Python函數式編程 map reduce filter

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函數式編程，使代碼簡潔高效。

 

Map函數：

　　map(func, *iterables)，作用是將一個列表映射到另一個列表。

class map(object):    """    map(func, *iterables) --> map object        Make an iterator that computes the function using arguments from    each of the iterables.  Stops when the shortest iterable is exhausted.    """

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使用方法：

def f(x):    return x**2li = range(1,10)res = map(f,li)print(res)print(list(res))"""<map object at 0x000000000117E2E8>[1, 4, 9, 16, 25, 36, 49, 64, 81]"""

 　　　　　　

 

map(function, iterable, ...)

map()函數接收兩個參數，一個是函數，一個是可迭代的對象，map將傳入的函數依次作用到序列的每個元素，返回一個map對象，不是list。

 

基本等價於 [f(x) for x in interable]，列表推導比map效率要高一些

map(lambda x: x+1, range(1, 3)) => [x+1 for x in range(1,3)]

 

str = ["far","foo","bar"]mp = map(lambda x:x.upper(),str)res = list(mp)print(res)"""[‘FAR‘, ‘FOO‘, ‘BAR‘]"""

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Reduce函數

　　reduce(function, sequence[, initial])，對可迭代對象依次做累計操作，如依次相加或相乘。

　　reduce()方法接收一個函數作為累加器（accumulator），數組中的每個值（從左至右）開始合并，最終為一個值。

def reduce(function, sequence, initial=None): # real signature unknown; restored from __doc__    """    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.    """

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直接使用會報錯

reduce(lambda x, y : x + y, [1, 3, 5, 7, 9])
"""
NameError: name ‘reduce‘ is not defined
"""

正確的使用是：reduce是functools中的一個函數，需要引用：from functools import reduce

 

使用方法：

from functools import reduceres1 = reduce(lambda x, y: x*y, [1, 2, 3])res2 = reduce(lambda x, y : x + y, [1, 3, 5])print(res1)print(res2)"""69"""

 

 

Filter函數

　　filter(function or None, iterable)，作用是按照所定義的函數過濾掉列表中的一些元素

class filter(object):    """    filter(function or None, iterable) --> filter object        Return an iterator yielding those items of iterable for which function(item)    is true. If function is None, return the items that are true.    """

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使用方法：

flt = filter(lambda x: x > 5, range(10))res = list(flt)print(flt)print(res)"""<filter object at 0x0000000000649A58>[6, 7, 8, 9]"""

 

Python函數式編程 map reduce filter