Map、Filter和Reduce函数（Python）

Map

map(function_to_apply, list_of_inputs)

设有以下代码：

>>> items = [1, 2, 3, 4, 5]
>>> squared = []
>>> for i in items:
...   squared.append(i**2)
... 
>>> squared
[1, 4, 9, 16, 25]

这段代码实现的功能用map函数可以两行完成：

items = [1, 2, 3, 4, 5]
squared = list(map(lambda x: x**2, items))

高级一点地，可以用一列函数作为输入：

def multiply(x):
    return (x*x)
def add(x):
    return (x+x)

funcs = [multiply, add]
for i in range(5):
    value = list(map(lambda x: x(i)+1, funcs))
    print(value)

>>>
[1, 1]
[2, 3]
[5, 5]
[10, 7]
[17, 9]

Filter

Filter creates a list of elements for which a function returns true.

>>> number_list = range(-5,5)
>>> less_than_zero = list(filter(lambda x: x<0, number_list))
>>> print(less_than_zero)
[-5, -4, -3, -2, -1]

经过_filter_函数_过滤_，条件判断为真的结果的存入list。另外值得注意的地，_filter_是_built-in_函数，运行速度更快。

Reduce

Reduce is a really useful function for performing some computation on a list and returning the result. It applies a rolling computation to sequential pairs of values in a list.

>>> from functools import reduce
>>> product = reduce((lambda x, y: x * y), [1, 2, 3, 4])
>>> product
24