1. 条件判断

计算机之所以能做很多自动化的任务，就是因为它可以自己做条件判断。每种语言都离不开条件判断，无非是语法不一致而已。故此处只给出具体实例，不做过多讲解。

# eg1  if ... else ...
>>> age = 3
>>> if age >= 18:
...     print('your age is',age)
...     print('adult')
... else:
...     print('your age is',age)
...     print('teenager')
... 
('your age is', 3)
teenager

# eg2 if ... elif ... else ...
>>> age = 3
>>> if age >= 18:
...     print('adult')
... elif age >= 6:
...     print('teenager')
... else:
...     print('kid')
... 
kid

2. 循环

• Python循环有两种，一种是for...in循环，依次把list或tuple中的每个元素迭代出来【类似PHP中的foreach】,for X in ...循环就是把每个元素代入变量，然后执行缩进块的语句

# 例1
>>> names = ['liheng','wenxiu','yuling']
>>> for name in names:
...     print(name)
... 
liheng
wenxiu
yuling

# 例2
>>> s = 0
>>> for i in (1,2,3,4,5):
...     s = s + i 
... 
>>> print(s)
15

• 知识点：range()函数，可以生成一个整数序列，再通过list()函数可以转换为list

>>> range(5)
[0, 1, 2, 3, 4]
>>> list(range(5))
[0, 1, 2, 3, 4]

>>> sum = 0
>>> for i in list(range(5)):
...     sum = sum + i
... 
>>> print(sum)
10

• 第二种是while循环，只要条件满足就不断循环，条件不满足就退出循环

>>> s = 0 
>>> n = 99
>>> while n > 0:
...     s = s + n
...     n = n - 2
... 
>>> print s
2500

• 知识点： break 和 continue 语句，与其他语言相似，不再赘述

 while n < 100:
...     n = n + 1
...     if n % 2 == 0:
...         continue
...     print(n)
... 
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