Python中4个强大的内置函数

python 四大利器

filter() and map() and reduce() and lambda()

filter(function, iterable)¶

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.

see https://docs.python.org/2/library/functions.html?highlight=filter#filter

对迭代器中function(iter)为True的组成一个新的迭代器(list/string/tuple等，取决于迭代器的类型)。

>>> def f(x): return x % 2 != 0 and x % 3 !=0
>>> filter(f,xrange(1,100)) 
[1, 5, 7, 11, 13, 17, 19, 23, 25, 29, 31, 35, 37, 41, 43, 47, 49, 53, 55, 59, 61, 65, 67, 71, 73, 77, 79, 83, 85, 89, 91, 95, 97]
# or
>>> filter(lambda x: x % 2 != 0 and x % 3 !=0, xrange(1,100))
[1, 5, 7, 11, 13, 17, 19, 23, 25, 29, 31, 35, 37, 41, 43, 47, 49, 53, 55, 59, 61, 65, 67, 71, 73, 77, 79, 83, 85, 89, 91, 95, 97]
# or
>>> [ x for x in xrange(1,100) if x % 2 != 0 and x % 3 !=0]
[1, 5, 7, 11, 13, 17, 19, 23, 25, 29, 31, 35, 37, 41, 43, 47, 49, 53, 55, 59, 61, 65, 67, 71, 73, 77, 79, 83, 85, 89, 91, 95, 97]

map(function, iterable, ...)¶

Apply function to every item of iterable and return a list of the results.

>>> def cube(x): return x*x*x 
>>> map(cube, xrange(0, 10))
[0, 1, 8, 27, 64, 125, 216, 343, 512, 729]
# or
>>> map(lambda x: x*x*x, xrange(0, 10))
# or
>>> [x*x*x for x in xrange(0, 10)]
# for multiple iterable
>>> map(lambda x,y: x+ y, xrange(0, 10), xrange(10, 20))
[10, 12, 14, 16, 18, 20, 22, 24, 26, 28]

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).

>>> def add(x,y): return x + y
>>> reduce(add, xrange(1, 11))
55
>>> reduce(add, xrange(1, 11),20)
75

lambda¶

lambda是Python中一种非常有趣的语法，它允许你快速定义单行的最小函数，类似与C语言中的宏，这些叫做lambda的函数，是从LISP借用来的，可以用在任何需要函数的地方,函数式编程的典范。
其实上面已经不自觉地在使用了，懒人必备。

>>> g = lambda x: x*x*x
>>> g(3)
27
>>> (lambda x: x*x*x)(3)

综合运用¶

我们也可以把filter map reduce lambda结合起来用，函数就可以简单的写成一行。

>>> filter(lambda x: x % 3 != 0 and x % 4 !=0, map(lambda x,y: x+ y, xrange(0, 100), xrange(100, 200)))
[106, 110, 118, 122, 130, 134, 142, 146, 154, 158, 166, 170, 178, 182, 190, 194, 202, 206, 214, 218, 226, 230, 238, 242, 250, 254, 262, 266, 274, 278, 286, 290, 298]
# 再求和试试
>>> reduce(lambda a,b:a+b, filter(lambda x: x % 3 != 0 and x % 4 !=0, map(lambda x,y: x+ y, xrange(0, 100), xrange(100,200))))
6634