Project Revision 1

Due: December 18, 2020
Points: 100

1. (50 points) A web crawler is a program that starts at a web site. It then follows the links on that page recursively. If you think of the links as branches of a tree, and the original web site as the root, the crawler follows the branches to the next “root” (i.e., web site) and then repeats this process some number of times (the depth).

   Your job is to write a simple web crawler. We will focus only on one type of link, that which begins with “http:”.

   Write a program that reads in a URL and the depth as an integer. The program should print the integer and the links on the page.

   All links will look like this: href="link’s url" so you have to extract the “link’s url” part. You can do this in two ways. First, you can use the string method find() to look for “href="”, and then for the closing “"”, and extract the string between the two. Or, you can use the regular expression package. To do this, import “re” and then use the line:

   re.findall(’href="(http://.*?)"’, webpagetextstring)

   where webpagetextstring is the contents of the web page you are checking. This returns a list of links, for example

   
   [ ’http://nob.cs.ucdavis.edu/mhi289i/sub1/index.html’,
   ’http://nob.cs.ucdavis.edu/secure-exer/index.html’,
   ’http://nob.cs.ucdavis.edu/mhi289i/sub2/index.html’,
   ’http://nob.cs.ucdavis.edu/mhi289i/next.html’ ]
   

   Print the links in the following form (your listing may differ):

   
   http://nob.cs.ucdavis.edu/mhi289i/index.html contains:
        http://nob.cs.ucdavis.edu/mhi289i/sub1/index.html
        http://nob.cs.ucdavis.edu/secure-exer/index.html
        http://nob.cs.ucdavis.edu/mhi289i/sub2/index.html
        http://nob.cs.ucdavis.edu/mhi289i/next.html
   

   or, if there are no links, print:

   
   http://nob.cs.ucdavis.edu/secure-exer/index.html contains no links
   

   To turn in: Please call your program “crawler1.py” , and submit it to Canvas.

2. (50 points) Extend your program in problem 1. Recursively visit each link and print the links on the page, as above. Do this to the indicated depth. Be sure you don’t repeat pages; once you print the links, do not print them again should you revisit the page.

   A good web site to test your program on is http://nob.cs.ucdavis.edu/mhi289i/index.html.

   Print the links in the following form (your listing may differ):

   
   http://nob.cs.ucdavis.edu/mhi289i/index.html contains:
        http://nob.cs.ucdavis.edu/mhi289i/sub1/index.html
        http://nob.cs.ucdavis.edu/secure-exer/index.html
        http://nob.cs.ucdavis.edu/mhi289i/sub2/index.html
        http://nob.cs.ucdavis.edu/mhi289i/next.html
   http://nob.cs.ucdavis.edu/mhi289i/sub1/index.html contains:
        http://nob.cs.ucdavis.edu/mhi289i/sub2/index.html
        http://nob.cs.ucdavis.edu/mhi289i/index.html
   

   Hint: Use a dictionary for this. The key would be the URL of the current web page and the value would be the list of links. Then, when you visit a web page, check that its URL is not in the dictionary. If it is, you already visited it and all its links, so simply return.

   To turn in: Call this program “crawler2.py” when you submit it.

Matt Bishop Office: 2209 Watershed Sciences Phone: +1 (530) 752-8060 Email: mabishop@ucdavis.edu

MHI 289I, Programming for Health Informatics Version of December 2, 2020 at 11:53PM

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