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:
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