removed
    return(imgPath)
}

S <- SE %>% 
    rowwise() %>%
    mutate(imgPath = getImgPath(url))

Answer 2

感谢您的帮助和耐心以及@Jubbles。为了其他人的利益，这里是完整的答案。

library(rvest)
library(dplyr)

SE <- read.csv("~/Desktop/FILE.txt", stringsAsFactors = FALSE)

getImgPath <- function(x) {

  if (try(url.exists(x))) {
  imgPath <- html(x) %>% 
            html_nodes(".photo") %>% 
              html_attr("src")
} 
else {
imgPath = "NA"
}
 #imgPath
 return(imgPath)
}

SE1 <- SE %>% 
  rowwise() %>%
  mutate(imgPath = getImgPath(url))

R dplyr 与HTML（或XML）文档、节点或节点集发生变异

R dplyr mutate with HTML (or XML) documents, nodes, or node sets

r

dplyr

rvest