返回语句返回None而不是值

我遇到了一个奇怪的问题。此代码返回None，而不是真正的，即使它进入到正确的分支和评估为真：返回语句返回None而不是值

edges = { (1, 'a') : [2, 3], 
      (2, 'a') : [2], 
      (3, 'b') : [4, 3], 
      (4, 'c') : [5] } 
accepting = [2, 5] 
loc = [] 
def nfsmsim(string, current, edges, accepting): 

    if string != "": 
     if ((current, string[0]) in edges.keys()): 
      global loc 
      loc = edges[(current, string[0])] 
      print "edge found:",loc 

    if (string == ""): 
     print "string is over",current,accepting 
     print type(current), type(accepting) 
     if current in accepting : 
      print "1" 
      return True 
     else: 
      print "2" 
      return 2 
    # fill in your code here 
    elif (current, string[0]) in edges.keys(): 
     global loc 
     string = string[1:] 
     nfsmsim(string, loc[0], edges, accepting) 
    elif len(loc)>1: 
     global loc 
     nfsmsim(string, loc[1], edges, accepting) 


# This problem includes some test cases to help you tell if you are on 
# the right track. You may want to make your own additional tests as well. 
print nfsmsim("abc", 1, edges, accepting)

的这个输出是：

string is over 5 [2, 5] 
<type 'int'> <type 'list'> 
1 
None (<< instead of True)

来源

2012-04-23 fixxxer

你应该包括你的'的功能 – jamylak 2012-04-23 04:30:10

def'那么我们就知道你应该做的是打印/返回！ – Colleen 2012-04-23 04:32:14

那里。更新了问题。代码中只有return语句 - 事实是我错过了这个问题。 – fixxxer 2012-04-23 04:33:15

这是一个递归函数。当您到达终端机箱（string == ""）时，您将返回1或2。这会返回到调用函数 - 之前的调用nfsmsim。但nfsmsim的呼叫不返回任何东西！您需要从nfsmsim的终端呼叫中获取该值，然后再次将其返回。

换句话说，你需要在每个if声明的这两个分支的return语句：

elif (current, string[0]) in edges.keys(): 
    global loc 
    string = string[1:] 
    nfsmsim(string, loc[0], edges, accepting) 
elif len(loc)>1: 
    global loc 
    nfsmsim(string, loc[1], edges, accepting)

来源

2012-04-23 04:38:19 senderle

[编辑]我不想给硬件太多;） – 2012-04-23 04:44:02

@ Thr4wn，谢谢:) fixxxer，不要居高临下，但如果你找出自己的回报表。 – senderle 2012-04-23 04:44:29

感谢您的指针！ :)我最终设置了一个标志，并在函数结尾处基于它返回True。但它看起来并不是最干净的方式。 – fixxxer 2012-04-23 05:28:54

函数结束时是一样的使用收益无不使用返回指令。

由于功能是递归的，并且您使用它的结果，你必须返回的每呼吁也是其体内的价值：

elif (current, string[0]) in edges.keys(): 
    global loc 
    string = string[1:] 
    return nfsmsim(string, loc[0], edges, accepting) 
elif len(loc)>1: 
    global loc 
    return nfsmsim(string, loc[1], edges, accepting)

你应该忘记使用全球禄。只要通过参数传递它。这是无论如何参考：

edges = { (1, 'a') : [2, 3], 
      (2, 'a') : [2], 
      (3, 'b') : [4, 3], 
      (4, 'c') : [5] } 
accepting = [2, 5] 
loc = [] 
def nfsmsim(string, current, edges, accepting, loc): 

    if string != "": 
     if ((current, string[0]) in edges.keys()): 
      loc = edges[(current, string[0])] 
      print "edge found:",loc 

    if (string == ""): 
     print "string is over",current,accepting 
     print type(current), type(accepting) 
     if current in accepting : 
      print "1" 
      return True 
     else: 
      print "2" 
      return 2 
    # fill in your code here 
    elif (current, string[0]) in edges.keys(): 
     string = string[1:] 
     return nfsmsim(string, loc[0], edges, accepting, loc) 
    elif len(loc)>1: 
     return nfsmsim(string, loc[1], edges, accepting, loc) 


# This problem includes some test cases to help you tell if you are on 
# the right track. You may want to make your own additional tests as well. 
print nfsmsim("abc", 1, edges, accepting, loc)

它打印出我的控制台上执行以下操作：

c:\tmp\___python\fixxxer\so10274792>python a.py 
edge found: [2, 3] 
edge found: [4, 3] 
edge found: [5] 
string is over 5 [2, 5] 
<type 'int'> <type 'list'> 
1 
True

来源

2012-04-23 08:55:04 pepr

返回语句返回None而不是值

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