从Python中的递归函数返回

Question

我对编程有点不熟悉，并且对返回函数的工作原理有些困惑。我正在尝试编写一个将函数映射到嵌套列表的元素的程序。变量级别表示列表中嵌套级别的次数。目前，我可以得到程序通过打印我的最终映射列表，totlist工作：

def map_nested(listbasket, function, levels): #listbasket is the list that contains lists 
    totlist=[] #this list will store the list after the function has been mapped to it 
    for listelement in listbasket: 

     if levels<=2: #once we get to the level that just contains a list of lists 
      newlist=list(map(function,listelement)) 
      totlist.append(newlist) #add to final mapped list 

     else: 
      map_nested(listelement, function, levels-1) #recursively call for next level 
    print(totlist) 

map_nested([[[1,2],[3,4]],[[5,6],[7,8]]], math.sqrt, 3) # my test function 

相反，我想要的东西，返回totlist，但我无法弄清楚如何做到这一点。每次我尝试返回它时，它都会返回一个空列表或列表的一部分。我觉得我已经尝试了我能想到的每种回报配置。

Answer 1

这将工作：

import math 

def map_nested(listbasket, function, levels): #listbasket is the list that contains lists 
    totlist=[] #this list will store the list after the function has been mapped to it 
    for listelement in listbasket: 

     if levels<=2: #once we get to the level that just contains a list of lists 
      newlist=list(map(function,listelement)) 
      totlist.append(newlist) #add to final mapped list 
     else: 
      totlist.append(map_nested(listelement, function, levels-1)) 
    return totlist 

map_nested([[[1,2],[3,4]],[[5,6],[7,8]]], math.sqrt, 3) # my test function 

或稍微整洁的解决方案：

import math 

def map_nested(input, function): 
    if type(input) is list: 
     return [map_nested(e, function) for e in input] 
    else: 
     return function(input) 

print map_nested([[[1,2],[3,4]],[[5,6],[7,8]]], math.sqrt) 

这是递归应用map_nested方法在层次结构中的每个列表。当递归到达列表中的元素时，它将应用原始调用中提供的函数。

请注意，这适用于任意深度嵌套的列表，也适用于不平衡的嵌套列表（例如，[1, 2, [3, 4]]）。

Answer 2

我会让totlist参数：

def map_nested(listbasket, function, levels, totlist=None): 
    if totlist is None: 
     totlist = [] 
    for listelement in listbasket: 
     if levels <= 2: 
      newlist = list(map(function, listelement)) 
      totlist.append(newlist) #add to final mapped list 
     else: 
      map_nested(listelement, function, levels-1, totlist) 
    return totlist 

现在：

>>> map_nested([[[1,2],[3,4]],[[5,6],[7,8]]], math.sqrt, 3) 
[[1.0, 1.4142135623730951], 
[1.7320508075688772, 2.0], 
[2.23606797749979, 2.449489742783178], 
[2.6457513110645907, 2.8284271247461903]] 

如果你想简化（不手动传递水平）和扁平巢，当您去，是这样的：

def map_nested_2(lst, f, out=None): 
    if out is None: 
     out = [] 
    for item in lst: 
     if isinstance(item, list): 
      map_nested_2(item, f, out) 
     else: 
      out.append(f(item)) 
    return out 

会给：

[1.0, 1.4142135623730951, 1.7320508075688772, 2.0, 2.23606797749979, 
2.449489742783178, 2.6457513110645907, 2.8284271247461903]