Python中的Map/Reduce

　　MapReduce是一种函数式编程模型，用于大规模数据集（大于1TB）的并行运算。概念"Map（映射）"和"Reduce（归约）"，是它们的主要思想，都是从函数式编程语言里借来的，还有从矢量编程语言里借来的特性。它极大地方便了编程人员在不会分布式并行编程的情况下，将自己的程序运行在分布式系统上。

　　Map（映射）函数，用来把一组键值对映射成一组新的键值对，指定并发的Reduce（归约）函数，用来保证所有映射的键值对中的每一个共享相同的键组。

　　然而在python中，map就是 :将一个函数映射到所有可枚举类型上，reduce就是归约。

#map/reduce

from  functools import reduce
print(list(map(str,[-1,-2,-3,-4,-5])))
def fn(x, y):
    print(x, y)
    return x*10 + y
r = reduce(fn,[1,3,5,7,9])
print(r)

def func_sum(x, y):
    return x + y
def square(x):
    return x*x
list_r = map(square,[-1,-2,-3,-4,-5])
ll_r = list(list_r)
print('r = ',ll_r)
ans = reduce(func_sum, ll_r)
print('sum is ',ans)

def add_100(a, b, c):
    print(a,b,c)
    return a * 10000+ b *100 + c
list1 = [11,22,33]
list2 = [44,55,66]
list3 = [77,88,99]
rec = map(add_100,list1, list2, list3)
print(list(rec))

from functools import reduce
def fn(x, y):
    print(x, y)
    return x * 10 + y
def char2num(s):
    return {'': 0, '': 1, '': 2, '': 3, '': 4, '': 5, '': 6, '': 7, '': 8, '': 9}[s]
    '''{} is a dictionary, [] is index of dict, so [key] return  value'''
print(reduce(fn, map(char2num, '')))

def str2int(s):
    def fn(x, y):
        return x * 10 + y
    def char2num(s):
        return {'': 0, '': 1, '': 2, '': 3, '': 4, '': 5, '': 6, '': 7, '': 8, '': 9}[s]
    return reduce(fn, map(char2num, s))
print(str2int(''))
print(int(''))
print(str(13572))

def str2int_(s):
    def char2num(s):
        return {'': 0, '': 1, '': 2, '': 3, '': 4, '': 5, '': 6, '': 7, '': 8, '': 9}[s]
    return reduce(lambda x,y: x * 10 + y, map(char2num, s))
    '''lambda 匿名函数，有些简单函数只需要用1次，所以不给起名字'''
print(str2int_(''))

#联系
#1 规范化英文名
def normalize(name):
    return name.capitalize()
L1 = ['adam', 'LISA', 'barT']
L2 = list(map(normalize, L1))
print(L2)
#2 请编写一个prod()函数，可以接受一个list并利用reduce()求积：
from functools import  reduce
def prod(L):
    return reduce(lambda x,y : x * y, L)
print('3 * 5 * 7 * 9 =', prod([3, 5, 7, 9]))
#3 利用map和reduce编写一个str2float函数，把字符串'123.456'转换成浮点数123.456：
from functools import  reduce
def str2float(s):
    def char2num(s):
        return {'': 0, '': 1, '': 2, '': 3, '': 4, '': 5, '': 6, '': 7, '': 8, '': 9}[s]
    pos = s.find('.')
    s_num = s.split('.')[0] + s.split('.')[1]
    print(pos, s_num)
    L = reduce(lambda x,y : x * 10 + y, map(char2num, s_num))
    return  L/ math.pow(10, pos)
print('str2float(\'123.456\') =', str2float('123.456'))
#Another solution
def str2float_(s):
    def char2num(s):
        return {'': 0, '': 1, '': 2, '': 3, '': 4, '': 5, '': 6, '': 7, '': 8, '': 9}[s]
    a, b = s.split('.')
    L = reduce(lambda  x,y: x * 10 + y, map(char2num, a + b))
    return L/10**len(b)
print('str2float(\'123.456\') =', str2float('123.456'))

　　以上代码都是liaoxuefeng教程中的内容实现和练习题。