在列表中查找最大值的无意外值 - Python 3递归

Question

I am trying to write a simple recursive function to find the maximum in a list without using any built in functions, with the exceptions being 'print' and 'len'. I made a simple linear search using recursion that compares each member of the list to the current Max.

x=[1,2,3]
Max=x[-1]

def Max_list(Max, x, c=2):

    if len(x)==1:
        return Max

    else:

        if c==(len(x)+1):
            print('hi')
            return Max

        elif x[len(x)-c]>Max:
            Max=x[len(x)-c]
            c+=1
            Max_list(Max, x, c)

        elif x[len(x)-c]<=Max:
            c+=1
            Max_list(Max, x, c)

print(Max_list(Max, x))

What confuses me is that my program prints 'hi' (this was a verification that my if condition was satisfied), but returns None. I could have made it attempt to return anything, and it would still return 'None'. I am wondering how to fix it obviously, but if anyone could give me an explanation as to why my code will always return None in its current state, that would be wonderful.

Answer 1

As one of the users pointed out in the comments, you need to add a return statement to the two calls to Max_list(Max, x, c) . Why is this the case?

In Python, if you have call a function that has no return statement, the default return value is None . If we look at your recursion, what it's actually doing is this:

    Initial Call:
    Max_list(3, x, 3)
              vvvvv
            Max_list(3, x, 4)
                      vvvvv
                     print('hi')
                     return 3 
            return None
    return None 

Upon adding those return statements, we get something in the vein of:

    Initial Call:
    Max_list(3, x, 3)
               vvvvvv
            Max_list(3, x, 4)
                      vvvvv
                     print('hi')
                     return 3 
            return the value of the above (3)
    return the value of the above (3)

Answer 2

As user2357112 said, you must explicity use return . Also Patrick Gallagher does make a good point about using x[-1] to retrieve the last element in the list.

x=[1,2,3]
Max=x[-1]

def Max_list(Max, x, c=2):

    if len(x)==1:
        return Max

    else:

        if c==(len(x)+1):
            return Max

        elif x[len(x)-c]>Max:
           Max=x[len(x)-c]
            c+=1
            return Max_list(Max, x, c)

        elif x[len(x)-c]<=Max:
            c+=1
            return Max_list(Max, x, c)

print(Max_list(Max, x))