Python中使用推导式创建序列

一时失言乱红尘 2023-10-18 11:52 38阅读 0赞

### 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}