python中filter()，map()和reduce()的区
filter() 返回true的，插入list
map（） 返回结果插入list
和map及list联合使用
>>> import sys
>>> showall = lambda x:list(map(sys.stdout.write,x))
>>> showall(['Jerry\n','Sherry\n','Alice\n'])
Jerry
Sherry
Alice
>>> showall(['Jerry','Sherry','Alice'])
JerrySherryAlice
等价于下面
>>> showall = lambda x: [sys.stdout.write(line) for line in x]
>>> showall(('I\t','Love\t','You!'))
I Love You![None, None, None]
例06: 在Tkinter中定义内联的callback函数
import sys
from Tkinter import Button,mainloop
x = Button(text='Press me',
command=(lambda:sys.stdout.write('Hello,World\n')))
x.pack()
x.mainloop()
>>>
Hello,World!
Hello,World!
例07: lambda和map联合使用,
>>> out = lambda *x: sys.stdout.write(' '.join(map(str,x)))
>>> out('This','is','a','book!\n')
This is a book!
例08: 判断字符串是否以某个字母开头
>>> print (lambda x: x.startswith('B'))('Bob')
True
-----
>>> Names = ['Anne', 'Amy', 'Bob', 'David', 'Carrie', 'Barbara', 'Zach']
>>> B_Name= filter(lambda x: x.startswith('B'),Names)
>>> B_Name
['Bob', 'Barbara']
例09: lambda和map联合使用:
>>> squares = map(lambda x:x**2,range(5))
>>> squares
[0, 1, 4, 9, 16]
例10. lambda和map,filter联合使用:
>>> squares = map(lambda x:x**2,range(10))
>>> filters = filter(lambda x:x>5 and x<50,squares)
>>> filters
[9, 16, 25, 36, 49]
例11. lambda和sorted联合使用
#按death名单里面，按年龄来排序
#匿名函数的值返回给key，进来排序
>>> death = [ ('James',32),
('Alies',20),
('Wendy',25)]
>>> sorted(death,key=lambda age:age[1]) #按照第二个元素，索引为1排序
[('Alies', 20), ('Wendy', 25), ('James', 32)]
例12. lambda和reduce联合使用
>>> L = [1,2,3,4]
>>> sum = reduce(lambda x,y:x+y,L)
>>> sum
10
例13. 求2-50之间的素数
#素数:只能被1或被自己整除的数
>>> nums = range(2,50)
>>> for i in nums:
nums = filter(lambda x:x==i or x % i,nums)
>>> nums
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]
例14. 求两个列表元素的和
>>> a = [1,2,3,4]
>>> b = [5,6,7,8]
>>> map(lambda x,y:x+y, a,b)
[6, 8, 10, 12]
例15. 求字符串每个单词的长度
>>> sentence = "Welcome To Beijing!"
>>> words = sentence.split()
>>> lengths = map(lambda x:len(x),words)
>>> lengths
[7, 2, 8]
写成一行:
>>> print map(lambda x:len(x),'Welcome To Beijing!'.split())
部分参考玩蛇网：http://www.iplaypy.com/wenda/lambda.html
部分参考csdn：http://blog.csdn.net/csdnstudent/article/details/40112803
