rvest, How to have NA values in html_nodes for creating datatables

问题

So I'm trying to make a data table of some information on a website. This is what I've done so far.

library(rvest)
url <- 'https://uws-community.symplicity.com/index.php?s=student_group'
page <- html_session(url)

name_nodes <- html_nodes(page,".grpl-name a")
name_text <- html_text(name_nodes)

df <- data.frame(matrix(unlist(name_text)), stringsAsFactors = FALSE)

library(tidyverse)
df <- df %>% mutate(id = row_number())

desc_nodes <- html_nodes(page, ".grpl-purpose")
desc_text <- html_text(desc_nodes)

df <- left_join(df, data.frame(matrix(unlist(desc_text)), 
                               stringsAsFactors = FALSE) %>% 
                  mutate(id = row_number()))

email_nodes <- html_nodes(page, ".grpl-contact a")

email_text <- html_text(email_nodes)
df <- left_join(df, data.frame(matrix(unlist(email_text)), 
                               stringsAsFactors = FALSE) %>% 
                  mutate(id = row_number()))

This has been working until I got to the emails part. A few of the entries do not have emails. In the data frame, instead of the appropriate rows showing the NA value for the email, the last three rows show an NA value.

How do I make it so the appropriate rows show have the NA value instead of just the last 3 rows?

回答1:

The key for solving this problem is to find the 20 parent nodes which are known to exist for each student group. With this list of parent nodes, use the html_node function on each parent node. The html_node function will return one result or NA depending if the desired tag exists. I would recommend this technique, any time there is a variable number of sub nodes.

library(rvest)
library(dplyr)
url <- 'https://uws-community.symplicity.com/index.php?s=student_group'
page <- html_session(url)

#find group names
name_text <- html_nodes(page,".grpl-name a") %>% html_text()
df <- data.frame(name_text, stringsAsFactors = FALSE)
df <- df %>% mutate(id = row_number())

#find text description
desc_text <- html_nodes(page, ".grpl-purpose") %>% html_text()
df$desc_text <- trimws(desc_text)

#find emails
#  find the parent nodes with html_nodes
#  then find the contact information from each parent using html_node
email_nodes<-html_nodes(page, "div.grpl-grp") %>% html_node( ".grpl-contact a") %>% html_text()
df$emails<-email_nodes

I also took the opportunity to simplify your code, since the lists are all 20 elements long, there is no reason for the unlist/ matrix/ mutate function do add the additional columns onto the data frame.

来源：https://stackoverflow.com/questions/51336519/rvest-how-to-have-na-values-in-html-nodes-for-creating-datatables