Question

我已经读过，python中的递归工作非常慢。是否可以将此递归更改为循环函数，这将更快地工作？我的功能有点复杂，但我会尝试展示最重要的部分：

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模块构建优先级队列。如果我有一个递归函数，例如计算Fibonnaci数，我可以很容易地将递归转换为循环。但在我的功能中，我没有回复声明，所以我不确定如何做到这一点。

Answer 1

我认为你可以直接这样做，实际上。只需先用root初始化堆，然后就可以使用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)

如果将实际的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并跳过该初始步骤，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，您始终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:将在代码中出现两次。

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)

现在换行do … while，删除尾调用并将条件谓词放入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：创建top不需要是有条件的。注意：我没有修复任何错误

更改循环的递归函数

2 个答案: