[Python] functions Lambda (), filter (), map (), reduce ()

ArticleDirectory

• 1. Lambda ()
• 2. Filter ()
• 3. Map ()
• 4. Reduce ()
• 5. Comprehensive examples

1. Lambda ()

Lambda () is an anonymous function in Python. Its syntax is as follows:

Lambda [arg1 [, arg2,... argn]: Expression

The following is an example of 1 + 2 = 3.

 

 
>>> Fun = Lambda X, Y: x + y >>> fun (1, 2) 3
 

 
>>> (Lambda X, Y: X + Y) (1, 2) 3
 

 

2. Filter ()

Filter (function, sequence): Execute function (item) in sequence for items in sequence, and combine items whose execution result is true into a list/string/tuple (depending on the sequence type) to return:

 

 
>>> Def f (x): Return X % 2! = 0 and X % 3! = 0 >>> filter (F, range (2, 25) [5, 7, 11, 13, 17, 19, 23] >>> def f (x ): return X! = 'A'> filter (F, "abcdef") 'bcdef'
 

 

3. Map () map (function, sequence): Execute function (item) in sequence for items in sequence. See the execution result to form a list and return:

 
>>> Def cube (x): Return x * x >>> map (cube, range (1, 11) [1, 8, 27, 64,125,216,343,512,729,100 0] >>> def cube (x): Return x + X... >>> map (cube, "ABCDE") ['A', 'bb ', 'cc', 'dd', 'ee']
 

In addition, MAP also supports multiple sequence, which requires that the function also supports the corresponding number of parameter inputs:

>>> Def add (x, y): Return x + y >>> map (ADD, range (8), range (8) [0, 2, 4, 6, 8, 10, 12, 14]
 

4. Reduce ()

Reduce (function, sequence, starting_value): calls the function sequentially for items in sequence. If starting_value exists, it can also be called as an initial value. For example, it can be used to sum the list:

 

 
>>> Def add (x, y): Return x + y >>> reduce (ADD, range (1, 11) 55 (note: 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10)> reduce (ADD, range (1, 11), 20) 75 (note: 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 20)
 

5. Comprehensive examples

The following are two examples of using the above four functions:

Example 1: Computing 5! + 4! + 3! + 2! + 1!

 
A = [5, 4, 3, 2, 1] def fun (x): Result = 1 while x> = 1: result = Result * x = x-1 return resultprint reduce (lambda X, Y: X + Y, map (fun, ))
 

Example 2:Select a prime number of less than 100

 
Import math def isprime (n): If n <= 1: Return false for I in range (2, INT (math. SQRT (n) + 1): If n % I = 0: Return false return True Print filter (isprime, range (1,100 ))