笨方法学Python（21-25）

习题21、函数可以返回东西

#!/usr/bin/python
# -*- coding:utf-8 -*-

def add(a, b):
    print "ADDING %d + %d" % (a, b)
    return a + b

def subtract(a, b):
    print "SUBTRACTING %d - %d" % (a, b)
    return a - b

def multiply(a, b):
    print "MULTIPLYING %d * %d" % (a, b)
    return a * b

def divide(a, b):
    print "DIVIDING %d / %d" % (a, b)
    return a / b

print "Let's do some math wit just functions!"

age = add(30, 5)
height = subtract(78, 4)
weight = multiply(90, 2)
iq = divide(100, 2)

print "Age: %d, Height: %d, Weight: %d, IQ: %d" % (age, height, weight, iq)

# A puzzle for the extra credit , type it in anyway.
print "Here is a puzzle."

what = add(age, subtract(height, multiply(weight, divide(iq,2))))

print "That becomes: ", what, "Can you do it by hand?"

小结

1. 为什么 Python 会把函数或公式倒着打印出来？
   其实不是倒着打印，而是自内而外打印。如果你把函数内容逐句看下去，你会发现这里的规律。试着搞清楚为什么说它是“自内而外”而不是“自下而上”。
2. 怎样使用raw_input()输入自定义值？
   记得int(raw_input())吧？不过这样也有一个问题，那就是你无法输入浮点数，所以你可以试着使用float(raw_input())。
3. 你说的“写一个公式”是什么意思？
   来个简单的例子吧：24 + 34 / 100 - 1023——把它用函数的形式写出来。然后自己想一些数学式子，像公式一样用变量写出来。

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习题22、到现在你学到了哪些东西？

习题23、读代码

习题24、更多练习

#!/usr/bin/python
# -*- coding:utf-8 -*-

print "Let's practice everything."
print 'You\'d need to konw \'bout escapes with \\ that do \n newlines and \t tabs.'

poem = """
\tThe lovely world
with logic so firmly planted
cannot discern \n the needs of love
nor comprehend passion fromm intuition
and requires an explantion
\n\t\twhere there is none.
"""

print "------------------"
print poem
print "------------------"

five = 10 - 2 + 3 - 6
print "This should be five: %s" % five

def secret_formula(started):
    jelly_beans = started * 500
    jars = jelly_beans / 1000
    crates = jars / 100
    return jelly_beans, jars, crates

start_point = 10000
beans, jars, crates = secret_formula(start_point)

print "With a starting point of: %d" % start_point
print "We'd have %d beans, %d jars, and %d crates." % secret_formula(start_point)   

start_point = start_point / 10

print "We can also do that this way:"
print "We'd have %d beans, %d jars, and %d crates." % secret_formula(start_point)

小结

1. 为什么你在后面把jelly_beans这个变量名又叫成了beans？
   这是函数的工作原理。记住函数内部的变量都是临时的，当你的函数返回以后，返回值可以被赋予一个变量。我这里是创建了一个新变量，用来存放函数的返回值。
2. 倒着阅读代码是什么意思？
   从最后一行开始，把你写的和我写的代码进行比较。如果这一行完全一样，就接着比较上一行，直到全部比较完为止。

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习题25、更多更多的练习

#!/usr/bin/python
# -*- coding:utf-8 -*-

def break_words(stuff):
    """This function will break up words for us."""
    words = stuff.split(' ')
    return words

def sort_words(words):
    """Sorts the words."""
    return sorted(words)

def print_first_word(words):
    """Prints the first word after popping it off."""
    word = words.pop(0)
    print word

def print_last_word(words):
    """Prints the last word after popping it off."""
    word = words.pop(-1)
    print word

def sort_sentence(sentence):
    """Takes in a full sentence and returns the sorted words."""
    words = break_words(sentence)
    return sort_words(words)

def print_first_and_last(sentence):
    """Print the first and last words of the sentence."""
    words = break_words(sentence)
    print_first_word(words)
    print_last_word(words)

def print_first_and_last_sorted(sentence):
    """Sorts the words then prints the first and last one."""
    words = sort_sentence(sentence)
    print_first_word(words)
    print_last_word(words)

运行截图：