Python有很多有用有趣的内置函数，比如reduce，map，filter，lambda，zip等。已经写过了lambda和zip相关的博客。继续写关于reduce，map，filter。

Map

首先用help方法看一下map的具体用法。

help(map)

# result

Help on built-in function map in module __builtin__:

map(...)
    map(function, sequence[, sequence, ...]) -> list
    
    Return a list of the results of applying the function to the items of
    the argument sequence(s).  If more than one sequence is given, the
    function is called with an argument list consisting of the corresponding
    item of each sequence, substituting None for missing values when not all
    sequences have the same length.  If the function is None, return a list of
    the items of the sequence (or a list of tuples if more than one sequence).

l1 = [1,2,3,4,5]

def func(x):
    return x*x

print map(func, l1)

# result

[1, 4, 9, 16, 25]

l1 = [1,2,3,4,5]
l2 = [6,7,8,9,10]

def func(x,y):
    return x+y

print map(func, l1, l2)

# result

[7, 9, 11, 13, 15]

对于不等长的sequences

l1 = [1,2,3,4,5,6]
l2 = [6,7,8,9,10]

def func(x,y):
    return y

print map(func, l1, l2)

# result

[6, 7, 8, 9, 10, None]

map可以和lambda一起用

l1 = [1,2,3,4,5]

print map((lambda x: x **2), l1)

# result

[1, 4, 9, 16, 25]

def square(x):
        return (x**2)
def cube(x):
        return (x**3)

funcs = [square, cube]
for r in range(5):
    value = http://www.mamicode.com/map(lambda x: x(r), funcs)>

如果function是None的话
l1 = [1,2,3,4,5]
l2 = [6,7,8,9,10]

print map(None, l1, l2)

# result

[(1, 6), (2, 7), (3, 8), (4, 9), (5, 10)]

map就像是一个for loop，对sequence中的元素依次进行操作。
from math import sqrt

def mymap(aFunc, aSeq):
	result = []
	for x in aSeq: result.append(aFunc(x))
	return result

print list(map(sqrt, [1, 2, 3]))

print mymap(sqrt, [1, 2, 3])

# result

[1.0, 1.4142135623730951, 1.7320508075688772]
[1.0, 1.4142135623730951, 1.7320508075688772]




Filter
用help看一下filter的用法
help(filter)

# result

Help on built-in function filter in module __builtin__:

filter(...)
    filter(function or None, sequence) -> list, tuple, or string
    
    Return those items of sequence for which function(item) is true.  If
    function is None, return the items that are true.  If sequence is a tuple
    or string, return the same type, else return a list.

filter函数，同样是filter(function，sequence)，返回list，tuple，或者string。返回经过function判断过之后是true的元素。如果function是None，那么就返回true的元素。如果sequence是tuple或者string，返回同样的类型，如果是其他类型，就返回list。
print list(range(-5,5))

print list(filter((lambda x: x<0), range(-5,5)))

# result

[-5, -4, -3, -2, -1, 0, 1, 2, 3, 4]
[-5, -4, -3, -2, -1]

如果function是None
print list(filter(None, range(-5,5)))

# result

[-5, -4, -3, -2, -1, 1, 2, 3, 4]

只有0没有输出，因为0这里被认为是False。
其他类型
def f(x):
    return x != 'a'

print filter(f, 'abcdef')

# result

bcdef



Reduce
用help看一下reduce的用法
help(reduce)

# result

Help on built-in function reduce in module __builtin__:

reduce(...)
    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.

reduce函数，reduce(function，sequence[，initial])返回一个值。把sequence中的元素从左往右两个两个根据function操作起来，最终剩下一个值。比如reduce(lambda x，y：x+y，[1,2,3,4,5]) 运行的过程就是((((1+2)+3)+4)+5). initial可以设置作为一个default，如果sequence是空的话。

def add(x,y):    return x+yprint reduce(add, range(1,11))print reduce(add, range(1,11), 20)print reduce(add, [],20)# result55 (1+2+3+4+5+6+7+8+9+10)75 (20+1+2+3+4+5+6+7+8+9+10)20 (default)<span style="font-family: Arial, Helvetica, sans-serif;"></span>

同样的也可以配合lambda一起使用
print reduce((lambda x, y: x * y), [1, 2, 3, 4])

# result

24 (1*2*3*4)


Python interview - reduce & map & filter