课程页面：https://www.udacity.com/course/intro-to-computer-science--cs101
授课教师：Dave Evans https://www.cs.virginia.edu/~evans/
如下内容包含课程笔记和自己的扩展折腾
factorial (recursion)
# -*- coding: utf-8 -*-
# Define a procedure, factorial, that takes a natural number as its input, and
# returns the number of ways to arrange the input number of items.
def factorial(n):
if n == 0:
return 1
else:
return n * factorial(n-1)
print factorial(0)
#>>> 1
print factorial(5)
#>>> 120
print factorial(10)
#>>> 3628800
Palindromes
# -*- coding: utf-8 -*-
# Define a procedure is_palindrome, that takes as input a string, and returns a
# Boolean indicating if the input string is a palindrome.
# Base Case: '' => True
# Recursive Case: if first and last characters don't match => False
# if they do match, is the middle a palindrome?
# 下面的是我的算法：
def is_palindrome(s):
if s == "":
return True
else:
return s[0] == s[-1] and is_palindrome(s[1:-1])
'''
这里是Udacity的算法，和我的思路稍许不一样：
def is_palindrome(s):
if s == "":
return True
else:
if s[0] == s[-1]:
return is_palindrome(s[1:-1])
else:
return False
'''
print is_palindrome('')
#>>> True
print is_palindrome('abab')
#>>> False
print is_palindrome('abba')
#>>> True
print is_palindrome('aba')
# True
Evans有提及，相比于interactive way，对python来说，the recursive way is fairly expensive... 有时候还是最好用数学上的一些算法优化一下。
Fibonacci numbers (recursive)
# -*- coding: utf-8 -*-
# Define a procedure, fibonacci, that takes a natural number as its input, and
# returns the value of that fibonacci number.
# Two Base Cases:
# fibonacci(0) => 0
# fibonacci(1) => 1
# Recursive Case:
# n > 1 : fibonacci(n) => fibonacci(n-1) + fibonacci(n-2)
def fibonacci(n):
if n == 0:
return 0
elif n == 1:
return 1
else:
return fibonacci(n-1) + fibonacci(n-2)
print fibonacci(0)
#>>> 0
print fibonacci(1)
#>>> 1
print fibonacci(15)
#>>> 610
Fibonacci numbers (interactive)
# -*- coding: utf-8 -*-
#Define a faster fibonacci procedure that will enable us to computer
#fibonacci(36).
# 下面是我的算法，写的很臃肿
def fibonacci(n):
fibo1 = 0
fibo2 = 1
count = 1
if n == 1:
return 1
if n == 0:
return 0
while count < n:
r = fibo1 + fibo2
fibo1 = fibo2
fibo2 = r
count +=1
return r
'''
# Udacity的算法相较之下好多了
def fibonacci(n):
current = 0
after = 1
for i in range(n):
current, after = after, current+after
return current
'''
print fibonacci(36)
#>>> 14930352
Implementing Urank
# -*- coding: utf-8 -*-
# Modify the crawl_web procedure so that instead of just returning the
# index, it returns an index and a graph. The graph should be a
# Dictionary where the key:value entries are:
# url: [list of pages url links to]
def crawl_web(seed): # returns index, graph of outlinks
tocrawl = [seed]
crawled = []
graph = {} # :[list of pages it links to]
index = {}
while tocrawl:
page = tocrawl.pop()
if page not in crawled:
content = get_page(page)
add_page_to_index(index, page, content)
outlinks = get_all_links(content)
graph[page] = outlinks
#Insert Code Here
union(tocrawl, outlinks)
crawled.append(page)
return index, graph
cache = {
'http://udacity.com/cs101x/urank/index.html': """
Dave's Cooking Algorithms
Here are my favorite recipes:
For more expert opinions, check out the
Nickel Chef
and Zinc Chef.
""",
'http://udacity.com/cs101x/urank/zinc.html': """
The Zinc Chef
I learned everything I know from
the Nickel Chef.
For great hummus, try
this recipe.
""",
'http://udacity.com/cs101x/urank/nickel.html': """
The Nickel Chef
This is the
best Hummus recipe!
""",
'http://udacity.com/cs101x/urank/kathleen.html': """
Kathleen's Hummus Recipe
- Open a can of garbanzo beans.
- Crush them in a blender.
- Add 3 tablespoons of tahini sauce.
- Squeeze in one lemon.
- Add salt, pepper, and buttercream frosting to taste.
""",
'http://udacity.com/cs101x/urank/arsenic.html': """
The Arsenic Chef's World Famous Hummus Recipe
- Kidnap the Nickel Chef.
- Force her to make hummus for you.
""",
'http://udacity.com/cs101x/urank/hummus.html': """
Hummus Recipe
- Go to the store and buy a container of hummus.
- Open it.
""",
}
def get_page(url):
if url in cache:
return cache[url]
else:
return None
def get_next_target(page):
start_link = page.find('>> ['http://udacity.com/cs101x/urank/hummus.html',
#'http://udacity.com/cs101x/urank/arsenic.html',
#'http://udacity.com/cs101x/urank/kathleen.html',
#'http://udacity.com/cs101x/urank/nickel.html',
#'http://udacity.com/cs101x/urank/zinc.html']
Computer Ranks
# -*- coding: utf-8 -*-
#Finishing the page ranking algorithm.
def compute_ranks(graph):
d = 0.8 # damping factor
numloops = 10
ranks = {}
npages = len(graph) # 6
for page in graph:
ranks[page] = 1.0 / npages # 0.16667
for i in range(0, numloops):
newranks = {}
for page in graph:
newrank = (1 - d) / npages
for item in graph:
if page in graph[item]:
newrank += d*(ranks[item]/len(graph[item]))
newranks[page] = newrank
ranks = newranks
return ranks
cache = {
'http://udacity.com/cs101x/urank/index.html': """
Dave's Cooking Algorithms
Here are my favorite recipies:
For more expert opinions, check out the
Nickel Chef
and Zinc Chef.
""",
'http://udacity.com/cs101x/urank/zinc.html': """
The Zinc Chef
I learned everything I know from
the Nickel Chef.
For great hummus, try
this recipe.
""",
'http://udacity.com/cs101x/urank/nickel.html': """
The Nickel Chef
This is the
best Hummus recipe!
""",
'http://udacity.com/cs101x/urank/kathleen.html': """
Kathleen's Hummus Recipe
- Open a can of garbonzo beans.
- Crush them in a blender.
- Add 3 tablesppons of tahini sauce.
- Squeeze in one lemon.
- Add salt, pepper, and buttercream frosting to taste.
""",
'http://udacity.com/cs101x/urank/arsenic.html': """
The Arsenic Chef's World Famous Hummus Recipe
- Kidnap the Nickel Chef.
- Force her to make hummus for you.
""",
'http://udacity.com/cs101x/urank/hummus.html': """
Hummus Recipe
- Go to the store and buy a container of hummus.
- Open it.
""",
}
def crawl_web(seed): # returns index, graph of inlinks
tocrawl = [seed]
crawled = []
graph = {} # , [list of pages it links to]
index = {}
while tocrawl:
page = tocrawl.pop()
if page not in crawled:
content = get_page(page)
add_page_to_index(index, page, content)
outlinks = get_all_links(content)
graph[page] = outlinks
union(tocrawl, outlinks)
crawled.append(page)
return index, graph
def get_page(url):
if url in cache:
return cache[url]
else:
return None
def get_next_target(page):
start_link = page.find('>> {'http://udacity.com/cs101x/urank/kathleen.html': 0.11661866666666663,
#'http://udacity.com/cs101x/urank/zinc.html': 0.038666666666666655,
#'http://udacity.com/cs101x/urank/hummus.html': 0.038666666666666655,
#'http://udacity.com/cs101x/urank/arsenic.html': 0.054133333333333325,
#'http://udacity.com/cs101x/urank/index.html': 0.033333333333333326,
#'http://udacity.com/cs101x/urank/nickel.html': 0.09743999999999997}