map/reduce

t = list(map(str, [1, 2, 3, 4, 5, 6, 7, 8, 9]))
print(t)

t = (x * x for x in range(10))
print(t)
for value in t:
    print(value)
from collections import Iterator
t2 = isinstance(t, Iterator)
print(t2)

reduce把一个函数作用在一个序列[x1, x2, x3, ...]上，这个函数必须接收两个参数，reduce把结果继续和序列的下一个元素做累积计算，其效果就是：

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

比方说对一个序列求和，就可以用reduce实现：

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

当然求和运算可以直接用Python内建函数sum()，没必要动用reduce。

但是如果要把序列[1, 3, 5, 7, 9]变换成整数13579，reduce就可以派上用场：

>>> 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
   
t = reduce(fn, [1,3, 5, 7, 9])
print(t)
print(type(t))

def str2int(s):
    def fn(x, y):
        return x * 10 + y
    def char2num(s):
        return {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}[s]
    return reduce(fn, map(char2num, s))
   
   
t2 = str2int('12345')
print(t2)
print(type(t2))

from functools import reduce
def char2num(s):
    return {'0':0, '1':1, '2':2, '3':3, '4':4, '5':5, '6':6, '7':7, '8':8, '9':9}[s] #这里是什么意思??????????
def str2int(s):
    return reduce(lambda x, y: x * 10 + y, map(char2num, s)) #label##########

t = str2int('11112222')
print(t)