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#### map/reduce

Python内建了map()reduce()函数。

            f(x) = x * x

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┌───┬───┬───┬───┼───┬───┬───┬───┐
│   │   │   │   │   │   │   │   │
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[ 1   2   3   4   5   6   7   8   9 ]

│   │   │   │   │   │   │   │   │
│   │   │   │   │   │   │   │   │
▼   ▼   ▼   ▼   ▼   ▼   ▼   ▼   ▼

[ 1   4   9  16  25  36  49  64  81 ]


>>> def f(x):
...     return x * x
...
>>> r = map(f, [1, 2, 3, 4, 5, 6, 7, 8, 9])
>>> list(r)
[1, 4, 9, 16, 25, 36, 49, 64, 81]


map()传入的第一个参数是f，即函数对象本身。由于结果r是一个IteratorIterator是惰性序列，因此通过list()函数让它把整个序列都计算出来并返回一个list。

L = []
for n in [1, 2, 3, 4, 5, 6, 7, 8, 9]:
L.append(f(n))
print(L)


>>> list(map(str, [1, 2, 3, 4, 5, 6, 7, 8, 9]))
['1', '2', '3', '4', '5', '6', '7', '8', '9']


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


>>> from functools import reduce
>>> def add(x, y):
...     return x + y
...
>>> reduce(add, [1, 3, 5, 7, 9])
25


>>> from functools import reduce
>>> def fn(x, y):
...     return x * 10 + y
...
>>> reduce(fn, [1, 3, 5, 7, 9])
13579


>>> from functools import reduce
>>> def fn(x, y):
...     return x * 10 + y
...
>>> def char2num(s):
...     digits = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}
...     return digits[s]
...
>>> reduce(fn, map(char2num, '13579'))
13579


from functools import reduce

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

def str2int(s):
def fn(x, y):
return x * 10 + y
def char2num(s):
return DIGITS[s]
return reduce(fn, map(char2num, s))


from functools import reduce

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

def char2num(s):
return DIGITS[s]

def str2int(s):
return reduce(lambda x, y: x * 10 + y, map(char2num, s))


lambda函数的用法在后面介绍。

### 练习

# -*- coding: utf-8 -*-
----
def normalize(name):
pass
----
# 测试:
L1 = ['adam', 'LISA', 'barT']
L2 = list(map(normalize, L1))
print(L2)


Python提供的sum()函数可以接受一个list并求和，请编写一个prod()函数，可以接受一个list并利用reduce()求积：

# -*- coding: utf-8 -*-
from functools import reduce
----
def prod(L):
pass
----
print('3 * 5 * 7 * 9 =', prod([3, 5, 7, 9]))
if prod([3, 5, 7, 9]) == 945:
print('测试成功!')
else:
print('测试失败!')


# -*- coding: utf-8 -*-
from functools import reduce

def str2float(s):
----
pass
----
print('str2float(\'123.456\') =', str2float('123.456'))
if abs(str2float('123.456') - 123.456) < 0.00001:
print('测试成功!')
else:
print('测试失败!')


do_map.py

do_reduce.py