Python中的map、reduce和filter淺析

更新時間：2014年04月26日 10:46:29 作者：

這篇文章主要介紹了Python中的map、reduce和filter,用實例來理解這3個函數(shù),需要的朋友可以參考下

1、先看看什么是 iterable 對象

以內(nèi)置的max函數(shù)為例子，查看其doc：

復(fù)制代碼代碼如下:

>>> print max.__doc__
max(iterable[, key=func]) -> value
max(a, b, c, ...[, key=func]) -> value

With a single iterable argument, return its largest item.
With two or more arguments, return the largest argument.

在max函數(shù)的第一種形式中，其第一個參數(shù)是一個 iterable 對象，既然這樣，那么哪些是 iterable 對象呢？

復(fù)制代碼代碼如下:

>>> max('abcx')
>>> 'x'
>>> max('1234')
>>> '4'
>>> max((1,2,3))
>>> 3
>>> max([1,2,4])
>>> 4

我們可以使用yield生成一個iterable 對象（也有其他的方式）:

復(fù)制代碼代碼如下:

def my_range(start,end):
    ''' '''
    while start &lt;= end:
        yield start
        start += 1

執(zhí)行下面的代碼：

復(fù)制代碼代碼如下:

for num in my_range(1, 4):
    print num
print max(my_range(1, 4))

將輸出：

復(fù)制代碼代碼如下:

2、map

在http://docs.python.org/2/library/functions.html#map中如此介紹map函數(shù)：

復(fù)制代碼代碼如下:

map(function, iterable, ...)
Apply function to every item of iterable and return a list of the results. If additional iterable arguments are passed, function must take that many arguments and is applied to the items from all iterables in parallel. If one iterable is shorter than another it is assumed to be extended with None items. If function is None, the identity function is assumed; if there are multiple arguments, map() returns a list consisting of tuples containing the corresponding items from all iterables (a kind of transpose operation). The iterable arguments may be a sequence or any iterable object; the result is always a list.

map函數(shù)使用自定義的function處理iterable中的每一個元素，將所有的處理結(jié)果以list的形式返回。例如：

復(fù)制代碼代碼如下:

def func(x):
''' '''
return x*x

print map(func, [1,2,4,8])
print map(func, my_range(1, 4))

運行結(jié)果是：

復(fù)制代碼代碼如下:

[1, 4, 16, 64]
[1, 4, 9, 16]

也可以通過列表推導(dǎo)來實現(xiàn)：

復(fù)制代碼代碼如下:

print [x*x for x in [1,2,4,8]]

3、reduce

在http://docs.python.org/2/library/functions.html#reduce中如下介紹reduce函數(shù)：

復(fù)制代碼代碼如下:

reduce(function, iterable[, initializer])
Apply function of two arguments cumulatively to the items of iterable, from left to right, so as to reduce the iterable to a single value. For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5). The left argument, x, is the accumulated value and the right argument, y, is the update value from the iterable. If the optional initializer is present, it is placed before the items of the iterable in the calculation, and serves as a default when the iterable is empty. If initializer is not given and iterable contains only one item, the first item is returned.

這個已經(jīng)介紹的很明了，

復(fù)制代碼代碼如下:

reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])

相當(dāng)于計算

復(fù)制代碼代碼如下:

((((1+2)+3)+4)+5)

而：

復(fù)制代碼代碼如下:

reduce(lambda x, y: x+y, [1, 2, 3, 4, 5],6)

相當(dāng)于計算

復(fù)制代碼代碼如下:

(((((6+1)+2)+3)+4)+5)

4、filter

在http://docs.python.org/2/library/functions.html#filter中如下介紹filter函數(shù)：

復(fù)制代碼代碼如下:

filter(function, iterable)
Construct a list from those elements of iterable for which function returns true. iterable may be either a sequence, a container which supports iteration, or an iterator. If iterable is a string or a tuple, the result also has that type; otherwise it is always a list. If function is None, the identity function is assumed, that is, all elements of iterable that are false are removed.

Note that filter(function, iterable) is equivalent to [item for item in iterable if function(item)] if function is not None and [item for item in iterable if item] if function is None.

參數(shù)function（是函數(shù)）用于處理iterable中的每個元素，如果function處理某元素時候返回true，那么該元素將作為list的成員而返回。比如，過濾掉字符串中的字符a：

復(fù)制代碼代碼如下:

def func(x):
''' '''
return x != 'a'

print filter(func, 'awake')

運行結(jié)果是：

復(fù)制代碼代碼如下:

wke

這也可以通過列表推導(dǎo)來實現(xiàn)：

復(fù)制代碼代碼如下:

print ''.join([x for x in 'awake' if x != 'a'])

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