Working with Forms and Dropdown Lists using rvest and httr in R

When scraping websites for data using rvest and httr in R, one common challenge is dealing with forms that require selecting an item from a dropdown list. In this article, we will explore how to use rvest and httr to interact with these types of forms, specifically focusing on the select function and form submission.

Introduction

rvest and httr are two popular R packages used for web scraping and HTTP requests. While they provide efficient ways to scrape websites, handling forms that require user input can be tricky. In this article, we will delve into the world of rvest and httr to learn how to work with dropdown lists and form submissions.

Understanding Forms in rvest and httr

When interacting with a website using rvest and httr, you may encounter forms that require selecting an item from a dropdown list. These forms typically contain HTML elements such as select, option, and input fields.

In the given Stack Overflow question, we see the following code snippet:

library(rvest)
library(httr)

url <- "http://www.ahw.gov.ab.ca/IHDA_Retrieval/ihdaData.do"
sess <- html_session(url)

# Step 1: Follow links to specific pages
sess %>% follow_link(css="#content > div > p:nth-child(8) > a") %>%
  follow_link(css="#content > div > table:nth-child(3) > tbody > tr:nth-child(10) > td > a")

In this code snippet, we are following links to specific pages on the website using the follow_link function. This allows us to navigate through the website’s structure and find relevant data.

Extracting Dropdown List Options

To extract the options from a dropdown list, we can use the html_nodes function in combination with the option selector:

library(tidyverse)
library(rvest)

url <- "http://www.ahw.gov.ab.ca/IHDA_Retrieval/ihdaData.do"

# Get HTML page
page <- GET(url) %>% read_html()

# Extract dropdown list options
pages <- tibble(id = page %>% html_nodes("option") %% html_attr("value"),
                 item = page %>% html_nodes("option") %% html_text())

# Remove empty rows
pages <- pages[which(pages$item != ""), ]

In this code snippet, we are extracting the options from a dropdown list using html_nodes and option. We then create a tibble to store the extracted data.

Submitting Forms with rvest

To submit a form, we need to send an HTTP request with the required parameters. In rvest, we can use the POST function to achieve this:

params <- list(command = "doSelect", displayObject.id = pages$id[1])

next_page <- POST(paste0(url, "selectSubCategory.do"), body = params)

In this code snippet, we are creating a list of parameters (command and displayObject.id) to be sent with the HTTP request. We then use the POST function to send the request and retrieve the response.

Handling Dropdown List Selection

Once we have extracted the options from a dropdown list, we can select an item by sending the corresponding command parameter in the form:

params <- list(command = "doSelect", displayObject.id = pages$id[1])

next_page <- POST(paste0(url, "selectSubCategory.do"), body = params)

In this code snippet, we are selecting the first item from the dropdown list by sending the command parameter with the corresponding value.

Conclusion

Working with forms and dropdown lists using rvest and httr in R can be challenging, but it is not impossible. By understanding how to extract options from dropdown lists and submit forms, you can unlock valuable data from websites that would otherwise be inaccessible.

In this article, we have explored the select function and form submission in rvest and httr. We hope that this guide has provided you with a solid foundation for working with forms and dropdown lists in R.

Last modified on 2024-03-26