Table of Contents
2.Filter" >2.Filter
3.Reduce" >3.Reduce
Home Backend Development Python Tutorial Detailed explanation of python functions map, filter, and reduce

Detailed explanation of python functions map, filter, and reduce

Apr 02, 2018 am 10:13 AM
filter map python

This article shares with you a detailed explanation of the python functions map, filter, and reduce. The content is quite good. I hope it can help friends in need

##1 .map

Map will map a function to all elements of an input list. This is its specification:

Specification

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map(function_to_apply, list_of_inputs)

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Most of the time, we want to pass all the elements in the list to a function one by one and collect the output. For example:

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items = [1, 2, 3, 4, 5]

squared = []for i in items:

    squared.append(i**2)

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Map allows us to achieve it in a much simpler and more beautiful way. That's it:

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items = [1, 2, 3, 4, 5]

squared = list(map(lambda x: x**2, items))

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Most of the time, we use anonymous functions (lambdas) to match

map, So I did the same thing above. Not only for a list of inputs, we can even use it for a list of functions!

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def multiply(x):

        return (x*x)def add(x):

        return (x+x)

 

funcs = [multiply, add]for i in range(5):

    value = map(lambda x: x(i), funcs)

    print(list(value))    

    # 译者注:上面print时,加了list转换,是为了python2/3的兼容性

    #        在python2中map直接返回列表,但在python3中返回迭代器

    #        因此为了兼容python3, 需要list转换一下

    # Output:

    # [0, 0]

    # [1, 2]

    # [4, 4]

    # [9, 6]

    # [16, 8]

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2.Filter

顾名思义,filter过滤列表中的元素,并且返回一个由所有符合要求的元素所构成的列表,符合要求即函数映射到该元素时返回值为True. 这里是一个简短的例子:

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number_list = range(-5, 5)

less_than_zero = filter(lambda x: x < 0, number_list)

print(list(less_than_zero))  

# 译者注:上面print时,加了list转换,是为了python2/3的兼容性

#        在python2中filter直接返回列表,但在python3中返回迭代器

#        因此为了兼容python3, 需要list转换一下

# Output: [-5, -4, -3, -2, -1]

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这个filter类似于一个for循环,但它是一个内置函数,并且更快。

注意:如果mapfilter对你来说看起来并不优雅的话,那么你可以看看另外一章:列表/字典/元组推导式


3.Reduce

当需要对一个列表进行一些计算并返回结果时,Reduce 是个非常有用的函数。举个例子,当你需要计算一个整数列表的乘积时。

通常在 python 中你可能会使用基本的 for 循环来完成这个任务。

现在我们来试试 reduce:

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from functools import reduce

product = reduce( (lambda x, y: x * y), [1, 2, 3, 4] )

# Output: 24

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