Think Python/Answers

< Think Python

Contents

Chapter 1Edit

See below for Chapter 1 exercises.

Exercise 1.4Edit

If you run a 10 kilometer race in 43 minutes 30 seconds, what is your average time per mile? What is your average speed in miles per hour? (Hint: there are about 1.61 kilometers in a mile.)

>>> 10 / 1.61 # Convert kilometers to miles
6.2111801242236018
>>> (43 * 60) + 30 # Convert time to seconds
2610
>>> 2610 / 6.2111801242236018 # what is your average time (seconds) per mile
420.21000000000004
>>> 420.21000000000004 / 60 # what is your average time (minutes) per mile
7.0035000000000007
>>> 60 / 7.0035000000000007 # Miles per hour
8.5671449989291055

How about
>>> 10 / 43.5 # avg kilometers per minute
0.22988505747126436
>>> 0.22988505747126436*60 # kilometers per hour
13.793103448275861
>>> 13.793103448275861 / 1.61 # convert to M.P.H
8.567144998929106

or a one-liner

>>> (10 / 1.61) / (43.5 / 60)   # (distance in miles) / (time in hours)
8.567144998929106         # miles/hour

making it 'pretty' & exploring how print works....

>>> print round((10 / 1.61) / (43.5 / 60), 2), 'mph'    # (distance in miles)/(time in hours) rounded to 2 places
8.57 mph

Chapter 2Edit

Exercise 2.1Edit

If you type an integer with a leading zero, you might get a confusing error:

>>> zipcode = 02492
                  ^
SyntaxError: invalid token

Other number seem to work, but the results are bizarre:

>>> zipcode = 02132
>>> print zipcode
1114

So python is assuming you want to convert an octal number to a decimal number. In the base 8 numbering system where valid numbers are 0, 1, 2, 3, 4, 5, 6 and 7.

Base  8: 00 01 02 03 04 05 06 07 10 11 12 13 14 15 16 17 20 21 22 23 24
Base 10: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20

Every 8 numbers we increment the left hand columns. This means that the right most column is the number of 'ones'. The one to the left of that is a tally of the number of 'eights', the one next to that is a tally of a full column of 'eight' times the 'eight column' - 64. The one next to that is 64*8 - 512 and so on. For more information read Base Eight math.

That is why zipcode = 02492 is invalid as the digit 9 is not a valid octal number. We can do the conversion manually as follows:

>>> print 02132
1114
>>> (2*512)+(1*64)+(3*8)+(2*1)
1114
>>> 

Exercise 2.4Edit

The volume of a sphere with radius r is 4/3 π r3. What is the volume of a sphere with radius 5?

>>> pi = 3.1415926535897931
>>> r = 5
>>> 4/3*pi*r**3 # This is the wrong answer
392.69908169872411
>>> r = 5.0 # Radius can be a float here as well, but is not _necessary_.
>>> 4.0/3.0*pi*r**3 # Using floats give the correct answer
523.59877559829886
>>> 

Suppose the cover price of a book is $24.95, but bookstores get a 40% discount. Shipping costs $3 for the first copy and 75 cents for each additional copy. What is the total wholesale cost for 60 copies?

 $24.95  Cost
  $9.98  Discount per book
 $14.97  Cost per book after discount
  60     Total number of books
$898.20  Total cost not inc delivery

  $3.00  First book delivery
  59     Remaining books
  $0.75  Delivery cost for extra books
 $44.25  Total cost for extra books
 $47.25  Total Delivery cost
        
$945.45  Total Bill

This answer is wrong because 40.0/100.0 return wrong value 0.40000000000000002 for more info see IEEE 754 (Standard for Floating-Point Arithmetic)
>>> (24.95-24.95*40.0/100.0)*60+3+0.75*(60-1)
945.44999999999993
>>> 24.95*0.6*60+0.75*(60-1)+3
945.45

If I leave my house at 6:52 am and run 1 mile at an easy pace (8:15 per mile), then 3 miles at tempo (7:12 per mile) and 1 mile at easy pace again, what time do I get home for breakfast?

Answer: 7:30 am

How I did it:

>>> start = (6*60+52)*60
>>> easy = (8*60+15)*2
>>> fast = (7*60+12)*3
>>> finish_hour = (start + easy + fast)/(60*60.0)
>>> finish_floored = (start + easy + fast)//(60*60)  #int() function can also be used to get integer value, but isn't taught yet.
>>> finish_minute  = (finish_hour - finish_floored)*60
>>> print 'Finish time was %d:%d' % (finish_hour,finish_minute)
Finish time was 7:30

*** ANOTHER WAY ***
start_time_hr = 6 + 52 / 60.0
easy_pace_hr = (8 + 15 / 60.0 ) / 60.0
tempo_pace_hr = (7 + 12 / 60.0) / 60.0
running_time_hr = 2 * easy_pace_hr + 3 * tempo_pace_hr
breakfast_hr = start_time_hr + running_time_hr
breakfast_min = (breakfast_hr-int(breakfast_hr))*60
breakfast_sec= (breakfast_min-int(breakfast_min))*60

print ('breakfast_hr', int(breakfast_hr) )
print ('breakfast_min', int (breakfast_min) )
print ('breakfast_sec', int (breakfast_sec) )
>>> 

Chapter 3Edit

Exercise 3.3Edit

Python provides a built-in function called len that returns the length of a string, so the value of len('allen') is 5. Write a function named right_justify that takes a string named s as a parameter and prints the string with enough leading spaces so that the last letter of the string is in column 70 of the display.

>>> def right_justify(s):
...     print (' '*(70-len(s))+s)
... 
>>> right_justify('allen')
                                                                 allen
>>> 

Exercise 3.5Edit

You can see my solution at http://thinkpython.com/code/grid.py.

"""
Solution to Exercise 3.5 on page 27 of Think Python
Allen B. Downey, Version 1.1.24+Kart [Python 3.2]

"""

# here is a mostly-straightforward solution to the
# two-by-two version of the grid.

def do_twice(f):
    f()
    f()

def do_four(f):
    do_twice(f)
    do_twice(f)

def print_beam():
    print('+ - - - -', end='')

def print_post():
    print('|        ', end='')

def print_beams():
    do_twice(print_beam)
    print('+')

def print_posts():
    do_twice(print_post)
    print('|')

def print_row():
    print_beams()
    do_twice(print_posts)

def print_grid():
    do_twice(print_row)
    print_beams()

print_grid()
 ____________   

# another solution

def do_twice(f):
    f()
    f()

def do_four(f):
    do_twice(f)
    do_twice(f)

def print_column():
    print '+----+----+'

def print_row():
    print '|    |    |'

def print_rows():
    do_four(print_row)

def do_block():
    print_column()
    print_rows()

def print_block():
    do_twice(do_block)
    print_column()

print_block()

# nathan moses-gonzales
_________

# straight-forward solution to 4x4 grid

def do_twice(f):
    f()
    f()

def do_four(f): # not needed for 2x2 grid
    do_twice(f)
    do_twice(f)

def print_beam():
    print('+----', end='')

def print_post():
    print('|    ', end='')

def print_beams():
    do_twice(print_beam)
    print('+')

def print_posts():
    do_twice(print_post)
    print('|')

def print_row():
    print_beams()
    do_twice(print_posts)

def print_grid2x2():
    do_twice(print_row)
    print_beams()

def print_beam4():
    do_four(print_beam)
    print('+')

def print_post4():
    do_four(print_post)
    print('|')

def print_row4():
    print_beam4()
    do_twice(print_post4)

def print_grid4x4():
    do_four(print_row4)
    print_beam4()

print_grid4x4()
-----------------------

# here is a less-straightforward solution to the
# four-by-four grid 

def one_four_one(f, g, h):
    f()
    do_four(g)
    h()

def print_plus():
    print '+',

def print_dash():
    print '-',

def print_bar():
    print '|',

def print_space():
    print ' ',

def print_end():
    print

def nothing():
    "do nothing"

def print1beam():
    one_four_one(nothing, print_dash, print_plus)

def print1post():
    one_four_one(nothing, print_space, print_bar)

def print4beams():
    one_four_one(print_plus, print1beam, print_end)

def print4posts():
    one_four_one(print_bar, print1post, print_end)

def print_row():
    one_four_one(nothing, print4posts, print4beams)

def print_grid():
    one_four_one(print4beams, print_row, nothing)

print_grid()

comment = """
After writing a draft of the 4x4 grid, I noticed that many of the
functions had the same structure: they would do something, do
something else four times, and then do something else once.

So I wrote one_four_one, which takes three functions as arguments; it
calls the first one once, then uses do_four to call the second one
four times, then calls the third.

Then I rewrote print1beam, print1post, print4beams, print4posts,
print_row and print_grid using one_four_one.

Programming is an exploratory process.  Writing a draft of a program
often gives you insight into the problem, which might lead you to
rewrite the code to reflect the structure of the solution.

--- Allen
"""

print comment

# another solution
def beam():
    plus = "+"
    minus = "-"*4
    print(plus, minus, plus,minus, plus, minus, plus, minus, plus)

def straight():
    straight = "|"
    space = " "*4
    print(straight, space, straight, space, straight, space, straight, space,
          straight, space)

def quad_straight():
    straight()
    straight()
    straight()
    straight()

def twice():
    beam()
    quad_straight()
    beam()
    quad_straight()

def quad():
    twice()
    twice()
    beam()

quad()
-- :)
------------------

# Without functions. 
print("+ - - - - " * 2 + "+")
print("|\t\t  | \t\t|\n" * 3 + "|\t\t  | \t\t|")
print("+ - - - - " * 2 + "+")
print("|\t\t  | \t\t|\n " *3 + "|\t\t  | \t\t|")
print("+ - - - - " * 2 + "+")

------------------

Why not using the first solution and adapt it to the number of rows

def do_twice(f):
    f()
    f()

def do_four(f):
    do_twice(f)
    do_twice(f)

def print_column():
    print '+----+----+----+----+'

def print_row():
    print '|    |    |    |    |'

def print_rows():
    do_four(print_row)

def do_block():
    print_column()
    print_rows()

def print_block():
    do_twice(do_block)
    # print_column()
    do_twice(do_block)
    print_column()

print_block()


-----------------------
# mteodor

def draw_line(bar, middle = ' ', repeat = 2, lenght = 2):
  """ Draw a single line like this:
  [ (B M*repeat)*lenght B]
  """
  for k in range(lenght):
    print("%s %s " % (bar, middle*repeat), end='')
  print(bar)

def draw_grid(lenght = 2, height = 2, width = 2):
  """ Draw a grid like this:
  + -- + -- +
  |    |    |
  |    |    |
  + -- + -- +
  |    |    |
  |    |    |
  + -- + -- +
  where:
  * lenght x heigth are the table size
  * width is the size of a cell/column
  """
  for i in range(height):
    draw_line('+', '-', width, lenght)
    for j in range(lenght):
      draw_line('|', ' ', width, lenght)
  draw_line('+', '-', width, lenght)

draw_grid(4, 4, 3)

Chapter 4Edit

4.3 Exercise 1Edit

from TurtleWorld import *

world = TurtleWorld()
bob = Turtle()

def square(t):
    for i in range(4):
        fd(t, 100)
        lt(t)

square(bob)
wait_for_user()

4.3 Exercise 2Edit

from TurtleWorld import *

world = TurtleWorld()
bob = Turtle()
print(bob)

def square(t, length):
    t = Turtle()
    for i in range(4):
        fd(t, length)
        lt(t)

square(bob, 200)
wait_for_user()

4.3 Exercise 3Edit

from TurtleWorld import *

world = TurtleWorld()
bob = Turtle()
print(bob)

def polygon(t, length, n):
    t = Turtle()
    for i in range(n):
        fd(t, length)
        lt(t, 360 / n)

polygon(bob, 50, 8)
wait_for_user()

Chapter 5Edit

Exercise 5.2Edit

def countdown(a):         # A typical countdown function
    if a < 0:
        print("Blastoff")
    elif a > 0:
        print(a)
        countdown(a - 1)

def call_function(n,a):    # The countdown function is called "n" number of times. Any other function can be used instead of countdown function.
    for i in range(n):
        countdown(a)
    
call(3, 10)

Chapter 9Edit

Exercise 9.1Edit

fin = open('words.txt')
for line in fin:
	word = line.strip()
	if len(word) > 20:
		print (word)

Exercise 9.2Edit

fin = open('words.txt')

def has_no_e(word):
    for char in word:
        if char in 'Ee':
            return False
    return True  

count = 0
for line in fin:
    word = line.strip()
    if has_no_e(word):
        count += 1
        print word

percent = (count / 113809.0) * 100

print str(percent) + "% of the words don't have an 'e'."

Exercise 9.3Edit

fin = open('words.txt')

def avoids(word,letter):
    for char in word:
        if char in letter:
            return False
    return True


letter = raw_input('What letters to exclude? ')
count = 0
for line in fin:
    word = line.strip()
    if avoids(word, letter):
        count += 1
        print word

percent = (count / 113809.0) * 100

print str(percent) + "% of the words don't have " + letter + '.'

Chapter 10Edit

Exercise 10.1Edit

Write a function called nested_sum that takes a nested list of integers and add up the elements from all of the nested lists.

def nested_sum(nestedList, newList = [0]):
        '''
        nestedList: list composed of nested lists containing int.
        newList: list. The flat list composed of all the items present
        in the nested lists.
        Returns the sum of all the int in the nested list
        '''
        #Helper function to flatten the list
        def flatlist(nestedList):
                '''
                Returns a flat list
                '''
                for i in range(len(nestedList)):
                        if type(nestedList[i]) == int:
                                newList.append(nestedList[i])
                        else:
                                flatlist(nestedList[i])
                return newList

        flatlist(nestedList)
        print sum(newList)
        
nested_sum(nestedList)

Exercise 10.2Edit

Write a function named "capitalize_nested" that takes a nested list of strings and returns a new nested list with all strings capitalized.

>>> def capitalize_nested(l): 
        def capitalize(s):
            return s.capitalize()
        for n, i in enumerate(l):       
            if type(i) is list:
                l[n] = capitalize_nested(l[n])
            elif type(i) is str:
                l[n] = capitalize(i)
        return l

Exercise 10.3Edit

Write a function that takes a list of numbers and returns the cumulative sum.

>>> def cumulative(l):
    cumulative_sum = 0
    new_list = []
    for i in l:
        cumulative_sum += i
        new_list.append(cumulative_sum)
    return new_list

Exercise 10.4Edit

Write a function called middle that takes a list and returns a new list that contains all but the first and last elements.

>>> def middle(x):
	res = []
	i = 1
	while i <= len(x)-2:
		res.append(x[i])
		i += 1
	return res

This can also be done simply with a slice.

>>> def middle(x):
        return x[1:-1]

Exercise 10.5Edit

Write a function called chop that takes a list and modifies it, removing the first and last elements, and returns None.

>>> def chop(x):
	del x[:1]
	del x[-1:]

Chapter 11Edit

Exercise 11.1Edit

Write a function that reads the words in words.txt and stores them as keys in a dictionary. It doesn’t matter what the values are. Then you can use the in operator as a fast way to check whether a string is in the dictionary.

fin = open('words.txt')
englishdict = dict()


def create_diction():
    counter = 0
    dictionairy = dict()
    for line in fin:
        word = line.strip()
        dictionairy[word] = counter
        counter += 1
    return dictionairy

Exercise 11.2Edit

def histogram(s):
    d = dict()
    for c in s:
        d[c] = 1 + d.get(c, 0)
    return d

Exercise 11.3Edit

Dictionaries have a method called keys that returns the keys of the dictionary, in no particular order, as a list. Modify print_hist to print the keys and their values in alphabetical order.

v = {'p' : 1, 'a' : 1, 'r' : 2, 'o' : 1, 't' : 1}
def print_hist(h):
    d = []
    d += sorted(h.keys())
    for c in d:
        print(c, h[c])

OR

v = {'p' : 1, 'a' : 1, 'r' : 2, 'o' : 1, 't' : 1}
def print_hist(h):
    for c in sorted(h.keys()):
        print c, h[c]

Exercise 11.4Edit

Modify reverse_lookup so that it builds and returns a list of all keys that map to v, or an empty list if there are none.

def reverse_lookup(d,v):
    l = list()
    for c in d:
        if d[c] == v:
            l.append(c)
    return l

Chapter 12Edit

Exercise 12.1Edit

numbers = (1,2,3)
def sumall(numbers):
    x = 0
    for i in numbers:
        x = x + i
    print x
sumall(numbers)

or

def sumall(*t):
    x = 0
    for i in range(len(t)):
        x += t[i]
    return x

or

def sumall(*args):
	t = list(args)
	return sum(t)

or

def sumall(*args):
    return sum(args)

Exercise 12.2Edit

import random

def sort_by_length(words):
	t = []
	for word in words:
		t.append((len(word),word))
	t.sort(reverse=True)
	res = []
	for length, word in t:
		res.append(word)
	i=0
	final = []
	while i <= len(res)-2:
		if len(res[i]) == len(res[i+1]):
			y_list = [res[i], res[i+1]]
			random.shuffle(y_list)
			final = final + y_list
			i += 2
		else:
			final.append(res[i])
			i += 1
	if i == len(res)-1:
		final.append(res[i])
	return final

or

from random import shuffle

def sort_by_length(words):
	r = []
	d = dict()
	for word in words:
		d.setdefault(len(word), []).append(word)
	for key in sorted(d, reverse=True):
		if len(d[key]) > 1:
			shuffle(d[key])
		r.extend(d[key])
	return r

Exercise 12.3Edit

import string

def most_frequent(s):
	d = dict()
	inv = dict()
	for char in s:
		if char in string.ascii_letters:
			letter = char.lower()		
			d[letter] = d.get(letter, 0) + 1
			
	for letter, freq in d.items():
		inv.setdefault(freq, []).append(letter)
		
	for freq in sorted(inv, reverse=True):
		print('{:.2%}:'.format(freq/(sum(list(inv)*len(inv[freq])))), ', '.join(inv[freq]))

Chapter 13Edit

Exercise 13.7Edit

from string import punctuation, whitespace, digits
from random import randint
from bisect import bisect_left

def process_file(filename):
	h = dict()
	fp = open(filename)
	for line in fp:
		process_line(line, h)
	return h

def process_line(line, h):
	line = line.replace('-', ' ')
	for word in line.split():
		word = word.strip(punctuation + whitespace + digits)
		word = word.lower()
		if word != '':
			h[word] = h.get(word, 0) + 1

hist = process_file('emma.txt')

def cum_sum(list_of_numbers):
	cum_list = []
	for i, elem in enumerate(list_of_numbers):
		if i == 0:
			cum_list.append(elem)
		else:
			cum_list.append(cum_list[i-1] + elem)
	return cum_list

def random_word(h):
	word_list = list(h.keys())
	num_list = []
	for word in word_list:
		num_list.append(h[word])
	cum_list = cum_sum(num_list)
	i = randint(1, cum_list[-1])
	pos = bisect_left(cum_list, i)
	return word_list[pos]

print(random_word(hist))

Chapter 14Edit

Exercise 14.3Edit

import shelve

def dict_of_signatures_and_words(filename='words.txt'):
	d = dict()
	for line in open(filename):
		word = line.lower().strip()
		signature = ''.join(sorted(word))
		d.setdefault(signature, []).append(word)
	return d

def db_of_anagrams(filename='anagrams', d=dict_of_signatures_and_words()):
	db = shelve.open(filename)
	for key, values in d.items():
		if len(values)>1:
			for index, value in enumerate(values):
				db[value]=values[:index]+values[index+1:]
	db.close()

def print_contents_of_db(filename='anagrams'):
	db = shelve.open(filename, flag='r')
	for key in sorted(db):
		print(key.rjust(12), '\t<==>\t', ', '.join(db[key]))
	db.close()

db_of_anagrams()
print_contents_of_db()

Exercise 14.5Edit

# Replace urllib.request with urllib if you use Python 2.
# I would love to see a more elegant solution for this exercise, possibly by someone who understands html.

import urllib.request

def check(zip_code):
	if zip_code == 'done':
		return False
		
	else:
                if len(zip_code) != 5:
		      print('\nThe zip code must have five digits!')
		return True

def get_html(zip_code):
	gibberish = urllib.request.urlopen('http://www.uszip.com/zip/' + zip_code)
	less_gib = gibberish.read().decode('utf-8')
	return less_gib

def extract_truth(code, key, delimiter):
	pos = code.find(key) + len(key)
	nearly_true = code[pos:pos+40]
	truth = nearly_true.split(delimiter)[0]
	return truth

while True:
	zip_code = input('Please type a zip code (5 digits) or "done" if want to stop:\n')
	
	if not check(zip_code):
                 break
	
	code = get_html(zip_code)
	
	invalid_key = '(0 results)'
	if invalid_key in code:
		print('\nNot a valid zip code.')
		continue
	
	name_key = '<title>'
	name_del = ' zip'
	name = extract_truth(code, name_key, name_del)
	
	pop_key = 'Total population</dt><dd>'
	pop_del = '<'
	pop = extract_truth(code, pop_key, pop_del)
	
	if not 1 < len(pop) < 9:
		pop = 'not available'

	print('\n' + name)
	print('Population:', pop, '\n')

Chapter 15Edit

Exercise 15.1Edit

import math

class Point(object):
	"""represents a point in 2-D space"""

def distance(p1, p2):
	distance = math.sqrt((p2.x - p1.x)**2 + (p2.y - p1.y)**2)
	return distance

p1 = Point()
p2 = Point()

p1.x = 3
p1.y = 2
p2.x = 4
p2.y = 3

print(distance(p1, p2))

Chapter 16Edit

Exercise 16.1Edit

def print_time(t):
    print '%.2d:%.2d:%.2d' % (t.hour, t.minute, t.second)

or

# Solution for Python3
# More on string formatting: http://docs.python.org/py3k/library/string.html#formatspec

def print_time(t):
    # 0 is a fill character, 2 defines the width
    print('{}:{:02}:{:02}'.format(t.hour, t.minute, t.second))

Exercise 16.2Edit

def is_after(t1, t2):
    return (t1.hour, t1.minute, t1.second) > (t2.hour, t2.minute, t2.second)

Exercise 16.3Edit

# Comment not by the author: This will give a wrong result, if (time.second + seconds % 60) > 60

def increment(time, seconds):

    n = seconds/60
    time.second += seconds - 60.0*n
    time.minute += n

    m = time.minute/60
    time.minute -= m*60
    time.hour += m

or

# Solution for Python3
# Replace '//' by '/' for Python2

def increment(time, seconds):
	time.second += seconds
	time.minute += time.second//60
	time.hour += time.minute//60
	
	time.second %= 60
	time.minute %= 60
	time.hour %= 24
# A different way of going about it

def increment(time, seconds):
    # Converts total to seconds, then back to a readable format
    time.second = time.hour*3600 + time.minute*60 + time.second + seconds
    (time.minute, time.second) = divmod(time_in_seconds, 60)
    (time.hour, time.minute) = divmod(time.minute, 60)

Exercise 16.4Edit

# Solution for Python3
# Replace '//' by '/' for Python2

from copy import deepcopy

def increment(time, seconds):
	r = deepcopy(time)
	
	r.second += seconds
	r.minute += r.second//60
	r.hour += r.minute//60
	
	r.second %= 60
	r.minute %= 60
	r.hour %= 24

	return r

Exercise 16.5Edit

class Time(object):
    """represents the time of day.
        attributes: hour, minute, second"""

time = Time()
time.hour = 11
time.minute = 59
time.second = 30

def time_to_int(time):
    minutes = time.hour * 60 + time.minute
    seconds = minutes * 60 + time.second
    return seconds

def int_to_time(seconds):
    time = Time()
    minutes, time.second = divmod(seconds, 60)
    time.hour, time.minute = divmod(minutes, 60)
    return time

def increment(time, addtime):
    seconds = time_to_int(time)
    return int_to_time(seconds + addtime)

def print_time (x):
    print 'The time is %.2d : %.2d : %.2d' % (x.hour, x.minute, x.second)
print_time (time)

newtime = increment (time, 70)

print_time (newtime)

Exercise 16.6Edit

def time_to_int(time):
	minutes = time.hour * 60 + time.minute
	seconds = minutes * 60 + time.second
	return seconds

def int_to_time(seconds):
	time = Time()
	minutes, time.second = divmod(seconds, 60)
	time.hour, time.minute = divmod(minutes, 60)
	return time

def mul_time(time, factor):
	seconds = time_to_int(time)
	seconds *= factor
	seconds = int(seconds)
	return int_to_time(seconds)
	
def average_pace(time, distance):
	return mul_time(time, 1/distance)

Exercise 16.7Edit

Write a class definition for a Date object that has attributes day, month and year. Write a function called increment_date that takes a Date object, date, and an integer, n, and returns a new Date object that represents the day n days after date. Hint: “Thirty days hath September...” Challenge: does your function deal with leap years correctly? See wikipedia.org/wiki/Leap_year.

class Date(object):
	"""represents a date.
	attributes: day, month, year"""

def print_date(date):
	# German date format

	print('{}.{}.{}'.format(date.day, date.month, date.year))

def is_leap_year(year):
        # http://en.wikipedia.org/wiki/Leap_year#Algorithm

	if year % 4 == 0:
		if year % 100 == 0:
			if year % 400 == 0:
				return True
			return False
		return True
	return False

def month_list(year):
	if is_leap_year(year):
		return [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
	return [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

def days_of_year(year):
	if is_leap_year(year):
		return 366
	return 365

def date_to_int(date):
	days = 0
	for year in range(1, date.year):
		days += days_of_year(year)
	
	month_days = month_list(date.year)
	for month in range(1, date.month):
		days += month_days[month - 1]
	
	days += date.day - 1
	return days

def int_to_date(days):
	date = Date()
	
	date.year = 1
	next_days = 365
	while days >= next_days:
		date.year += 1
		days -= next_days
		next_days = days_of_year(date.year)
		
	date.month = 1
	next_days = 31
	month_days = month_list(date.year)
	while days >= next_days:
		date.month += 1
		days -= next_days
		next_days = month_days[date.month - 1]
	
	date.day = days + 1
	return date

def increment_date(date, n):
	days = date_to_int(date)
	return int_to_date(days + n)

d1 = Date()
d1.day, d1.month, d1.year = 8, 3, 2012
print_date(d1)

d2 = increment_date(d1, 7)
print_date(d2)

Exercise 16.8Edit

1. Use the datetime module to write a program that gets the current date and prints the day of the week.

from datetime import date

def current_weekday():
	i = date.today()
	print i.strftime('%A')

current_weekday()

2. Write a program that takes a birthday as input and prints the user’s age and the number of days, hours, minutes and seconds until their next birthday.

# Python3 solution. Replace "input" by "raw_input" for Python2.
from datetime import datetime

def time_until_birthday():
    dob_input = input(('Please enter the date of your birth in '
                       'the format "mm/dd/yyyy": '))
    dob = datetime.strptime(dob_input, '%m/%d/%Y')
    now = datetime.now()
    if now > datetime(now.year, dob.month, dob.day):
        age = now.year - dob.year
        next_year = True
    else:
        age = now.year - dob.year - 1
        next_year = False
    time_to_birthday = datetime(now.year + next_year,
                                dob.month, dob.day) - now
    days = time_to_birthday.days
    hours, remainder = divmod(time_to_birthday.seconds, 3600)
    minutes, seconds = divmod(remainder, 60)
    print("\nYou are {} years old.".format(age))
    print(("You have {0} days, {1} hours, {2} minutes and {3} "
           "seconds left until your next birthday.").format(
               days, hours, minutes, seconds))

time_until_birthday()

Chapter 17Edit

Exercise 17.8Edit

2.

from visual import scene, sphere

scene.range = (256, 256, 256)
scene.center = (128, 128, 128)

t = range(0, 256, 51)

for x in t:
    for y in t:
        for z in t:
            pos = x, y, z
            color = (x/255., y/255., z/255.)
            sphere(pos=pos, radius=10, color=color)

3. Download http://thinkpython.com/code/color_list.py and use the function read_colors to generate a list of the available colors on your system, their names and RGB values. For each named color draw a sphere in the position that corresponds to its RGB values.

# As there currently (2013-04-12) is no function read_colors in color_list.py
# I use a workaround and simply import the variable COLORS from color_list.py.
# I then use the function all_colors() on COLORS to get a list of the colors.

from color_list import COLORS
from visual import scene, sphere

def all_colors(colors_string=COLORS):
    """Extract a list of unique RGB-tuples from COLORS.
    The tuples look like (r, g, b), where r, g and b are each integers in
    [0, 255].
    """

    # split the string into lines and remove irrelevant lines
    lines = colors_string.split('\n')[2:-2]  

    # split the individual lines and remove the names
    numbers_only = [line.split()[:3] for line in lines]
    
    # turn strings into ints and rgb-lists into tuples
    rgb_tuples = [tuple([int(s) for s in lst]) for lst in numbers_only]

    # return a list of unique tuples
    return list(set(rgb_tuples))

def make_spheres(color_tuples=all_colors()):
    scene.range = (256, 256, 256)
    scene.center = (128, 128, 128)
    for (r, g, b) in color_tuples:
        sphere(pos=(r, g, b), radius=7, color=(r/255., g/255., b/255.))

if __name__ == '__main__':
    make_spheres()

Chapter 3.5Edit

calculatorEdit

#recursion or recursive
print "\n	INDEX\n""\n	C=1 for addition\n""\n	C=2 for substraction\n""\n	
C=3 for multiplication\n""\n	C=4 for division\n""\n	C=5 for to find modulus\n""\n	C=6 to find factorial\n"
C=input("Enter your choice here: ")
def add(x,y):
	c=x+y
	print x,"+",y,"=",c
def sub(x,y):
	c=x-y
	print x,"-",y,"=",c
def mul(x,y):
	c=x*y
	print x,"*",y,"=",c
def div(x,y):
	c=x/y
	print x,"/",y,"=",c
def mod(x,y):
	c=x%y
	print x,"%",y,"=",c
if C==6:
	def f(n):
		if n==1:
			print n
			return n
		else:
			print n,"*",
			return n*f(n-1)
	n=input("enter your no here: ")
	print f(n)
if C==1:
	a=input("Enter your first no here: ")
	b=input("Enter your second no here: ")
	add(a,b)
elif C==2:
	a=input("Enter your first no here: ")
	b=input("Enter your second no here: ")
	sub(a,b)
elif C==3:
	a=input("Enter your first no here: ")
	b=input("Enter your second no here: ")
	mul(a,b)
elif C==4:
	a=input("Enter your first no here: ")
	b=input("Enter your second no here: ")	
	div(a,b)
elif C==5:
	a=input("Enter your first no here: ")
	b=input("Enter your second no here: ")
	mod(a,b)

palindromeEdit

def first(word):
	return word[0]
def last(word):return word[-1]
def middle(word):
	return word[1:-1]
def palindrome(word):
	if first(word)==last(word):
		word = middle(word)
		n=len(word)
		if n<2:
			print "palindrome"
		else:		
			return palindrome(word)
	else:
		print "not palindrome"


word=raw_input("Enter the  string:")
palindrome(word)

sum of all digitsEdit

def sum_of_n_numbers(number):
	if(number==0):
		return 0
	else:
		return number + sum_of_n_numbers(number-1)
num = raw_input("Enter a number:")
num=int(num)
sum = sum_of_n_numbers(num)
print sum
###another answer in case of while loops
def sum_of_Digits(number):
	sum=0
	while number>0:
		digit=number%10
		sum=sum+digit
		number=number/10
	return sum
num=raw_input("enter the number")
num=int(num)
sum_of_digits=sum_of_Digits(num)
print sum_of_digits

Exercise 18.5Edit

class Card(object):
	
	suit_names = ['Clubs', 'Diamonds', 'Hearts', 'Spades']
	rank_names = [None, 'Ace', '2', '3', '4', '5', '6', '7', 
					'8', '9', '10', 'Jack', 'Queen', 'King']
			
	def __init__(self, suit = 0, rank = 2):
		self.suit = suit
		self.rank = rank
	
	def __str__(self):
		return '%s of %s' % (Card.rank_names[self.rank], 
							Card.suit_names[self.suit])
							
	def __cmp__(self, other):
		c1 = (self.suit, self.rank)
		c2 = (other.suit, other.rank)
		return cmp(c1, c2)
		
	def is_valid(self):
		return self.rank > 0
		
class Deck(object):
	
	def __init__(self, label = 'Deck'):
		self.label = label
		self.cards = []
		for i in range(4):
			for k in range(1, 14):
				card = Card(i, k)
				self.cards.append(card)
				
	def __str__(self):
		res = []
		for card in self.cards:
			res.append(str(card))
		print self.label
		return '\n'.join(res)
		
		
	def deal_card(self):
		return self.cards.pop(0)
		
	def add_card(self, card):
		self.cards.append(card)
	
	def shuffle(self):
		import random
		random.shuffle(self.cards)
		
	def sort(self):
		self.cards.sort()
		
	def move_cards(self, other, num):
		for i in range(num):
			other.add_card(self.deal_card())
	
	def deal_hands(self, num_hands, num_cards):
		if num_hands*num_cards > 52:
			return 'Not enough cards.'
		
		l = []
		
		for i in range(1, num_hands + 1):
			hand_i = Hand('Hand %d' % i)
			self.move_cards(hand_i, num_cards)
			l.append(hand_i)
		
		return l

class Hand(Deck):
	
	def __init__(self, label = ''):
		self.cards = []
		self.label = label

# 18-6, 1-4:
class PokerHand(Hand):
	
	def suit_hist(self):
		self.suits = {}
		for card in self.cards:
			self.suits[card.suit] = self.suits.get(card.suit, 0) + 1
		return self.suits
		
	def rank_hist(self):
		self.ranks = {}
		for card in self.cards:
			self.ranks[card.rank] = self.ranks.get(card.rank, 0) + 1
		return self.ranks
	
	def P(self):
		self.rank_hist()
		for val in self.ranks.values():
			if val >= 2:
				return True
		return False
	
	def TP(self):
		self.rank_hist()
		count = 0
		for val in self.ranks.values():
			if val == 4:
				return True
			elif val >= 2 and val < 4:
				count += 1
		return count >= 2
		
	def TOAK(self):
		self.rank_hist()
		for val in self.ranks.values():
			if val >= 3:
				return True
		return False
	
	def STRseq(self):
		seq = []
		l = STRlist()
		self.rank_hist()
		h = self.ranks.keys()
		h.sort()
		if len(h) < 5:
			return []
		
		# Accounts for high Aces:
		if 1 in h:
			h.append(1)
		
		for i in range(5, len(h)+1):
			if h[i-5:i] in l:
				seq.append(h[i-5:i])
		return seq
	
	def STR(self):
		seq = self.STRseq()
		return seq != []

	def FL(self):
		self.suit_hist()
		for val in self.suits.values():
			if val >= 5:
				return True
		return False

	def FH(self):
		d = self.rank_hist()
		keys = d.keys()
		
		for key in keys:
			if d[key] >= 3:
				keys.remove(key)
				for key in keys:
					if d[key] >= 2:
						return True
		return False
		
	def FOAK(self):
		self.rank_hist()
		for val in self.ranks.values():
			if val >= 4:
				return True
		return False
		
	def SFL(self):
		seq = self.STRseq()
		if seq == []:
			return False
		for list in seq:
			list_suits = []
			for index in list:
				for card in self.cards:
					if card.rank == index:
						list_suits.append(card.suit)
			list_hist = histogram(list_suits)
			for key in list_hist.keys():
				if list_hist[key] >= 5:
					return True
		return False
				
	def classify(self):
		self.scores = []
		hands = ['Pair', 'Two-Pair',
		'Three of a Kind', 'Straight',
		'Flush', 'Full House',
		'Four of a Kind', 'Straight Flush']
		if self.P():
			self.scores.append(1)
		if self.TP():
			self.scores.append(2)
		if self.TOAK():
			self.scores.append(3)
		if self.STR():
			self.scores.append(4)
		if self.FL():
			self.scores.append(5)
		if self.FH():
			self.scores.append(6)
		if self.FOAK():
			self.scores.append(7)
		if self.SFL():
			self.scores.append(8)
		if self.scores != []:
			return hands[max(self.scores)-1]
	
def STRlist():
	s = []
	for i in range(0,9):
		s.append(range(1,14)[i:i+5])
	s.append([10,11,12,13,1])	
	return s

def histogram(l):
	d = dict()
	for k in range(len(l)):
		d[l[k]] = 1 + d.get(l[k],0)
	return d

# 18-6, 5:
def p(config = '', trials = 10000, n = 1):
	"""Estimates probability that the
	nth dealt hand will be config. A hand
	consists of seven cards."""
	
	successes = 0
	
	for i in range(1, trials + 1):
		deck = Deck('Deck %d' % i)
		deck.shuffle()
		
		box = Hand()
		deck.move_cards(box, (n-1)*7)
		
		hand = PokerHand('Poker Hand %d' % i)
		deck.move_cards(hand, 7)
		if hand.classify() == config:
			successes += 1
	
	return 1.0*successes/trials

#Iterate until first desired config.:
if __name__ == '__main__':
	
	c = 1
	
	while True:
		deck = Deck()
		deck.shuffle()
		hand = PokerHand('Poker Hand %d' % c)
		deck.move_cards(hand, 5)
		print hand
		print hand.SFL()
		if hand.SFL():
			print hand.STRseq()
			break
		print ''
		c += 1

Code by Victor Alvarez

Appendix BEdit

Exercise B.3Edit

Write a function called bisection that takes a sorted list and a target value and returns the index of the value in the list, if it’s there, or None if it’s not.

from bisect import bisect_left

def bisection(sorted_list, item):
    i = bisect_left(sorted_list, item)
    if i < len(sorted_list) and sorted_list[i] == item:
        return i
    else:
        return None

if __name__ == '__main__':
    a = [1, 2, 3]
    print(bisection(a, 2))  # expect 1
    b = [1, 3]
    print(bisection(b, 2))  # expect None
    c = [1, 2]
    print(bisection(c, 3))  # expect None