Learn Python The Hard Way学习(32)

shangban

下面我们会做一些有趣的事情，如果跟着进度学习的话，你会发现你已经可以用if语句和布尔表达式做很多事情了。



不管怎样，程序会做一些重复的事情，下面我们就用for循环打印一个列表变量。做这个练习的时候你必须自己弄懂它们的含义和作用。




在使用for循环之前，我们需要一个东西保存循环的值，最好的方法是使用一个列表，列表就是按照顺序保存数据的容器，不是很复杂，就是一种新的语法而已，结构像下面这样：

hairs = ['brown', 'blond', 'red']

eyes = ['brown', 'blue', 'green']

weights = [1, 2, 3, 4]






list以 [ 号开头，里面的元素以 , 号分隔，像函数的参数一样，然后以 ] 结束，python把所有这些包含在一个变量中。




敬告：上一个练习中我们用if语句包含if语句，很多人不明白为什么一个东西可以包含另外一个东西，在程序中这是很常见的，你会发现函数可以包含函数，list可以包含list。如果你发现一种不理解的结构，用笔记录下来，以后弄明白它。




下面我们来看一些list，并且循环打印它们：

the_count = [1, 2, 3, 4, 5]
fruits = ['apples', 'oranges', 'pears', 'apricots']
change = [1, 'pennies', 2, 'domes', 3, 'quarters']

# this first kind of for-loop goes through a list
for number in the_count:
print "This is count %d" % number

# same as above
for fruit in fruits:
print "A fruit of type: %s" % fruit

# also we can go through mixed lists too
# notice we have to use %r since we don't know what's in it
for i in change:
print "I got %r" % i

# we can also build lists, first start with an empty on
elements = []

# then use the range function to do 0 to 5 counts
for i in range(0, 6):
print "Adding %d to the list." % i
# append is a function that lists understand
elements.append(i)

# now we can print them out too
for i in elements:
print "Elements was: %d" % i






运行结果

root@he-desktop:~/mystuff# python ex32.py

This is count 1

This is count 2

This is count 3

This is count 4

This is count 5

A fruit of type: apples

A fruit of type: oranges

A fruit of type: pears

A fruit of type: apricots

I got 1

I got 'pennies'

I got 2

I got 'domes'

I got 3

I got 'quarters'

Adding 0 to the list.

Adding 1 to the list.

Adding 2 to the list.

Adding 3 to the list.

Adding 4 to the list.

Adding 5 to the list.

Elements was: 0

Elements was: 1

Elements was: 2

Elements was: 3

Elements was: 4

Elements was: 5






加分练习

1. 弄懂range函数。

range([start], stop[, step])

产生一个整数列表，step默认是1，如果start没有赋值，默认是从0开始。




2. 你能不能不使用22行的for循环，而直接使用range给elemens赋值呢？

elements = range(0, 6)




3. 查看list的帮助文档，看看append附近还有什么函数。