Python's anonymous function lambda interpretation and usage

The syntax for a lambda function contains only one statement, as follows: Lambda arg1,arg2,..... argn:expression (mainly see the following example)
code example:

#-*-coding:utf-8-*-# __author__ = "www.iplaypy.com" # ordinary Python function def Func (a,b,c): return a+b+c print func (1,2,3) # return value 6 # lambda anonymous function f = lambda a,b,c:a+b+c print F (1,2,3#< Span style= "color: #008000;" > return result is 6                

# Look at the Python sample code above, f = the keyword lambda in lambda a,b,c:a+b+c represents an anonymous function,
Return returns a value, and the expression itself results in a return value.

(Here are some of the code I copied, it is through the code so that I have a more in-depth understanding of the anonymous function, so suggest that you take a serious look,)

No parameter anonymous function:
------
>>> t = lambda:true #分号前无任何参数
>>> T ()
True

Functions equivalent to the definition of def below
>>> def func (): Return True
>>> func ()
True

------

>>> s = "This Is\na\ttest" #建此字符串按照正常情形输出
>>> s
' This is\na\ttest '
>>> print S.split () #split函数默认分割: spaces, line breaks, TAB
[' This ', ' is ', ' a ', ' test ']
>>> ". Join (S.split ()) #用join函数转一个列表为字符串
' This is a test '

Equivalent to

>>> (lambda S: ". Join (S.split ())") ("This Is\na\ttest")




anonymous function with parameters

>>> Lambda x:x**3 #一个参数
>>> Lambda X,y,z:x+y+z #多个参数

>>> Lambda X,y=3:x*y #允许参数存在默认值



anonymous function calls

#直接赋值给一个变量, and then again like the general function call

------

>>> C = Lambda x,y,z:x*y*z
>>> C (2,3,4)
24

------

>>> C = Lambda x,y=2:x+y #使用了默认值
>>> c #不输的话, using the default value of 2
12

------

>>> a = Lambda *z:z #*z Returns a meta-ancestor
>>> A (' Testing1 ', ' Testing2 ')
(' Testing1 ', ' Testing2 ')

------

>>> C = Lambda **arg:arg #arg返回的是一个字典
>>> C ()
{}

#直接后面传递实参

------

>>> (Lambda x,y:x if x> y else y) (101,102)
102

------

>>> (Lambda x:x**2) (3)
9

#lambda返回的值, combined with map,filter,reduce

>>> filter (lambda x:x%3==0,[1,2,3,4,5,6])
[3, 6]

Equivalent to the following list derivation

>>> L = [x for x in [1,2,3,4,5,6] if x%3==0]
>>> L
[3, 6]


Nested use

#lambda嵌套到普通函数中, the lambda function itself is the value of return

------

>>> def increment (n):
... return Lambda x:x+n
...
>>> F=increment (4)
>>> F (2)
6

------

>>> def say ():
... title = ' Sir, '
... action= Lambda X:title + x
... return action
...
>>> act = Say ()
>>> Act (' smith! ')
' sir,smith! '



A large number of examples:

Example 01: String union with default value or x= (lambda ...) This format

>>> x = (lambda x= "Boo", y= "Too", z= "Zoo": x+y+z)
>>> x ("Foo")
' Footoozoo '



Example 02: Federated Use with lists

>>> L = [Lambda x:x**2,\
Lambda x:x**3,\
Lambda X:x**4]

>>> for F in L:
... print f (2)
...
4
8
16

It can also be called as follows

>>> Print L[0] (3)
9

Example 03: Working with Dictionaries

>>> key = ' B '
>>> dic = {' A ': lambda:2*2,\
... ' B ': lambda:2*4,\
... ' C ': lambda:2*8}
>>> Dic[key] ()
8

Example 04: Finding the Minimum value

>>> lower = lambda x,y:x if x<y else y
>>> lower (' AA ', ' AB ')
' AA '



Example 05: Combined with map and list

>>> 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

Equivalent to the following

>>> ShowAll = lambda x: [Sys.stdout.write (line) for line in X]
>>> showall (' i\t ', ' love\t ', ' you! ')
I Love you! [None, none, none]



Example 06: Defining an inline callback function in Tkinter

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!



Example 07:LAMBDA and map are used together,

>>> out = Lambda *x:sys.stdout.write (". Join (Map (str,x)))
>>> out (' The ', ' is ', ' a ', ' book!\n ')
This is a book!



Example 08: Judging whether a string starts with a letter

>>> 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 ']



Example 09:LAMBDA and map are used together:

>>> squares = map (lambda x:x**2,range (5))
>>> Squares
[0, 1, 4, 9, 16]



Example 10. Lambda and Map,filter are used together:

>>> squares = map (lambda x:x**2,range (10))
>>> filters = filter (lambda x:x>5 and X<50,squares)
>>> Filters
[9, 16, 25, 36, 49]



Example 11. Lambda and sorted are used together

#按death名单里面, sorted by age

#匿名函数的值返回给key, come in sort of

>>> death = [(' James ', 32),
(' Alies ', 20),
(' Wendy ', 25)]

>>> sorted (Death,key=lambda age:age[1]) #按照第二个元素, index 1 sorted
[(' Alies ', '), (' Wendy ', +), (' James ', 32)]



Example 12. Using lambda and reduce together

>>> L = [1,2,3,4]
>>> sum = reduce (lambda x,y:x+y,l)
>>> sum
10



Example 13. Ask for a prime number between 2-50

#素数: Numbers that can only be divisible by 1 or by themselves

>>> 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]



Example 14. The and of the two list elements

>>> a = [1,2,3,4]
>>> B = [5,6,7,8]
>>> map (Lambda X,y:x+y, A, b)
[6, 8, 10, 12]



Example 15. Find the length of each word in a string

>>> sentence = "Welcome to beijing!"
>>> words = Sentence.split ()

>>> lengths = Map (lambda x:len (x), words)
>>> Lengths
[7, 2, 8]

Write a line:

>>> Print map (lambda X:len (x), ' Welcome to beijing! '). Split ())

Some references play snake net: http://www.iplaypy.com/wenda/lambda.html

Section Reference csdn:http://blog.csdn.net/csdnstudent/article/details/40112803

Python's anonymous function lambda interpretation and usage