使用 Rvest 进行刮擦和循环

Question

我已经查看了与此类似主题相关的 SO 上类似问题的几个答案，但似乎都不适合我。

(loop across multiple urls in r with rvest)

(Harvest (rvest) multiple HTML pages from a list of urls)

我有一个 URL 列表，我希望从每个 URL 中获取表格并将其附加到主数据框。

## get all urls into one list
page<- (0:2)
urls <- list()
for (i in 1:length(page)) {
  url<- paste0("https://www.mlssoccer.com/stats/season?page=",page[i])
  urls[[i]] <- url
}


### loop over the urls and get the table from each page
table<- data.frame()
for (j in urls) {
  tbl<- urls[j] %>% 
    read_html() %>% 
    html_node("table") %>%
    html_table()
  table[[j]] <- tbl
}

第一部分按预期工作并获取我要抓取的网址列表。我收到以下错误：

 Error in UseMethod("read_xml") : 
  no applicable method for 'read_xml' applied to an object of class "list"

有关如何纠正此错误并将 3 个表循环到单个 DF 中的任何建议？我很感激任何提示或指示。

Answer 1

试试这个：

library(tidyverse)
library(rvest)

page<- (0:2)
urls <- list()
for (i in 1:length(page)) {
  url<- paste0("https://www.mlssoccer.com/stats/season?page=",page[i])
  urls[[i]] <- url
}

### loop over the urls and get the table from each page
tbl <- list()
j <- 1
for (j in seq_along(urls)) {
  tbl[[j]] <- urls[[j]] %>%   # tbl[[j]] assigns each table from your urls as an element in the tbl list
    read_html() %>% 
    html_node("table") %>%
    html_table()
  j <- j+1                    # j <- j+1 iterates over each url in turn and assigns the table from the second url as an element of tbl list, [[2]] in this case
}

#convert list to data frame
tbl <- do.call(rbind, tbl)

table[[j]] <- tbl 在原始代码中 for 循环的末尾是不必要的，因为我们将每个 url 分配为 tbl 列表的元素：tbl[[j]] <- urls[[j]]

Answer 2

这是你的问题：

for (j in urls) {
  tbl<- urls[j] %>% 

当您使用j in urls 时，j 的值不是整数，它们本身就是 url。

试试：

for (j in 1:length(urls)) {
  tbl<- urls[[j]] %>% 
    read_html() %>% 
    html_node("table") %>%
    html_table()
  table[[j]] <- tbl
}

你也可以使用seq_along():

for (j in seq_along(urls))