15-112 Fundamentals of Programming September 29 , 2013
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15-112 Fundamentals of Programming September 29th , 2013 Announcements Midterm next Sunday What are we doing today? Reading from URLs Recursion What is HTML? Hyper Text Markup Language <HTML> <TITLE> Hello World </TITLE> <H1> Welcome to My Webpage </H1> <img src="melatest.jpg“> </HTML> How does the www work? You enter a URL in the address bar of your web browser The web browser fetches the index.html file from that location Index.html file has HTML code that is displayed by the web browser Reading webpages We can read web pages using Python We use the library called urllib Open a url by using urlopen p = urllib.urlopen("http://www.cnn.com") line = p.readline() while line: print line line = p.readline() Let’s Try this Read UPC barcode from user and display the item using http://www.upcdatabase.com import urllib upc = raw_input("Enter UPC> ") p = urllib.urlopen("http://www.upcdatabase.com/item/"+upc) line = p.readline() while line: if "Description" in line: line = line.replace("Description","") line = line.replace("<td>","") line = line.replace("</td>","") line = line.replace("<tr>","") line = line.replace("</tr>","") print line line = p.readline() Recursion A method of simplification that involves dividing a problem into simpler subproblems of the same type. A function that calls itself is called a recursive functions Factorial Example def factorial(n): if n == 0: Base Case return 1 return n * factorial(n-1) Recursive Case General Form of Recursive Functions Function (input) base case – this will stop chain of calls recursive case – call Function with simpler input Definition of Factorial F(0) = 1 F(n) = n*F(n-1) Definition of power F(b,0) = 1 F(b,e) = b * F(b,e-1) Factorial Power def power(base, exp): if (exp == 0): return 1 else: return base * power(base, exp-1) Sum of All elements in a list F([ ]) = 0 F([x,….]) = x + F([….]) Sum of All elements in a list def listSum(list): if (len(list) == 0): return 0 else: return list[0] + listSum(list[1:]) Exercise We want to write a function that counts how many odd numbers exist in a list Write the function definition Write the recursive function Exercise 2 Following is the Fibonacci series 1, 1, 2, 3, 5, 8, 13, 21, 34 In general Fn = Fn-1 + Fn-2 Write the function definition Write a recursive function called Fib(n) that returns the nth Fibonacci number
