更改循环的递归函数

Question

我读过，python 中的递归工作非常缓慢。是否可以将此递归更改为循环函数，这样可以更快地工作？ 我的功能有点复杂，但我会尽量展示最重要的部分：

class Element(object):
    def __init__(self, name):
        self.name = name
        self.priority = randint(1000)
    # some other operations and functions

import heapq

def fun(name):

      if condition():
         e = Element(name)
         # make some operations
         for i in e.sublist:
             if condition2():
                   heapq.heappush(heap,e)
             else:
                   updatepriority(e)
      if heap:
         top = heapq.heappop(heap)
         fun(top.name)

所以我有一个递归函数，它搜索抛出许多子列表并使用 heapq 模块构建一个优先级队列。 如果我有一个递归函数，例如计算斐波那契数，我可以轻松地将递归转换为循环。但是在我的函数中，我没有返回语句，所以我不确定我该怎么做。

Answer 1

实际上，我认为您可以相当直接地做到这一点。只需先用你的 root 初始化堆，然后你就可以使用 while 进行迭代。

def fun(root):
  # Initialize the heap with your root element.
  heap = []
  if condition():
    e = Element(root)
    # make some operations
    for i in e.sublist:
      if condition2():
        heapq.heappush(heap,e)
      else:
        updatepriority(e)

  # Now we can iterate through the remainder of the heap.
  while heap:
    top = heapq.heappop(heap)
    if condition():
      e = Element(top.name)
      # make some operations
      for i in e.sublist:
        if condition2():
          heapq.heappush(heap,e)
        else:
          updatepriority(e)

如果您将实际堆元素传递给函数参数，而不是 name，那么您可以立即将其推送到 heap 并跳过初始步骤，从而使您的代码更简单。

def fun(root):
  heap = [root]

  # Now we can iterate through the remainder of the heap.
  while heap:
    top = heapq.heappop(heap)
    if condition():
      e = Element(top.name)
      # make some operations
      for i in e.sublist:
        if condition2():
          heapq.heappush(heap,e)
        else:
          updatepriority(e)

Answer 2

是的，您总是可以将递归转换为迭代：

• 首先确保所有递归调用都是尾调用，就像你所做的那样。

  def fun(name):
  
    if condition():
       e = new Element(name)
       # make some operations
       for i in e.sublist:
           if condition2():
                 heapq.heappush(heap,e)
           else:
                 updatepriority(e)
    if heap:
       top = heapq.heappop(heap)
       fun(top.name)
  

• 然后让尾调用有自己的条件，if heap: 会在代码中出现两次。

  def fun(name):
  
    if condition():
       e = new Element(name)
       # make some operations
       for i in e.sublist:
           if condition2():
                 heapq.heappush(heap,e)
           else:
                 updatepriority(e)
    if heap:
       top = heapq.heappop(heap)
    if heap:
       fun(top.name)
  

• 现在包裹do … while，移除尾调用并将条件谓词放在while中，并处理递归的参数。

  def fun(name):
    do #not python
  
       if condition():
           e = new Element(name)
           # make some operations
           for i in e.sublist:
               if condition2():
                   heapq.heappush(heap,e)
               else:
                   updatepriority(e)
        if heap:
           top = heapq.heappop(heap)
  
        name = top.name
    while heap # not python
  

• 现在转换为while（python没有do...while）

  def fun(name):
      heap=True
      while heap:
  
          if condition():
              e = new Element(name)
              # make some operations
              for i in e.sublist:
                  if condition2():
                      heapq.heappush(heap,e)
                  else:
                      updatepriority(e)
          if heap:
              top = heapq.heappop(heap)
  
          name = top.name
  

注意：top 的创建不需要是有条件的。 注意：我没有修复你的任何错误