转自 http://www.jb51.net/article/67157.htm
推导式是Python中很强大的、很受欢迎的特性，具有语言简洁，速度快等优点。推导式包括:
1.列表推导式
2.字典推导式
3.集合推导式
嵌套列表推导式
NOTE: 字典和集合推导是最近才加入到Python的(Python 2.7 和Python 3.1以上版). 下面简要介绍下:
【列表推导式】
列表推导能非常简洁的构造一个新列表:只用一条简洁的表达式即可对得到的元素进行转换变形
其基本格式如下:
代码如下:
[expr for value in collection ifcondition]过滤条件可有可无，取决于实际应用，只留下表达式；相当于下面这段for循环:
代码如下:
- result = []
- for value in collection:
- if condition:
- result.append(expression)
代码如下:
- >>> names = ['Bob','Tom','alice','Jerry','Wendy','Smith']
-
- >>> [name.upper() for name in names if len(name)>3]
- ['ALICE', 'JERRY', 'WENDY', 'SMITH']
代码如下:
- >>> [(x,y) for x in range(5) if x%2==0 for y in range(5) if y %2==1]
- [(0, 1), (0, 3), (2, 1), (2, 3), (4, 1), (4, 3)]
代码如下:
- >>> M = [[1,2,3],
- ... [4,5,6],
- ... [7,8,9]]
- >>> M
- [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
- >>> [row[2] for row in M]
- [3, 6, 9]
#或者用下面的方式
- >>> [M[row][2] for row in (0,1,2)]
- [3, 6, 9]
代码如下:
- >>> M
- [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
- >>> [M[i][i] for i in range(len(M))]
- [1, 5, 9]
例5: 求M,N中矩阵和元素的乘积
代码如下:
- >>> M = [[1,2,3],
- ... [4,5,6],
- ... [7,8,9]]
- >>> N = [[2,2,2],
- ... [3,3,3],
- ... [4,4,4]]
- >>> [M[row][col]*N[row][col] for row in range(3) for col in range(3)]
- [2, 4, 6, 12, 15, 18, 28, 32, 36]
- >>> [[M[row][col]*N[row][col] for col in range(3)] for row in range(3)]
- [[2, 4, 6], [12, 15, 18], [28, 32, 36]]
- >>> [[M[row][col]*N[row][col] for row in range(3)] for col in range(3)]
- [[2, 12, 28], [4, 15, 32], [6, 18, 36]]
代码如下:
- >>> bob
- {'pay': 3000, 'job': 'dev', 'age': 42, 'name': 'Bob Smith'}
- >>> sue
- {'pay': 4000, 'job': 'hdw', 'age': 45, 'name': 'Sue Jones'}
- >>> people = [bob, sue]
- >>> [rec['age']+100 if rec['age'] >= 45 else rec['age'] for rec in people] # 注意for位置
- [42, 145]
字典和集合推导式是该思想的延续，语法差不多，只不过产生的是集合和字典而已。其基本格式如下:
代码如下:
{ key_expr: value_expr for value in collection if condition }
代码如下:
- >>> strings = ['import','is','with','if','file','exception']
-
- >>> D = {key: val for val,key in enumerate(strings)}
-
- >>> D
- {'exception': 5, 'is': 1, 'file': 4, 'import': 0, 'with': 2, 'if': 3}
集合推导式跟列表推导式非常相似，唯一区别在于用{}代替[]。其基本格式如下:
代码如下:
{ expr for value in collection if condition }
代码如下:
- >>> strings = ['a','is','with','if','file','exception']
- >>> {len(s) for s in strings} #有长度相同的会只留一个，这在实际上也非常有用
- set([1, 2, 4, 9])
嵌套列表是指列表中嵌套列表，比如说:
代码如下:
- >>> L = [[1,2,3],
- [4,5,6],
- [7,8,9]]
代码如下:
- names = [['Tom','Billy','Jefferson','Andrew','Wesley','Steven','Joe'],
- ['Alice','Jill','Ana','Wendy','Jennifer','Sherry','Eva']]
用for循环实现:
代码如下:
- tmp = []
- for lst in names:
- for name in lst:
- if name.count('e') >= 2:
- tmp.append(name)
- print tmp
#输出结果
- >>>
- ['Jefferson', 'Wesley', 'Steven', 'Jennifer']
代码如下:
- >>> names = [['Tom','Billy','Jefferson','Andrew','Wesley','Steven','Joe'],
- ['Alice','Jill','Ana','Wendy','Jennifer','Sherry','Eva']]
-
- >>> [name for lst in names for name in lst if name.count('e')>=2] #注意遍历顺序，这是实现的关键
- ['Jefferson', 'Wesley', 'Steven', 'Jennifer']
