使用递归时出现“无类型”错误 function

Question

这是我写的程序：

1. count_occur()调用first_occur()以获取元素的第一次出现。
2. 然后count_occur()调用last_occur()得到最后一次出现
3. 然后减去第一次和最后一次出现的索引，得到出现的次数。

#calculates the first occurence of the element

def first_occur(lists,k,low,high):
    
    mid=(low+high)//2
    
    if low>high:
        
        return -1
    #condition to calculate the first occurence
    if lists[mid]==k:
        
        if mid==0 or lists[mid-1]!=lists[mid]:
            
            return mid
        
        else:
            
            return first_occur(lists,k,low,mid-1)
        
    elif lists[mid]<k:
        
        first_occur(lists,k,mid+1,high)
        
    else:
        
        first_occur(lists,k,low,mid-1)
        
#calculates the last occurence of the element
def last_occur(lists,k,low,high):
    
    mid=(low+high)//2
    
    if low>high:
        
        return -1
    #condition to check the last occurence
    if lists[mid]==k:
        
        if mid==len(lists)-1 or lists[mid+1]!=lists[mid]:
            
            return mid
        
        else:
            
            return first_occur(lists,k,mid+1,high)
        
    elif lists[mid]<k:
        
        first_occur(lists,k,mid+1,high)
        
    else:
        
        first_occur(lists,k,low,mid-1)
        
        
#called via the input
def count_occur(lists,k):
    
    l=len(lists)
    fo=first_occur(lists,k,0,l)
    print(fo)
    lo=last_occur(lists,k,0,l)
    print(lo)
    
    if fo==-1:
        
        print('Not found')
        
    else:
        #checks for the number of occurences
        print("Found",(fo-lo+1),"times.")
        
        
#getting input via the user of the list.
a=list(map(int,input().split()))
print(a)

k=int(input("Enter the number, the occurences of which to be found."))

count_occur(a,k)

这是我收到的 output。 它应该包括元素在列表中出现的次数。 但这并没有发生：

12 34 56
[12, 34, 56]
Enter the number, the occurences of which to be found.12
None
None
Traceback (most recent call last):
  File "/Users/somilsharma/Desktop/DSA/14.py", line 79, in <module>
    count_occur(a,k)
  File "/Users/somilsharma/Desktop/DSA/14.py", line 70, in count_occur
    print("Found",(fo-lo+1),"times.")
                   ~~^~~
TypeError: unsupported operand type(s) for -: 'NoneType' and 'NoneType'

Answer 1

问题是您在 function 末尾附近递归调用first_occur()和last_occur()函数，但您没有使用return返回它们的值。 如果没有这些 return 语句，函数默认返回None ，因此会出现有关None的错误和日志消息。 别担心，这是一个相当常见的错误。

只需更改 function 末尾的第 22 和 25 行：

first_occur(lists,k,mid+1,high)

到

return first_occur(lists,k,mid+1,high)

此外，对last_occur() function 执行相同的操作。