Python中使用推导式创建序列

推导式创建序列

推导式是从一个或者多个迭代器快速创建序列的一种方法。它可以将循环和条件判断结合，从而避免冗长的代码。推导式是典型的Python 风格。

列表推导式

列表推导式生成列表对象，语法如下：
           [表达式for item in 可迭代对象]
或者：{表达式for item in 可迭代对象if 条件判断}

>>> [x for x in range(1,5)]
[1, 2, 3, 4]
>>> [x*2 for x in range(1,5)]
[2, 4, 6, 8]
>>> [x*2 for x in range(1,20) if x%5==0 ]
[10, 20, 30]
>>> [a for a in "abcdefg"]
['a', 'b', 'c', 'd', 'e', 'f', 'g']

还可以使用两个循环

>>> cells =[(row,col) for row in range(1,10) for col in range(1,10)]
>>> for cell in cells:
 print(cell)


(1, 1)
(1, 2)
(1, 3)
(1, 4)
(1, 5)
(1, 6)
(1, 7)
(1, 8)
(1, 9)
(2, 1)
(2, 2)
(2, 3)
(2, 4)
(2, 5)
(2, 6)
(2, 7)
(2, 8)
(2, 9)
(3, 1)
(3, 2)
(3, 3)
(3, 4)
(3, 5)
(3, 6)
(3, 7)
(3, 8)
(3, 9)
(4, 1)
(4, 2)
(4, 3)
(4, 4)
(4, 5)
(4, 6)
(4, 7)
(4, 8)
(4, 9)
(5, 1)
(5, 2)
(5, 3)
(5, 4)
(5, 5)
(5, 6)
(5, 7)
(5, 8)
(5, 9)
(6, 1)
(6, 2)
(6, 3)
(6, 4)
(6, 5)
(6, 6)
(6, 7)
(6, 8)
(6, 9)
(7, 1)
(7, 2)
(7, 3)
(7, 4)
(7, 5)
(7, 6)
(7, 7)
(7, 8)
(7, 9)
(8, 1)
(8, 2)
(8, 3)
(8, 4)
(8, 5)
(8, 6)
(8, 7)
(8, 8)
(8, 9)
(9, 1)
(9, 2)
(9, 3)
(9, 4)
(9, 5)
(9, 6)
(9, 7)
(9, 8)
(9, 9)
>>>

字典推导式

字典的推导式生成字典对象，格式如下：
{key_expression : value_expression for 表达式in 可迭代对象}
类似于列表推导式，字典推导也可以增加if 条件判断、多个for 循环。

统计文本中字符出现的次数：

>>> my_text = ' i love you, i love sxt, i love gaoqi'
>>> char_count = {c:my_text.count(c) for c in my_text}
>>> char_count
{' ': 9, 'i': 4, 'l': 3, 'o': 5, 'v': 3, 'e': 3, 'y': 1, 'u': 1, ',': 2, 's': 1, 'x': 1, 't': 1, 'g': 1, 'a': 1, 'q': 1}

集合推导式

集合推导式生成集合，和列表推导式的语法格式类似：
           {表达式for item in 可迭代对象}
或者：{表达式for item in 可迭代对象if 条件判断}

>>> {x for x in range(1,100) if x%9==0}
{99, 36, 72, 9, 45, 81, 18, 54, 90, 27, 63}