高阶函数

定义：一个函数接收另一个函数作为参数，这种函数称为高阶函数

变量指向函数：函数本身赋值给变量。

>>> abs(10) #函数调用
10
>>> abs #函数本身
<built-in function abs>
>>> a = abs(10)
>>> a
10
>>> b = abs
>>> b
<built-in function abs>
>>> b(10)
10
>>> 

函数名也是变量：函数名其实就是指向函数的变量。如abs()中abs为变量，它指向一个可以计算绝对值的函数。

>>> abs = 10 #abs这个变量指向了一个整数10
>>> abs(10)
Traceback (most recent call last):
  File "<pyshell#10>", line 1, in <module>
    abs(10)
TypeError: 'int' object is not callable
>>> abs
10
>>> del abs #可以恢复abs的使用

传入函数：高阶函数，让函数的参数能够接收别的函数

>>> def add(x,y,f):
    return f(x) + f(y)

>>> add(-5,6,abs)
11
>>> 

>>> from math import sqrt
>>> def some(x,*fs):
    s = [f(x) for f in fs]
    return s

>>> print(some(2,sqrt,abs,bin,hex,oct))
[1.4142135623730951, 2, '0b10', '0x2', '0o2']
>>> 

 map():接收两个参数，一个函数，一个Iterable，map将传入的参数依次作用到序列的每个元素，并把结果作为新的Iterable返回

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]
>>> r
<map object at 0x02CB0BD0>
>>> 

 map()函数与for循环的区别：

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

    
>>> print(L)
[1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> L
[1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> 

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

 reduce():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
>>> def fn(x,y):
    return x*10 + y

>>> reduce(fn,[1,3,5,7,9])
13579
>>> 

结合使用map()和reduce():

>>> from functools import reduce
>>> 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]

>>> reduce(fn,map(char2num,'13579'))
13579
>>> map(char2num,'13579')
<map object at 0x02CB0BF0>
>>> list(map(char2num,'13579'))
[1, 3, 5, 7, 9]

 将字符串转换为数字：

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]
def str2int(s):
    return reduce(lambda x, y: x*10 + y, map(char2num,s))
>>> str2int('1024')
1024
>>> 

 使用map函数实现给定列表中字符串首字母大写。

from string import capwords
L1 = ['zyj','sl','SB']
L2 = list(map(capwords,L1))
print(L2)

def normalize(name):
    return name.title()
name = ['zyj','wh','SBj']
L = list(map(normalize,name))
print(L)
>>> 
['Zyj', 'Sl', 'Sb']
['Zyj', 'Wh', 'Sbj']
>>>

from functools import reduce
def prod(L):
    return reduce(lambda x,y: x*y, L)
L1= [1,3,5,7,9]
print(prod(L1))
>>> 
945
>>> 

将小数转换为浮点数：

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]
def str2folat(s):
    return reduce(lambda x,y: x*10+y, map(char2num,s))
s= '123.456'
i = s.find('.')
n = len(s)-(i+1)
s1 = s[:i]+s[(i+1):]
#s1,s2 = s.split('.')
#s1 = s1+s2
result = str2folat(s1)/10**n
print(result)
>>> 
123.456
>>>