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  • 【384】reduce归纳、map映射、filter筛选 的用法

    参考:4. Map, Filter and Reduce — Python Tips 0.1 documentation 

    参考:Python的functools.reduce用法


    Map:映射,对于列表的每个元素进行相同的操作

    filter:筛选,筛选列表中满足某一条件的所有元素

    reduce:归纳,连续操作,连加、连乘等


    python 3.0以后, reduce已经不在built-in function里了, 要用它就得from functools import reduce.

    reduce的用法

    reduce(function, sequence[, initial]) -> value

    Apply a function of two arguments cumulatively to the items of a sequence,
    from left to right, so as to reduce the sequence to a single value.
    For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates
    ((((1+2)+3)+4)+5).  If initial is present, it is placed before the items
    of the sequence in the calculation, and serves as a default when the
    sequence is empty.

    意思就是对sequence连续使用function, 如果不给出initial, 则第一次调用传递sequence的两个元素, 以后把前一次调用的结果和sequence的下一个元素传递给function. 如果给出initial, 则第一次传递initial和sequence的第一个元素给function.

    from functools import reduce
    reduce(lambda x,y: x+y, [1, 2, 3])
    reduce(lambda x,y: x+y, [1, 2, 3], 9)
    
    reduce(lambda x,y: x*y, [1, 2, 3, 4])
    reduce(lambda x,y: x*y, [1, 2, 3, 4], 5)
    
    reduce(lambda x,y: x**y, [2, 3, 4])
    
    output:
    6
    15
    24
    120
    4096
    

    Example from Ed of COMP9021

    question:

    For instance, dict1 = {'Lucy' : 'I am a Knight', 'Laser':'I am a Knaves'}

    list1 = [(0,0), (0,1), (1,0),(1,1)]

    how do I put the output like this:

    {'Lucy' : 0, 'Laser':0}

    {'Lucy' : 0, 'Laser':1}

    {'Lucy' : 1, 'Laser':0}

    {'Lucy' : 1, 'Laser':1}

    answers:

    dict1 = {'Lucy' : 'I am a Knight', 'Laser':'I am a Knaves'}
    list1 = [(0,0), (0,1), (1,0),(1,1)]
    answer = [i for i in map(lambda x:{[key for key in dict1][0]:x[0], [key for key in dict1][1]:x[1]}, list1)]
    for i in answer:
    	print(i)
    
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  • 原文地址:https://www.cnblogs.com/alex-bn-lee/p/10570970.html
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