Python_24_Udacity_Evans_Intro to CS_6_How to have infinite power

<a href="http://www.lxweimin.com/p/54870e9541fc">總目錄</a>


課程頁面:https://www.udacity.com/course/intro-to-computer-science--cs101
授課教師:Dave Evans https://www.cs.virginia.edu/~evans/
如下內容包含課程筆記和自己的擴展折騰

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])

'''
這里是Udacity的算法,和我的思路稍許不一樣:
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,對python來說,the recursive way is fairly expensive... 有時候還是最好用數學上的一些算法優化一下。

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,寫的很臃腫
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的算法相較之下好多了
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 = {}  # <url>:[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': """<html>
<body>
<h1>Dave's Cooking Algorithms</h1>
<p>
Here are my favorite recipes:
<ul>
<li> <a >Hummus Recipe</a>
<li> <a s Best Hummus</a>
<li> <a s Hummus Recipe</a>
</ul>

For more expert opinions, check out the
<a >Nickel Chef</a>
and <a >Zinc Chef</a>.
</body>
</html>






""",
   'http://udacity.com/cs101x/urank/zinc.html': """<html>
<body>
<h1>The Zinc Chef</h1>
<p>
I learned everything I know from
<a >the Nickel Chef</a>.
</p>
<p>
For great hummus, try
<a >this recipe</a>.

</body>
</html>






""",
   'http://udacity.com/cs101x/urank/nickel.html': """<html>
<body>
<h1>The Nickel Chef</h1>
<p>
This is the
<a >
best Hummus recipe!
</a>

</body>
</html>






""",
   'http://udacity.com/cs101x/urank/kathleen.html': """<html>
<body>
<h1>
Kathleen's Hummus Recipe
</h1>
<p>

<ol>
<li> Open a can of garbanzo beans.
<li> Crush them in a blender.
<li> Add 3 tablespoons of tahini sauce.
<li> Squeeze in one lemon.
<li> Add salt, pepper, and buttercream frosting to taste.
</ol>

</body>
</html>

""",
   'http://udacity.com/cs101x/urank/arsenic.html': """<html>
<body>
<h1>
The Arsenic Chef's World Famous Hummus Recipe
</h1>
<p>

<ol>
<li> Kidnap the <a >Nickel Chef</a>.
<li> Force her to make hummus for you.
</ol>

</body>
</html>

""",
   'http://udacity.com/cs101x/urank/hummus.html': """<html>
<body>
<h1>
Hummus Recipe
</h1>
<p>

<ol>
<li> Go to the store and buy a container of hummus.
<li> Open it.
</ol>

</body>
</html>




""",
}

def get_page(url):
    if url in cache:
        return cache[url]
    else:
        return None

def get_next_target(page):
    start_link = page.find('<a href=')
    if start_link == -1:
        return None, 0
    start_quote = page.find('"', start_link)
    end_quote = page.find('"', start_quote + 1)
    url = page[start_quote + 1:end_quote]
    return url, end_quote

def get_all_links(page):
    links = []
    while True:
        url, endpos = get_next_target(page)
        if url:
            links.append(url)
            page = page[endpos:]
        else:
            break
    return links


def union(a, b):
    for e in b:
        if e not in a:
            a.append(e)

def add_page_to_index(index, url, content):
    words = content.split()
    for word in words:
        add_to_index(index, word, url)

def add_to_index(index, keyword, url):
    if keyword in index:
        index[keyword].append(url)
    else:
        index[keyword] = [url]

def lookup(index, keyword):
    if keyword in index:
        return index[keyword]
    else:
        return None



index , graph = crawl_web('http://udacity.com/cs101x/urank/index.html')

if 'http://udacity.com/cs101x/urank/index.html' in graph:
    print graph['http://udacity.com/cs101x/urank/index.html']
#>>> ['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': """<html>
<body>
<h1>Dave's Cooking Algorithms</h1>
<p>
Here are my favorite recipies:
<ul>
<li> <a >Hummus Recipe</a>
<li> <a s Best Hummus</a>
<li> <a s Hummus Recipe</a>
</ul>

For more expert opinions, check out the
<a >Nickel Chef</a>
and <a >Zinc Chef</a>.
</body>
</html>






""",
   'http://udacity.com/cs101x/urank/zinc.html': """<html>
<body>
<h1>The Zinc Chef</h1>
<p>
I learned everything I know from
<a >the Nickel Chef</a>.
</p>
<p>
For great hummus, try
<a >this recipe</a>.

</body>
</html>






""",
   'http://udacity.com/cs101x/urank/nickel.html': """<html>
<body>
<h1>The Nickel Chef</h1>
<p>
This is the
<a >
best Hummus recipe!
</a>

</body>
</html>






""",
   'http://udacity.com/cs101x/urank/kathleen.html': """<html>
<body>
<h1>
Kathleen's Hummus Recipe
</h1>
<p>

<ol>
<li> Open a can of garbonzo beans.
<li> Crush them in a blender.
<li> Add 3 tablesppons of tahini sauce.
<li> Squeeze in one lemon.
<li> Add salt, pepper, and buttercream frosting to taste.
</ol>

</body>
</html>

""",
   'http://udacity.com/cs101x/urank/arsenic.html': """<html>
<body>
<h1>
The Arsenic Chef's World Famous Hummus Recipe
</h1>
<p>

<ol>
<li> Kidnap the <a >Nickel Chef</a>.
<li> Force her to make hummus for you.
</ol>

</body>
</html>

""",
   'http://udacity.com/cs101x/urank/hummus.html': """<html>
<body>
<h1>
Hummus Recipe
</h1>
<p>

<ol>
<li> Go to the store and buy a container of hummus.
<li> Open it.
</ol>

</body>
</html>




""",
}

def crawl_web(seed): # returns index, graph of inlinks
    tocrawl = [seed]
    crawled = []
    graph = {}  # <url>, [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('<a href=')
    if start_link == -1:
        return None, 0
    start_quote = page.find('"', start_link)
    end_quote = page.find('"', start_quote + 1)
    url = page[start_quote + 1:end_quote]
    return url, end_quote

def get_all_links(page):
    links = []
    while True:
        url, endpos = get_next_target(page)
        if url:
            links.append(url)
            page = page[endpos:]
        else:
            break
    return links


def union(a, b):
    for e in b:
        if e not in a:
            a.append(e)

def add_page_to_index(index, url, content):
    words = content.split()
    for word in words:
        add_to_index(index, word, url)

def add_to_index(index, keyword, url):
    if keyword in index:
        index[keyword].append(url)
    else:
        index[keyword] = [url]

def lookup(index, keyword):
    if keyword in index:
        return index[keyword]
    else:
        return None

index, graph = crawl_web('http://udacity.com/cs101x/urank/index.html')
ranks = compute_ranks(graph)
print ranks

#>>> {'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}
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