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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