Extracting Links with Ruby String Methods: index, rindex

Extracting links from web pages (often known as data harvesting or web scraping)  is an easy task with Ruby's string methods. With the index and rindex string methods, the position of the first delimiter and the last delimiter in a line of html code can be found with just two lines of code. Many links within html source code begin and end with a double quote character(").  This makes the double quote character the ideal delimiter to locate when attempting to extract a link. Using the index method to find the position of the first double quote character and the rindex method to find the position of the  last double quote character, one can then use the range method to extract the complete link.


Identifying the Link to Extract 

In this code example, the goal is to extract the link from the following string within an html file:

for background information visit <a href="http://www.linkedin.com/pub/mark- stansberry/60/828/826">linkedin page</a><br />

The link we want, http://www.linkedin.com/pub/mark- stansberry/60/828/826, is delimited with two double quotes.

The code used to find the positions of the first double quote character and the last double quote character is given below. Note, that we assign the string from which the link is extracted to the string variable, a.


The Code: File name: strings_lesson.rb

a = 'for background information visit <a href="http://www.linkedin.com/pub/mark- stansberry/60/828/826">linkedin page</a><br />'


#finding the position of the first occurrence of a quote symbol

puts "The first quote in the string variable, a, is at postion #{a.index('"')}"

#But what about the last quote(") position. for that we use the rindex string method

puts "The last quote in the string variable, a,  is at postion #{a.rindex('"')}"        


Output from the Code

The first quote in the string variable, a, is at position 41
The last quote in the string variable, a, is at position 97

The Next Step 

Now that the positions of the first and last double quote characters is known, that information can be used with Ruby's In the next lesson, the code to actually extract and store the link is given. This code uses Ruby's powerful  substring extraction methods. Ruby's substring extraction methods. These methods require as parameters the starting position and the ending position of the substring delimiters in order to extract the substring.

Error Messages 

Running the code, requires that you cut and paste the above code into a file with the Ruby extension, .rb and then run the program. If for some reason you are obtaining errors, feel free to comment on this blog. The code has been tested (actually cut and pasted from the blog) and runs correctly.


References

String: http://www.ruby-doc.org/core-2.1.0/String.html

Ruby Data Harvesting, Lesson 2, Web Page Downloading, Readlines and Arrays

In  lesson 1 the code was written to download the html source page from the website, www.bookmarktutoring.com. If you ran the example code, you will have noticed that this web page has quite a few lines.

In the example code below, instead of just downloading the html file and displaying it, instead we are going to download the file and place the individual lines of the file into an array. This will allow us to access each line of the web page individually. Not only that, it will allow us in the next lesson, to search for specific content within the file, extract the data, and replace data in the file. Being able to download a web page to a file and store it in an array effectively will give us part of the foundation to build a data harvesting or scraping tool

require 'open-uri'
f = open('http://www.bookmarktutoring.com/default.html')
file_contents = f.readlines
puts file_contents
puts file_contents[0]
puts file_contents[1]
puts file_contents[3]


In the above code, the open-uri gem was imported into  prthe ogram. The main page of bookmarktutoring.com was downloaded and stored into the open uri object variable, f, with the open command (Note single quotes around the URL).  Next the readlines method was invoked on the variable f. The readlnes method reads one line of a time of the downloaded web page and stores each line sequentially into an array called file_contents.

The first puts command, puts_file_contents, displays the entire page,  as in the last example. The next three put statements however display only the first, second and third line of the web page. When you run the program you will first see the entire contents of the web page fly by the screen. The last three lines displayed will contain the first three lines of the web page.

As an exercise, substitute a URL of your choice in the open command. Be aware that you may run into problems for web pages that have URLs with different extension names and prefixes.


TO LESSON 3: DATA HARVESTING, ARRAY ITERATION

Data Harvesting with Ruby: Lesson 1: Downloading Web Pages

In  order to harvest or scrape data from the world wide web, you have to learn how to download iles from the web. The sample code below, a simplifief method,  illustrates a program that extracts the default page of bookmarktutoring.com.

require 'net/http'
url = 'http://www.bookmarktutoring.com/default.html'
uri = URI.parse(url)
response = Net::HTTP.get_response(uri)
puts response.body


In Ruby, downloading a file from the Internet requires the net/http gem. This code package or gem allows you to access (read and write) methods on files stored on remote servers.

The code extracts the main page of  BookMarkTutoring.com and displays it on the screen. Note that in order to access the main page of bookmarktutoring the default or index page of the web site is used.

On the first line of the code, the net/http gem is imported into the program. Next the page  to read into a variable called URL. Note that the web page is required to be in single quote marks.

The parse method of the URI is then involved. This command splits up the URL into a parsed segments so that the server where the web page resides can locate the file.

The method  "get_response" is then called. This method retrieves  the response of the server. The  resulting response of the server  is stored in the variable called "response."

The body method is then invoked on the response object.  The body contains the data contained in the page returned.

Lesson 2 Ruby Data Harvesting, Downloading Web Pages, Readlines, Arrays