Lambda, map, filter, and reduce for Python functions

1. Lambda function

Lambda () is an anonymous function in Python with the following syntax:

Lambda [arg1[, Arg2, ... ArgN]: expression

When learning conditional operations, for simple if else statements, you can use the ternary operation to indicate that:

# Common conditional Statements if 1 = =1    :'evescn'else    :' GM '  # Ternary Operations ' EVESCN ' if Else ' GM '

For simple functions, there is also an easy way to represent a lambda expression

# ###################### Common function ###################### # defining functions (normal way) def func (ARG):     return arg + 1  #  execution function result = func (123)  #  ######## ############## Lambda ######################  #  definition function (lambda expression)Lambda Arg:arg + 1  #  execution function result = MY_LAMBDA (123)

Lambda existence means a concise representation of a simple function, as in the example of a list below

L = [One, one, ten, x:x = map (lambda + = List (new_l)print(a)#  Output Results [21, 32, 43, 54]

2. Map functionThe map function maps the specified sequence according to the provided function. definition of the map function:

The map function invokes the function function for each element in the sequence parameter sequence, returning the result of invoking the return value of the function functions for each element

L = [One, one, one, ten]def  myadd (x):    return x + ten== List (new _n)print(b)#  Output Results [21, 32, 43, 54]

The use map() of functions, the user entered the non-standard English name, the first letter capitalized, other lowercase canonical name. Input: [‘adam‘, ‘LISA‘, ‘barT‘] , output: [‘Adam‘, ‘Lisa‘, ‘Bart‘] :

3. Filter function

Filter the elements in a sequence, and finally get a sequence that matches the criteria

def myfunc (x):     if x >:        return  True    Else:        return = [ One, one, one== = List (new_a)print(b)#    Output result [33]

4. Reduce function

Cumulative operation for all elements within a sequence

reduceA function that acts on a sequence [x1, x2, x3, ...], this function must receive two parameters, reduceThe result continues and the next element of the sequence is accumulated, and the effect is:

Reduce (f, [X1, x2, X3, x4]) = f (f (f (x1, x2), x3), x4)

For example, to sum a sequence, it can be reduce implemented by:

 from Import Reduce def Add (x, y): ...      return x + y ... >>> reduce (add, [1, 3, 5, 7, 9])25

Of course, the sum operation can be directly built into Python functions sum() , no need to use reduce .

But if you want to [1, 3, 5, 7, 9] transform the sequence into integers 13579 , you can put reduce it in handy:

 from Import Reduce def fn (x, y): ...      return x * ten + y ... >>> reduce (FN, [1, 3, 5, 7, 9])13579

This example is not very useful in itself, but if we consider that the string str is also a sequence, with a slight change to the above example, map() we can write the str converted int function:

>>> fromFunctoolsImportReduce>>>deffn (x, y): ...returnX * 10 +y ...>>>defChar2num (s): ...return{'0': 0,'1': 1,'2': 2,'3': 3,'4': 4,'5': 5,'6': 6,'7': 7,'8': 8,'9'8 {}[s] ...>>> reduce (FN, map (Char2num,'13579'))13579

str2intthe function that is organized into one is:

 fromFunctoolsImportReducedefStr2Int (s):deffn (x, y):returnX * 10 +ydefChar2num (s):return{'0': 0,'1': 1,'2': 2,'3': 3,'4': 4,'5': 5,'6': 6,'7': 7,'8': 8,'9': 9}[s]returnReduce (FN, map (Char2num, s))

You can also use lambda functions to further simplify:

 fromFunctoolsImportReducedefChar2num (s):return{'0': 0,'1': 1,'2': 2,'3': 3,'4': 4,'5': 5,'6': 6,'7': 7,'8': 8,'9': 9}[s]defStr2Int (s):returnReduceLambdaX, y:x * + y, map (Char2num, s))

That is, assuming that Python does not provide a int() function, you can write a function that converts the string to an integer by itself, and only requires a few lines of code!

Reprinted from: Https://www.liaoxuefeng.com/wiki/0014316089557264a6b348958f449949df42a6d3a2e542c000/ 0014317852443934a86aa5bb5ea47fbbd5f35282b331335000

Http://www.cnblogs.com/wupeiqi/articles/4943406.html

Lambda, map, filter, and reduce for Python functions