推導(dǎo)式是Python中很強(qiáng)大的、很受歡迎的特性，具有語(yǔ)言簡(jiǎn)潔，速度快等優(yōu)點(diǎn)。推導(dǎo)式包括:
1.列表推導(dǎo)式
2.字典推導(dǎo)式
3.集合推導(dǎo)式
嵌套列表推導(dǎo)式
NOTE: 字典和集合推導(dǎo)是最近才加入到Python的(Python 2.7 和Python 3.1以上版). 下面簡(jiǎn)要介紹下:
【列表推導(dǎo)式】
列表推導(dǎo)能非常簡(jiǎn)潔的構(gòu)造一個(gè)新列表:只用一條簡(jiǎn)潔的表達(dá)式即可對(duì)得到的元素進(jìn)行轉(zhuǎn)換變形
其基本格式如下:
[expr for value in collection ifcondition]
過(guò)濾條件可有可無(wú)，取決于實(shí)際應(yīng)用，只留下表達(dá)式；相當(dāng)于下面這段for循環(huán):
result = []
for value in collection:
if condition:
result.append(expression)
例1: 過(guò)濾掉長(zhǎng)度小于3的字符串列表，并將剩下的轉(zhuǎn)換成大寫字母
>>> names = ['Bob','Tom','alice','Jerry','Wendy','Smith']
>>> [name.upper() for name in names if len(name)>3]
['ALICE', 'JERRY', 'WENDY', 'SMITH']
例2: 求(x,y)其中x是0-5之間的偶數(shù)，y是0-5之間的奇數(shù)組成的元祖列表
>>> [(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)]
例3: 求M中3,6,9組成的列表
>>> 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]
例4: 求M中斜線1,5,9組成的列表
>>> 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中矩陣和元素的乘積
>>> 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]]
例5: 講字典中age鍵，按照條件賦新值
>>> 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]
【字典推導(dǎo)式】字典和集合推導(dǎo)式是該思想的延續(xù)，語(yǔ)法差不多，只不過(guò)產(chǎn)生的是集合和字典而已。其基本格式如下:
{ key_expr: value_expr for value in collection if condition }
例1: 用字典推導(dǎo)式以字符串以及其長(zhǎng)度建字典
>>> 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}
【集合推導(dǎo)式】
集合推導(dǎo)式跟列表推導(dǎo)式非常相似，唯一區(qū)別在于用{}代替[]。其基本格式如下:
{ expr for value in collection if condition }
例1: 用集合推導(dǎo)建字符串長(zhǎng)度的集合
>>> strings = ['a','is','with','if','file','exception']
>>> {len(s) for s in strings} #有長(zhǎng)度相同的會(huì)只留一個(gè)，這在實(shí)際上也非常有用
set([1, 2, 4, 9])
【嵌套列表推導(dǎo)式】
嵌套列表是指列表中嵌套列表，比如說(shuō):
>>> L = [[1,2,3],
[4,5,6],
[7,8,9]]
例1: 一個(gè)由男人列表和女人列表組成的嵌套列表,取出姓名中帶有兩個(gè)以上字母e的姓名，組成列表
names = [['Tom','Billy','Jefferson','Andrew','Wesley','Steven','Joe'],
['Alice','Jill','Ana','Wendy','Jennifer','Sherry','Eva']]
用for循環(huán)實(shí)現(xiàn):
tmp = []
for lst in names:
for name in lst:
if name.count('e') >= 2:
tmp.append(name)
print tmp
#輸出結(jié)果
>>>
['Jefferson', 'Wesley', 'Steven', 'Jennifer']
用嵌套列表實(shí)現(xiàn):
>>> 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] #注意遍歷順序，這是實(shí)現(xiàn)的關(guān)鍵
['Jefferson', 'Wesley', 'Steven', 'Jennifer']
