Map / Reduce in Python

To understand the concept, please refer to this paper of Google: link

In python 2, map() and reduce() are both built-in functions. However, since Python3, reduce() is no longer a built-in function, one should import it from functools module.

map() applies a function to all the items in an input list and returns an iterator object. Such like:

>> a = map(lambda x:x+x, [1,2,3,4,5])
>> list(a)
Out: [2, 4, 6, 8, 10]

reduce() performs a rolling computation to sequential pairs of values in an input list and returning the result.

reduce(f, [x1, x2, x3, x4]) = f(f(f(x1, x2), x3), x4)

Here is an example written by myself today:

from functools import reduce # only in Python 3

DIGITS = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9,'.': -1}

def char2num(s):
    return DIGITS[s] 

def str2float(s): 
    L = map(char2num, s)
    r = reduce(func,L)
    print(r)
    return r[0]

def func(x, y):

    if type(x) == int:
        if x == -1:
            return (0 + y, 0.1)
        elif y == -1:
            return(x, 0.1)
        else:
            return (x*10+y, 1)
    elif type(x) == tuple:

        if y == -1:
            return(x[0], 0.1)
        else:
            if x[1]< 1:
                res = x[1]*0.1
                return(x[0]+y*x[1], res)
            else:
                res = 1
                return(x[0]*10+y, res)

if __name__ == "__main__":
    #print('str2float(\'123.456\') =', str2float('123.456'))
    if abs(str2float('123.4') - 123.4) < 0.00001:
        print('测试成功!')
    else:
        print('测试失败!')

    if abs(str2float('123456') - 123456) < 0.00001:
        print('测试成功!')
    else:
        print('测试失败!')

    if abs(str2float('0.123456') - 0.123456) < 0.00001:
        print('测试成功!')
    else:
        print('测试失败!')

reference:

Blog

Map and Reduce in Python

Z. QIU

Map / Reduce in Python

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